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The structure and morphology of samples of syndiotactic polypropylene (sPP) having different stereoregularity with concentration of rrrr pentads from 78 to 95.8%, obtained by quenching the melt to 0°C, is analyzed by wide angle X-ray scattering (WAXS) and atomic force microscopy (AFM). The fast cooling from the melt induces the formation of nanocrystals in the trans-planar mesomorphic form of sPP and results in a morphology characterized by the presence of globular entities (nodules) and no higher-order superstructure. It is shown that the nodule sizes depend on the stereoregularity of the sPP samples and the permanence time at 0°C. The observed nodular morphology is completely different from the lamellar morphology typically obtained in sPP samples melt crystallized at lower cooling rate. This result indicates that crystals of different shape, size and higher-order organization can be generated by changing the crystallization pathway.

The correlations between the thermal behavior and the crystal morphology of isotactic propene-pentene copolymers were studied through wide-angle (WAXS) and small-angle (SAXS) X-ray diffraction. Copolymers with pentene concentration lower than 11 mol% crystallize in the α form of isotactic polypropylene (iPP) and a concomitant decrease of melting temperature and of the thickness of crystalline lamellae with increasing pentene concentration has been observed. At higher pentene concentrations the trigonal form of iPP crystallizes and a neat increase crystalline lamellar thickness and of the long period, with a slower decrease of melting temperature and crystallinity have been observed. These results have been treated in the general framework of copolymer crystallization theories, using a method proposed by Crist and correlated with the different level of inclusion of pentene co-units in the crystals of α and trigonal forms. For copolymers with pentene concentration lower than 11 mol% pentene co-units are in part incorporated in the crystals of α form. For higher pentene concentrations the decrease of melting temperature coupled with the increase of lamellae thickness with increasing comonomer content is the hallmark of the almost total inclusion of pentene co-units in the crystals of the trigonal form.

The mechanical behavior of propylene−pentene isotactic copolymers synthesized with a single-center organometallic catalyst in a broad interval of pentene (Pe) concentration, from 3 to 50 mol %, is analyzed. All copolymers show flexible behavior with very high deformation at breaking. The presence and entrance of Pe counits in α and δ forms of isotactic polypropylene (iPP) transform a strong and fragile iPP homopolymer into ductile and flexible materials. The stress at yield decreases as the Pe content increases, but a different behavior has been observed for samples in the α form or in the δ form. In samples in the α form, from 3 to 8.8 mol % of Pe, the stress at yield increases as the thickness of crystalline lamellae increases, while in samples in the δ form containing higher Pe counits, it decreases as the thickness of lamellae increases and Pe content increases. Phase transition of the α form in the mesophase and crystallization of the δ form, in samples with 8−11 mol % of Pe, occur upon deformation and are correlated to the strain-hardening observed at high strain.

The crystallographic micromechanical model (CMM) for prediction of yield stress of semicrystalline polymers, based on the thermally activated nucleation of screw dislocations at the boundary of lamellar crystals, is employed to interpret the yield behavior by effect of uniaxial drawing of some isotactic copolymers of propylene with different comonomeric units such as ethylene, 1-butene, 1-pentene, 1-hexene, and 1-octadecene (iPPEt, iPPBu, iPPPe, iPPHe and iPPOc, respectively). The samples are characterized by a random distribution of the comonomeric units. The CMM predicts that the values of stress at yield depend on the thickness of the lamellar crystals and relies on two parameters, i.e. the critical value of the free energy needed for nucleation and activation of a screw dislocation in crystallographic planes parallel to the chain axes, and the shear modulus relative to the planes of slip for the dislocations, whereas the role of the interlamellar amorphous phase is neglected. The aim of this study is to analyze to which extend the thickness of the lamellar crystals influences the yield stress for a series of propylene-based copolymers having a well-defined chain microstructure and crystallized in the α and/or γ forms, but possessing different degree of crystallinity, thickness of the lamellar crystals, stability of the crystals and also intrinsic flexibility of the portions of chains belonging to the amorphous regions, in relation with the degree of inclusion (exclusion) of the co-monomers in (from) the crystals. It is shown that, in the case of copolymers with a comonomer concentration lower than a threshold, the yield stress increases with increasing the lamellar thickness regardless of type of comonomer, the relative amount of the two polymorphs (α and γ forms), and the degree of inclusion, in good agreement with the predictions of the CMM approach. For copolymers with comonomer concentration higher than a threshold and bulky side groups, which are excluded from the crystals, the thickness of the lamellar crystals becomes low and the role of the chains in the amorphous regions becomes not negligible. In the case of copolymers with high butene concentration, instead, the stress at yield decreases with increasing the thickness of the lamellar crystals, because the high degree of inclusion of butene units in the crystals induces not only an increase in the thickness of the lamellar crystals, but also a decrease in the stability of the crystals, and the role of the chains located in the interlamellar amorphous layers acting as tie-molecules may not be disregarded.

The development of a highly oriented fiber morphology by stretching low stereoregular isotactic polypropylene (iPP) samples is studied at two different temperatures. The structural and morphological transformations occurring during deformation are followed in real time by collecting in situ wide (WAXS) and small-angle (SAXS) X-ray scattering data. WAXS analysis reveals that the disordered γ form initially present in the samples gradually transforms by stretching into the oriented mesomorphic form of iPP at room temperature and in well-oriented crystals of α form at 60 °C. SAXS analysis indicates that regardless of the stretching temperature, a cascade of events occurs at mesoscale during deformation, consisting in interlamellar separation and lamellar reorientation at low deformations, followed by formation of chevron-like textures, lamellar fragmentation, and cavitation. Moreover, different fibrillary morphologies develop at the different temperatures. In fact, the fibrillary morphology that develops at room temperature is characterized by rod-like fibrillary entities containing mesomorphic aggregates separated by amorphous regions, placed at uncorrelated longitudinal and lateral distances. Instead, the morphology that develops at 60 °C consists of a fibrillary network for the more stereoregular sample and of rod-like fibrillary entities for the less stereoregular sample, and both morphologies include well-oriented crystals of α form alternating with amorphous regions which are periodically stacked along the stretching direction and are placed at uncorrelated lateral distance. We argue that the difference in structural arrangement of amorphous and crystalline regions at mesoscale in the stretched samples are mainly due to the increase of the mobility of the chains in the amorphous matrix at higher temperature, which facilitates the relaxation of the amorphous phase and the changes in the relative arrangement of the crystalline domains.

Innovation of the Ziegler–Natta polymerization process for polymerization of propene that allows tailoring of the mechanical properties of isotactic polypropylene (iPP) directly in the polymerization stage is presented. The catalyst is modified by prepolymerization of trimethylallylsilane or vinylcyclohexane so that the catalyst particles are coated by a thin skin of poly(trimethylallylsilane) or poly(vinylcyclohexane) that will act as a nucleating agent for the crystallization of iPP. The presence of the nucleating agent accelerates the crystallization of iPP and affords crystallization of the α form even upon fast crystallization by quenching the melt, a condition that generally produces crystallization of the mesomorphic form. Crystals of the α form so obtained show a nodular morphology and the absence of the spherulitic superstructure. This novel iPP material is characterized by outstanding and unexpected properties of high mechanical strength and modulus and contemporarily high ductility, flexibility and good transparency due to the nodular morphology of the α form.

Binary blends of samples of syndiotactic polypropylene of different stereoregularity have been prepared by mixing a more stereoregular and crystalline sample sPP1 having [ rrrr ] = 78%, with low stereoregular and poorly crystalline samples sam PP1 and sam PP2 having [ rrrr ] = 54.6 and 45.8%, respectively, and a nearly atactic fully amorphous sample am PP1 with [ rrrr ] = 26.5%. The blends show separate melting of the two components with the more stereoregular component sPP1 melting at high temperature and the poorly syndiotactic component melting at lower temperature. Crystals of sPP1 are formed by cooling from the melt whereas more defective crystals of sam PP1 and sam PP2 are formed by cold crystallization during aging at room temperature. The presence of the strereoirregular component disturbs the melt crystallization of the stereoregular component sPP1 and depression of the crystallization and melting temperatures of sPP1 in the blends is observed. On the other hand, the cold crystallization at room temperature of the irregular component is accelerated and favored by the presence of the more stereoregular component sPP1, whose crystals act as crystallization nuclei. All data indicate interactions between the components in the melt and amorphous state, compatible with a good degree of mixing.

This chapter focus on the polypropylene tacticity. The stereoregularity and regioregularity of the processes catalyzed by heterogeneous and homogeneous Ziegler-Natta catalysts are discussed. The amazing variety of molecular architectures available for polypropylene-based materials are summarized together with the catalytic mechanisms for tacticity control. The main techniques applied for the determination of PP tacticity are reported as well as the outstanding development of new catalyst systems able to achieve unprecedented PP microstructures.

The crystal structure and polymorphism of isotactic (iPP) and syndioactic polypropylene (sPP) are illustrated, highlighting the rich variety of phase behavior of these polymers, the conditions of obtainment of the different polymorphs and the disorder phenomena occurring in the crystals. After description of the concepts of packing and conformational polymorphism occurring in the case of iPP and sPP respectively, the crystal structure of the different polymorphs of iPP and sPP are described. In particular, the main structural features relative to the monoclinc α-, the trigonal β- and the orthorhombic γ-forms, of iPP including the mesomorphic form, and the trigonal form which develops in random isotactic copolymers of propylene with pentene or hexene units, are described at first, the chain conformation in all these polymorphs being the 3/1 helix. Then, the complex polymorphism of sPP and the crystal structure of the orthorhombic helical form I and II, the orthorhombic trans-planar form III, the monoclininc form IV, and the trans-planar and helical mesophases are illustrated. The implications of the crystal structure with the final properties are outlined for these polymers, the great fortune of which was the almost simultaneous discovery of the polymerization catalyst systems and the structural elucidation.

The synthesis and the crystal structure of isotactic poly((R,S)-3-methyl-1-pentene) (iP(R,S)3MP) are presented. The synthetic strategy based on hydrogenation of isotactic 1,2-poly(E-3-methyl-1,3-pentadiene) has allowed for the preparation of a purely random copolymer of the two enantiomeric (R)-and (S)-3-methyl-1-pentene monomers. X-ray diffraction analysis and conformational and packing energy calculations have shown that chains of iP(R,S)3MP are in 4/1 helical conformation and are packed in a monoclinic unit cell with parameters a = 10.02 Å, b = 18.48 Å, c = 6.87 Å, and γ = 109.9° according to the space group P2 1 /b or P2 1. A high degree of disorder is present in the crystals due to the random enchainment of the enantiomeric R and S monomeric units, whose chirality influences the handedness of the helical chains and the conformation assumed by the lateral groups. Disorder in the conformation of the lateral groups is also present. The crystal structure of the random copolymer iP(R,S)3MP is different from the tetragonal structure of the pure enantiomer homopolymer iP(S)3MP. This is due to different entropic effects, related to the presence of different types of disorder included in the crystals that drive crystallization in different packing modes. The crystal structure of iP(R,S)3MP provides an example of the effect of the chirality of side groups on the crystal packing and is an example of symmetry breaking in the structures of polymers.

Syndiotactic poly(3-methyl-1-butene) (sP3MB) has been obtained for the first time by hydrogenation of 3,4-dpoly(isoprene), which in turn has been synthesized with the catalyst FeCl2(2,2'-bipyridine)(2) activated with methylaluminoxane. The synthesized sample of sP3MB is amorphous but crystallizes by stretching and annealing of the oriented amorphous phase at 60-70 degrees C. The diffraction image of the annealed fibers shows only three equatorial reflections and a few weak reflections distributed on three nonequatorial layer lines. From these data a model of the conformation of the chains of sP3MB is proposed. The chains of sP3MB assume in the crystalline phase a 3/1 helical conformation, providing the first example of a syndiotactic polymer with 3-fold helical chains. A high degree of disorder is present in the crystals so that the structure can be described by an arrangement of parallel conformationally ordered 3/1 helical chains and disorder in the lateral correlations between the chains.

The development of a highly oriented fiber morphology by effect of tensile deformation of stereodefective isotactic polypropylene (iPP) samples, starting from the unoriented form, is studied by following the transformation in real time during stretching through wide angle X-ray scattering (WAXS) measurements. In the stretching process, after yielding, the initial form transforms into the mesomorphic form of iPP through mechanical melting and re-crystallization. The analysis of the scattering invariant measured in theWAXS region highlights that the size of the mesomorphic domains included in the well oriented fiber morphology obtained at high deformations increases through a process which involves the coalescence of the small fragments formed by effect of tensile stress during lamellar destruction with the domain of higher dimensions. info:eu-repo/semantics/publishedVersion

The structure and the mechanical properties of metallocene-made butene-pentene (iPBC5) and butene-hexene (iPBC6) isotactic copolymers are studied. The effect of the presence of pentene and hexene units on the crystallization behavior and the mechanical properties of isotactic polybutene (iPB) is analyzed. All iPBC5 copolymers crystallize from the melt in form II of iPB, which transforms into form I by aging at room temperature. On the contrary, in iPBC6 copolymers the presence of hexene units stabilizes the form II and, for hexene concentrations higher than 11 mol%, prevents the transformation of form II into form I at room temperature. Both iPBC5 and iPBC6 copolymers show mechanical behavior of highly flexible and ductile materials with enhanced ductility compared to iPB, with values of stress at yielding and of Young's modulus that decrease with increasing comonomer content. In all the iPBC5 copolymers, form II crystallized from the melt transforms into form I by stretching, whereas in iPBC6 copolymers form II is stabilized and the transformation of form II into form I by stretching is completely inhibited at high hexene concentrations.

We report the synthesis and the crystal structure of isotactic polyIJ(R,S)-3-methyl-1-pentene) IJiPIJR,S)3MP). A purely random achiral copolymer of two enantiomeric (R) and (S) 3-methyl-1-pentene monomers cannot be obtained by polymerization of the racemic mixture of the (R) and (S) monomers but has been obtained by stereospecific polymerization of 3-methyl-1,3-pentadiene to isotactic 1,2-poly(3-methyl-1,3- pentadiene) (iP3MPD12) and successive hydrogenation. The crystal structures of chiral iP(R,S)3MP and chiral polyIJ(S)-3-methyl-1-pentene) (iP(S)3MP) are different, indicating that crystal structures of polymers may be driven by different entropic effects related to chirality and type of crystal disorder. The conformational disorder of the chiral lateral groups prevails in chiral iP(S)3MP, inducing crystallization of chains of one helical chirality, whereas the entropic effect due to the statistical substitution of chains of different helical chirality in the sites of the lattice prevails in chiral iP(R,S)3MP.

A study of the mechanical properties and of the crystallization behaviour of samples of isotactic polybutene (iPB) of different stereoregularity prepared with metallocene catalysts, is presented. Stereoregular samples with concentration of rr defetcs lower than 2 mol% crystallize from the melt by compression-molding and fast cooling to room temperature in form II, whereas more stereoirregular samples crystallize surprisingly mainly in form III. The stereodefective iPB samples show interesting mechanical properties of high ductility and flexibility that increase with increasing concentration of rr stereodefects, with values of deformation at break higher than 1000% for the most stereoirregular sample containing about 5 mol% of stereodefects. The aging at room temperature produces transformation of form II into form I in more stereoregular samples, whereas no phase transformation occurs in more stereoirregular samples crystallized mainly in form III. A strong increase of Young’s modulus and of stress at yield is observed after transformation of form II into form I. For more stereoirregular samples with concentration of rr defects higher than 2 mol% crystallized in form III, the mechanical properties do not change significantly upon aging at room temperature because no phase transformations occur during aging. Also for the aged samples a remarkable increase of ductility and flexibility with increasing concentration of rr stereodefects is observed. The mechanical deformation is associated with stress induced transformations of form II and form III into form I. These results indicate that mechanical properties of iPB can be modified by the controlled incorporation of stereodefects and improvement of ductility is easily obtained while maintaining high crystallinity by inclusion of small amounts of rr stereodefects.

A set of syndiotactic polypropylene samples with tailored degree of stereoregularity and uniform distribution of stereodefects, obtained via organometallic catalysis, was used to study the relation between the syndiotactic pentad concentration and the viscoelastic plateau modulus in the melt state. The plateau modulus was found to increase with increasing the degree of stereoregularity, indicating that syndiotactic polypropylenes of different stereoregularity produce a different entanglement density of the amorphous phase. The change in plateau modulus was highly nonlinear for large values of syndiotactic pentad contents, suggesting that the presence of even a relatively small number of defects along the syndiotactic backbone significantly alters the space filling attitude of the polymer chain in the melt. This result, which confirms and extends those already obtained from experiments and numerical simulations, points out to the concept that the dynamics of macromolecular chains is largely controlled by the relative configuration of consecutive stereoisomeric centers along the chain, thus providing a link between chain dynamics and molecular architecture

The structure and crystallization properties of a norbornene end-functionalized syndiotactic polypropylene (sPP) macromonomer (MMsPP) with [rrrr] = 80 mol % and molecular mass Mn = 3600 g/mol and corresponding comb-like poly(macromonomer) (pMMsPP) with Mn = 100000 g/mol have been investigated using wide angle (WAXS) and small angle (SAXS) X-ray scattering, atomic force (AFM), transmission electron (TEM), and optical (OM) microscopy. The analysis reveals the tendency of the macromonomer and poly(macromonomer) to form structures characterized by layers of sPP chains alternated to layers occupied by the cyclic groups (norbornene or polynorbornene), and a small degree of interdigitation of sPP chains belonging to adjacent layers facing each other in an end-to-end arrangement. It is argued that this layering is dictated, upon crystallization, by nanophase separation of the flexible sPP chains in regions apart from those occupied by the cyclic groups coupled to formation of sPP crystals organized in the lamellar morphology. SAXS measurements indicate that in isothermally crystallized samples the layered structures are irregular and include sPP lamellar crystals of uniform thickness separated by amorphous layers whose thickness values have a multimodal distribution. These properties are intrinsic to the chemical structure of our systems and are only minimally influenced by the topological constrains of the covalent bonding of macromonomers in the corresponding comb-polymer.

A study of the morphology and the mechanical properties of the mesomorphic form of isotactic polypropylene (iPP) that crystallizes in samples of different stereoregularity prepared with metallocene catalysts is reported. Highly isotactic samples slowly crystallized from the melt in the α form show the typical lamellar morphology with organization in spherulites. Bundle-like elongated crystalline entities and needle-like crystals of γ form are instead observed for stereoirregular samples slowly crystallized from the melt in the γ form. All samples crystallize by fast quenching the melt at 0 °C in the mesomorphic form, regardless of stereoregularity. Crystals of the mesomorphic form always exhibit a nodular morphology and absence of lamellar spherulitic superstructures, independent of the stereoregularity. This morphology accounts for the similar good deformability of all the quenched samples, whatever the concentration of stereodefects. For all samples and any stereoregularity, the nodular morphology with absence of spherulites is preserved after annealing and transformation of the mesophase into α form. The formation of a nodular α form accounts for outstanding properties of high ductility and mechanical strength of the annealed samples crystallized in the α form. The most stereoirregular sample with rr concentration of 11% shows elastic behavior either when it is slowly crystallized in the γ form or when it is crystallized in the mesomorphic form and also after annealing and transformation of the mesophase into the nodular α form.

The resolution of the crystal structure and the physical properties of isotactic 1,2-poly(E-3-methyl-1,3-pentadiene) (iPE3MPD) are presented. This new polymer is one of the very few examples of known crystalline isotactic 1,2-polydienes. It does not crystallize by cooling from the melt but crystallizes by aging the amorphous samples at room temperature for several days and successive annealing. The stretching of amorphous or low crystalline compression-molded samples gives oriented fibers of a highly disordered mesomorphic form. Well-oriented fibers of the ordered crystalline form of iPE3MPD have been prepared by stretching amorphous or low crystalline compression-molded samples and successive annealing at 60-70 degrees C under tension. The crystalline structure of iPE3MPD is characterized by macromolecules in 7/2 helical conformation packed in an orthorhombic unit cell with parameters a = 17.4 angstrom, b = 16.5 angstrom, c = 15:3 angstrom according to the space group P2(1)ab. iPE3MPD has shown interesting mechanical properties of high deformability with elastic behavior associated with the crystallization during deformation of the mesophase that melts upon relaxation.

In recent years it has been introduced a new generation of complexes of transition metals and lanthanides with various ligands containing donor atoms such as P, N, and O (e.g., phosphines, imines, iminopyridines, and ketoimines) that, in combination with MAO, give highly stereoregular polymers (cis-1,4, iso- and syndiotactic; 1,2, iso- and syndiotactic) from various substituted butadienes (i.e., isoprene, 1,3-pentadiene, 1,3-hexadiene, 3-methyl-1,3-pentadiene, 1,3-heptadiene, 1,3-octadiene, and 5-methyl-1,3-hexadiene). The availability of all these stereoregular polymers has allowed us to have access, via hydrogenation reaction with p-toluenesulfonyl hydrazide, to a whole series of olefin homopolymers and perfectly alternating copolymers which, in some cases, were not obtainable through the simple stereospecific polymerization of the corresponding monomers. In this communication, we report on the synthesis and characterization (by NMR, DSC, GPC, and X-ray) of four perfectly alternating ethylene/2-butene copolymers having different stereoregularity degree, from completely isotactic to predominantly syndiotatctic; in particular, the racemo-di-isotactic ethylene/2-butene copolymer obtained by hydrogenation of diisotactic trans-1,4-poly(2,4-hexadiene) is a novel polymer which was never reported before.7

The yield behavior during uniaxial drawing of isotactic random copolymers of propene with ethylene (iPPEt), 1-butene (iPPBu), 1-pentene (iPPPe), 1-hexene (iPPHe), and 1-octadecene (iPPOc) is analyzed within the framework of our current understanding of deformation properties of semicrystalline polymers, that is, the intrinsic stability of lamellar crystals and related polymorphism phenomena, along with the ability of entangled amorphous chains to transmit stress. The samples selected for analysis were synthesized using single-site metalorganic catalysts, are highly stereoregular, and contain small amounts of regiodefects caused by secondary 2,1 erythro units. Moreover, the interchain and intrachain distribution of comonomeric units is uniform. In the case of iPPEt copolymers, samples containing ≈3.5 mol% stereodefects were also studied. The yield behavior of these samples depends on the kind and concentration of defects, and is directly related to the level of inclusion in and exclusion from crystals of the comonomeric units. Apart from iPPBu copolymer samples with high butene content, the yield stress of all samples increases with the thickness of lamellar crystals according to a common trend, regardless of comonomer. In the case of iPPBu copolymers containing a high concentration of butene units, the yield stress decreases with increasing lamellar thickness. The increase in yield stress with lamellar thickness is rationalized in terms of the micromechanical model of crystallographic slips, based on thermal activation of screw dislocations. The parameters of the model describing the yield behavior are the critical free energy required to form a screw dislocation and the shear modulus associated with the slip planes of the dislocations. These were set as identical to those deduced for isotactic polypropylene homopolymer samples (iPP) crystallized under different conditions. Study of the yield behavior of these copolymers extends the use of the dislocation model to a set of samples crystallized under similar conditions but characterized by differences in comonomeric unit, degree of crystallinity, lamellar thickness, polymorphism, and intrinsic flexibility of the chain backbone. The results indicate that for a homogeneous class of propene-based copolymers, namely crystallized in the α-form of iPP under similar conditions, lamellar thickness controls the level of plastic resistance provided that the concentration of structural irregularities in the crystals is not too high. iPPBu copolymers with high comonomer concentration do not obey this rule because of the high level of inclusion of comonomers in the crystals, which induces an increase in lamellar thickness but also a decrease in crystal stability.

A study of the nucleating effects of isotactic poly(trimethylallylsilane) (iPTMAS) and poly(vinylcyclohexane) (iPVCH) for isotactic polypropylene (iPP) and their influence on the mechanical behavior of iPP is reported. The particles of catalyst are covered by poly(trimethylallylsilane) or poly(vinylcyclohexane) by polymerization of the corresponding monomers before polymerization of propylene. The method guarantees that the nucleating agents are perfectly dispersed into the iPP samples, with their consequent high efficiency. Isotactic poly(vinylcyclohexane) and poly(trimethylallylsilane) favor crystallization from the melt of α and β forms, respectively. Both nucleating agents affect the morphology greatly reducing the size of shperulites. This in turn affects the mechanical properties improving ductility and flexibility. All nucleated samples crystallize by rapid cooling from the melt in the α form and not in the mesophase, as occurs in the pure iPP. The resulting α form presents nodular crystals and behaves as a stiff material with high deformability.

Using nonliving bis(phenoxyimine)titanium catalysts activated by methylaluminoxane (MAO) in the presence of propylene, allyl-terminated syndiotactic polypropylene macromonomers with varying tacticity ([rrrr] = 0.80 and 0.94) and molecular weight (Mn = 3600 and 5600 g/mol) were produced. The end-functionalized polymers were converted to hydroxyl- and subsequently norbornene-terminated macromonomers. Two series of syndiotactic polypropylene comb-polymers were synthesized through metathesis polymerization of the norbornene-functionalized polypropylene. The molecular weight (Mn = 46 000–172 000 g/mol) and polydispersity (Mw/Mn = 1.21–1.89) of the comb polymers was determined by gel permeation chromatography (GPC). Using differential scanning calorimetry (DSC), melting temperature (Tm) and crystallization temperature (Tc) were determined and both were observed to decrease with increasing conversion to poly(macromonomer). To the best of our knowledge, this is the first synthesis of comb-polymers from end-functionalized syndiotactic polypropylene.

A theoretical model has been derived for the calculation of small-angle X-ray scattering (SAXS) data from stacks of crystalline lamellae in which two or more kinds of lamellae are mixed within the same stack according to different statisticalmodels, ranging from the fully randommode, to alternated one, up to block configurations. The model has been used to study the complex lamellar morphology which develops from melt in some blends obtained bymixing two semicrystalline samples of isotactic polypropylene (iPP) characterized by different degree of stereoreoregularity, similar molecular masses, and melting temperatures of ≈162 C for the more stereoregular component iPP1 and ≈74 C for the less stereoregular one iPP2. It is shown that for samples crystallized in conditions far from thermodynamic equilibrium, the observed SAXS intensity profiles may easily show absence of correlation peaks, associated with high amount of diffuse scattering, not easy to unravel. Using our theoretical approach we show that featureless SAXS intensity distribution may correspond to formation of disordered lamellar stacks with lamellae of the two components randomly mixed within the same stacks. The tendency of iPP1 and iPP2 to form mixed lamellar stackswith a random configuration provides clear evidence that the two components are miscible in the melt.

Single crystals of syndiotactic poly(propene-co-1-butene) with 1-butene as the rich component have been obtained through ultrathin film growth from the melts. The morphological and structural studi...

A study of the relationships between the stress-induced phase transitions and the mechanical properties of isotactic propylene-hexene copolymers with hexene concentration in the range 1-26 mol%, prepared with metallocene catalysts, is reported. Hexene units are included in crystals of R form of isotactic polypropylene (iPP) and produce large disturbance of the crystalline lattice and a consequent decrease of melting temperature, degree of crystallinity, crystallite size, and plastic resistance of the crystals. Defective crystals of R form rapidly transform by stretching into the mesomorphic form of iPP that, in turn, facilitates further stretching up to very high values of deformation of nearly 900-1000%, resulting in high flexibility. This explains the experimental observation that the presence of hexene comonomeric units induces a strong enhancement of ductility, flexibility, and toughness, compared to the highly stereoregular homopolymer prepared with the same catalyst. These copolymers show mechanical properties of highly flexible materials with values of the tensile strength, elastic modulus, and resistance to the plastic deformation that depend on the degree of crystallinity and the occurrence of phase transitions during deformation and can be easily tailored by changing the hexene concentration.

A reversible stress-induced phase transition occurs by stretching syndiotactic copolymers of propene and 1-butene with 1-butene content higher than 70 mol-%. It involves a non-standard mode of distortion of the crystalline lattice in a direction transverse, rather than parallel, to the applied tensile stress. We demonstrate that this distortion reflects conformational and crystallographic restraints on the slip processes involved at high deformations. This transition plays an important role in the elasticity of this novel class of thermoplastic elastomers, which show rigidity and mechanical strength two orders of magnitude higher than those of conventional elastomers.

Isotactic polypropylene-based (iPP) materials find large applications for the food packaging. In this article the structure, thermal and mechanical properties of some commercial grades of ethylene/propene copolymers produced by heterogeneous Ziegler-Natta (ZN) catalysis, have been characterized. These grades can be potentially used for manufacturing food containers. However, despite the desired performances in terms of mechanical and optical properties, we found that the samples exceed the overall migration limit set by the EU Directive into isooctane, used as fatty food simulant. The migrating species are highly defective copolymer chains with low molecular mass and high ethylene content. The migration is intrinsically related to the presence of fractions generated by the heterogeneous multisite surface of the ZN catalysts. A simple chemical route to allow the iPP-based materials to fall within the overall migration limit is proposed.

A study of the structure and properties of binary blends of syndiotactic polypropylene (sPP) having different degrees of stereoregularity and synthesized by different metallocene catalysts, is reported. The blends have been prepared by mixing a stereoregular and crystalline sample with [ rrrr ] = 78% with low stereoregular and poorly crystalline samples having [ rrrr ] = 54.6 and 45.8%, and a nearly atactic fully amorphous sample with [ rrrr ] = 26.5%. All blends show outstanding elastomeric properties with rigidity and tensile strength that can be tailored by changing the stereoregularity of the components and blend composition. A continuous change of properties with composition is observed, from those of high-modulus thermoplastic elastomers for blends having high concentration of the most syndiotactic sample sPP1, to those of low-strength elastomers for blends with high content of the less syndiotactic component. The addition of the stereoirregular component to the more stereoregular sample produces an effect on the crystallization behavior and mechanical properties analogous to that produced by incorporation of defects of stereoregularity. Blends behave as a single sPP component having an average degree of stereoregularity intermediate between those of the two components. All data indicates a good miscibility of the sPP chains of different stereoregularity in the melt and in the amorphous state.

In semi-crystalline polymers, polymorphic transitions between different crystalline forms can be triggered not only by thermal treatments but also by mechanical deformation. The transformations related to phase changes of the crystals, and those occurring at lamellar length scales by the effect of tensile deformation are studied in detail, focusing on a set of isotactic polypropylene samples, having a molecular mass in the range 100–200 kg/mol, polydispersity index close to two, and different concentration stereodefects along the chains with a uniform distribution. This enables the effect of the microstructure of the chains on the deformation behaviour to be illustrated. The transformations which occur by effect of deformation are followed in real time during stretching through wide and small angle X-ray scattering measurements, made using the high flux of X-rays available at a Synchrotron light sources. The stream of data obtained in the measurements performed in continuum are analysed in the framework of our current understanding of the deformation mechanism of semi-crystalline polymers. This study shows that, during the transformations of a spherulitic morphology into a fibrillar morphology, the stress-induced phase transitions which occur during plastic deformation are regulated by the same factors that govern the textural and morphological changes that is the ability of the entangled amorphous chains to transmit the stress and the intrinsic stability of the lamellar crystals. Since the relative stability of the different polymorphic forms involved in the structural transformations and the intrinsic flexibility of the chains depend on the stereoregularity, we are able to make precise correlations between the stereoregularity of the chains, and the deformation behaviour, paving the way for understanding the material properties at molecular level.

A method for building the phase diagram of syndiotactic polypropylene in stretched fibers as a function of stereoregularity, degree of deformation, and temperature is reported. The method is based on the analysis of the X-ray fiber diffraction patterns of samples of different stereoregularity, prepared with different metallocene catalysts and stretched at different degrees of deformation and stretching temperatures. For all the analyzed samples the boundary lines between the different regions of stability of the different polymorphic forms have been determined by the emergence or disappearance of X-ray reflections typical of the different forms. Although the phase diagram is given from a dynamic rather than a thermodynamic perspective, the polymorphic forms observed by X-ray diffraction correspond to the equilibrium modifications, thermodynamically stable at those values of stereoregularity and deformation.

A study of the structure of isotactic propylene-hexene random copolymers, prepared with single-center metallocene catalysts, is reported. For low concentrations of hexene comonomeric units, up to nearly 10 mol%, copolymers crystallize in the alpha form of isotactic polypropylene. Copolymers with hexene contents higher than 10 mol% crystallize in a new polymorphic form that melts at nearly 50 °C. The hexene units are partially included in both crystals of alpha form and of the new form. The crystallization of the new crystalline form allows incorporation of a high amount of hexene units. The melting temperature and the crystallinity decrease rapidly with increasing hexene content. A slower decrease of melting temperature and crystallinity is observed at high hexene concentrations because of the crystallization of the new form. The crystal structure of the new form has been studied by analysis of X-ray powder and fiber diffraction patterns of samples containing hexane concentration higher than 10 mol%. Chains in three-fold helical conformation of propylene-hexene copolymers are packed in a trigonal unit cell according to the space group R3c or R3menoc. The values of a and b axes of the unit cell depend on hexene concentration, and values of a = b = 17.5 Å and c = 6.5 Å have been found for the sample with 26 mol% of hexene. The structure contains high degree of disorder due to the constitutional disorder of the random copolymer chains that produces disorder in the positioning of the lateral groups in the unit cell. Moreover statistical disorder in the up-down positioning of the helical chains and slight disorder in the orientation of chains around the three-fold axes are present. The inclusion of hexene units in the crystals induces a suitable increase of density that allows crystallization of three-fold helical chains in the trigonal form, where the helical symmetry of the chains is maintained in the crystal lattice. The structure is similar to that of form I of isotactic polybutene. This form does not crystallize, and has never been observed so far for the polypropylene homopolymer because, in the absence of bulky side groups, it would have a too low density. This structure represents an example of entropy driven phase formation.

The role of crystals in the elasticity of semicrystalline polymers is discussed in the case of syndiotactic polypropylene, which provides an example of a thermoplastic elastomer with a degree of crystallinity that can be tailored by changing and controlling the stereoregularity. This can be achieved using metallocene catalysts with different structures and stereoselectivity. The comparison of crystallization and physical properties of samples of syndiotactic polypropylene of different stereoregularity, with rrrr pentad concentrations being variable in the wide range 26-96%, prepared with different catalysts, has shown that syndiotactic polypropylenes present different types of elastic behavior, depending on the degree of crystallinity. For the most-stereoregular and crystalline samples with high melting temperatures, crystals actively participate to the elastic response of the material and elasticity has a mainly enthalpic character attributable to the metastability of the trans-planar form III that transforms into the more-stable helical form II during elastic recovery. For less-crystalline samples, with low melting temperatures, elasticity has instead a pure entropic origin as in conventional thermoplastic elastomers, and crystals act only as knots of the physical elastomeric network.

Syndiotactic propylene–ethylene copolymers have been synthesized with a single-center Cs-symmetric syndiospecific metallocene catalyst. A study of the effect of the presence of ethylene comonomeric units on the polymorphic behavior of syndiotactic polypropylene (sPP) and on the structural transitions occurring during stretching is reported. For copolymer samples with low ethylene contents, in the range 2–7 mol%, crystals of the helical form I, present in the melt-crystallized samples, transform into the trans-planar form III by stretching at high deformation. Form III transforms in part into the helical form II by releasing the tension, as it occurs for sPP. Samples with ethylene contents in the range 8–10 mol% are crystallized from the melt as a mixture of crystals of helical form I and form II. Both helical forms transform by stretching at low values of deformation (lower than 300%) into the trans-planar mesomorphic form, which transforms into the trans-planar form III by further stretching at higher deformations (higher than 500%). For these samples form III transforms back into the mesomorphic form, rather than into the helical forms, by releasing the tension. Unoriented samples of copolymers with ethylene content in the range 13–18 mol% are mainly crystallized in the helical form II, which transforms into the trans-planar mesomorphic form by stretching. Upon releasing the tension the mesomorphic form remains stable and no polymorphic transition is observed. The presence of ethylene comonomeric units stabilizes the trans-planar forms in fibers of the copolymer samples. This has been confirmed by the result that for high ethylene contents the trans-planar form III and the mesomorphic form do not transform in helical forms by annealing of fibers stretched at high deformations.

Highly isotactic polypropylene samples, containing a very low amount of rr stereodefects (0.1-0.2%) and slightly higher concentration of defects of regioregularity (0.8-0.9% of 2,1 erythro units), with different molecular masses have been prepared with an isospecific but not fully regioselective metallocene catalyst. The effects of the presence of rr defects and of 2,1 regiodefects, and the effect of the molecular mass on the mechanical properties and crystallization behavior of polypropylene have been analyzed. Samples containing 2,1 regiodefects are very stiff and much more fragile than the samples containing only rr stereodefects. The presence of rr defects, even for low concentrations, produces instead increase of ductility and improvement of drawability at room temperature. The different effect of stereo- and regio-defects is probably related to their different levels of inclusion inside the crystalline phase. The uniform inclusion of rr defects in the crystals makes the stereodefective and regioregular samples more homogeneous, where crystalline and amorphous phases have the same composition. The study of the crystallization behavior has shown that the molecular mass strongly influences the amount of alpha and gamma forms that crystallize from the melt. Samples containing regiodefects and with high molecular masses (higher than 200000) show the same amount of gamma form and the same crystallization rate. A lower amount of gamma form and higher crystallization rates are instead observed for the sample having similar microstructure but lower molecular mass. This indicates that the crystallization of the gamma form is favored over the alpha form when the crystallization is slow, that is, for samples with high molecular mass. When the molecular mass is lower than 100000 the crystallization is faster and the formation of the alpha form is kinetically favored. The comparison with regioregular samples containing only rr stereodefects has shown that the effects of rr stereodefects and 2,1-regiodefects on the crystallization properties of polypropylene are very similar, at least when the concentration of defects is small (1% of rr stereodefects or 2,1 erythro units). Both defects produce a shortening of the regular crystallizable isotactic and regioregular sequences, inducing crystallization of the gamma form.

An analysis of the mechanical properties and polymorphic transformations occurring during plastic deformation of isotactic polypropylene with variable stereoregularity, containing only rr stereo-defects, is presented. Thermoplastic materials showing high stiffness, or high flexibility, or elastic properties can be produced depending on the concentration of defects. Relationships between the different mechanical behavior and the different observed polymorphic transformations occurring during tensile deformation are discussed. The elastic behavior of the poorly isotactic samples and the values of the corresponding mechanical parameters are related to the structural transformations occurring during stretching. The γ-form present in the unstretched sample transforms by stretching into the α-form, which in turn, transforms into the mesomorphic form at very high deformations. The mesomorphic form transforms back into the crystalline α-form upon releasing the tension and elastic recovery is observed. The crystallization of the mesomorphic form into the α-form upon releasing the tension is not observed in the case of flexible or stiff-plastic samples, which do not show elastic behavior. This indicates that in the elastomeric samples elasticity is probably partially due to the enthalpic contribution associated with the crystallization of the mesomorphic form into the α-form.

A study of the structure and mechanical properties of syndiotactic polypropylene prepared with non-metallocene catalysts, based on titanium chlorides bearing phenoxyimine ligands, is reported. This catalyst promotes living polymerization of propylene, producing high molecular weight syndiotactic polypropylene, through a secondary (2,1) mechanism of enchainment of monomer and chain-end control of stereoselectivity. The effect of the presence of m dyads defects of stereoregularity, consistent with the chain-end mechanism, and defects of regioregularity, due to (1,2) misinsertions, on the crystallization behavior and mechanical properties is analyzed. As-prepared samples crystallize in conformationally disordered modifications of the isochiral helical form II containing kink-bands defects. This is probably due to the presence of regiodefects, producing pairs of vicinal methylene groups along the polymer chains, which favors, locally, the formation of trans-planar sequences that, in turn, induces the crystall...

An analysis of the crystallization properties of fractions of elastomeric polypropylene (ELPP) prepared with Al 2 O 3 -supported tetraalkyl zirconium catalyst is presented. A comparison with the polymorphic behavior of isotactic polypropylene (iPP) samples prepared with a single center homogeneous metallocene catalyst is also shown. The ELPP sample has been fractionated by extraction with boiling solvents. The irregular fraction insoluble in pentane and soluble in hexane crystallizes from the melt almost totally in the γ form, whereas the more stereoregular fraction, insoluble in n -heptane, crystallizes mainly in the α form. The relative amount of γ form crystallized from the melt is much lower than the one formed in samples of metallocene-made iPP samples with a similar average content of isotactic stereosequences. Since the γ form crystallizes in chains having short regular isotactic sequences, these data indicate that in the fractions of the ELPP sample the regular isotactic sequences are longer than those present in chains of metallocene-made iPP. In particular, in the more irregular crystalline fractions of the ELPP sample the chains are characterized by a stereo-block microstructure, consisting in the presence of quite regular isotactic sequences alternating with irregular sequences, the latter containing most of the defects. The presence of stereoblocks allows crystallization of these highly irregular fractions, even in the presence of a very high content of defects, and accounts for the elastomeric properties shown by this sample. The whole ELPP sample is constituted mostly by amorphous ether soluble (40%) and pentane soluble (26%) fractions and shows elastic properties due to the high molecular weight of chains in all the fractions and the crystallization of isotactic sequences present in the stereoblocks.

A study of polymorphic transitions occurring during the stretching of samples of syndiotactic polypropylene (s-PP) in the trans-planar mesomorphic form is presented. Samples in the pure transplanar mesomorphic form or in mixture with crystals of the helical form I have been stretched at room temperature and at 4 °C. Both the stretching temperature and the crystalline modification of the starting material strongly influence the polymorphism of oriented s-PP fibers. Stretching procedures at room temperature induce the crystallization of the trans-planar form III, regardless of the crystalline modification of the starting unoriented sample. Both the trans-planar mesomorphic form and the helical form I transform at high deformation into the crystalline trans-planar form III. When the stretching is instead performed at 4 °C the structural transitions occurring upon stretching depend on the crystalline modification of the starting unoriented material. Indeed, if s-PP samples in the pure trans-planar mesomorphic form are stretched at 4 °C, only an orientation of the mesomorphic crystals occurs, and the transition into the crystalline form III is never observed, also for high deformation. When the s-PP samples are, instead, in mixtures of crystals of the mesomorphic and helical forms, stretching at 4 °C induces the transition of the helical form into the trans-planar form III at low deformations. At high deformations the mesomorphic form also transforms into the crystalline form III. In all the experiments, the removal of the tension in fibers stretched at room temperature and at 4 °C produces structural changes, which depend on the crystalline form of the starting unoriented samples and on the crystalline modifications formed during the stretching. The trans-planar form III transforms, indeed, into the crystalline forms present in the unoriented samples or formed during the stretching (helical form and/or mesomorphic form). These results indicate that the structural evolution of the trans-planar form III upon removing the tension in stretched fibers strongly depends on the memory of the crystalline forms present in the starting unoriented material and on the memory of the crystalline forms produced during the stretching.

A compared analysis of the polymorphic behavior of samples of isotactic polypropylene (iPP) prepared with heterogeneous Ziegler-Natta catalysts and with a single center homogeneous metallocene catalyst is presented. Different samples of Ziegler-Natta iPP samples, prepared with MgCl2-supported catalysts modified by adding different Lewis bases, have been fractionated by extraction with boiling solvents. The irregular fraction, insoluble in diethyl ether and soluble in hexane, crystallizes from the melt almost totally in the gamma form. The more stereoregular fractions crystallize instead basically in the alpha form. This confirms that, even in the case of Ziegler-Natta iPP samples the gamma form may develop by melt-crystallization at atmospheric pressure in fractions containing high concentration of defects. The relative amount of gamma form crystallized from the melt is, however, much lower that that observed in samples of metallocene-made iPP containing comparable amount of defects. Since the gamma form crystallizes in chains having short regular isotactic sequences, these data indicate that in Ziegler-Natta iPP samples the regular isotactic sequences are longer than those present in chains of metallocene-made iPP having a similar overall concentration of defects. The different polymorphic behavior of metallocene and Ziegler-Natta iPP samples is related to the different distribution of defects in the polymeric chains, generated by the different kinds of catalytic systems. While in the metallocene-made iPP the distribution of defects along the chains is random, in Ziegler-Natta iPP samples the majority of the defects are segregated in a small fraction of poorly crystallizable macromolecules or in more irregular portions of the chain, so that much longer fully isotactic sequences can be produced, leading to the crystallization of the alpha form, even for a relatively high overall concentration of defects. These results confirm the idea that the structural analysis of iPP, in particular the crystallization of the gamma form, may give information about the microstructure of the polymer chains. The measure of the maximum amount of gamma form crystallized from the melt may be used as an indirect method to evaluate the average length of isotactic sequences. This analysis allows concluding that some fractions of Ziegler-Natta iPP are characterized by chains with a stereo-block microstructure, consisting in regular isotactic sequences linked to more irregular sequences. The latter contain the major part of stereodefects mainly consisting in isolated rr triads, r diads and longer …rrrr… syndiotatic sequences. The hypothesis of a stereo-block microstructure for some of these less stereoirregular fractions is also consistent with the high degree of crystallinity observed in the samples crystallized from solution or from the melt, despite the high concentration of defects.

The structural transformations occurring in fibers of syndiotactic polypropylene (sPP) during stretching have been studied by wide-angle X-ray diffraction measurements, using syncrotron radiation. sPP samples were cyclically stretched and relaxed at controlled rate, while recording X-ray fiber diffraction patterns and stress-strain curves. Clear evidences that the polymorphic transition between the helical form II and the trans-planar form III of sPP is a fast process and occurs on the same time scale as the rate the material is stretched, are provided. The transition of form II into form III starts during the stretching in correspondence to critical values of the stress-strain parameters. As the strain increases, crystals of form 11 transform into form III and the inverse transition occurs, releasing the tension. Below a critical strain, the sample is almost completely in the helical form, as in the initial unstrained state. A complete recovery of the initial dimensions of the specimen is observed, upon releasing the tension. Moreover, the total amount of crystallinity does not change during cyclic elengation and recovery. This suggests that the reversible phase transition between form II and form III does not involve the formation of any disordered, intermediate phase, i.e., it is a direct and cooperative process, implying conformational and structural rearrangements of large bundles of close neighboring chains and occurs instantaneously. These data indicate that while the driving force leading the conventional elastomers to recover the initial dimensions is merely entropic, in the case of sPP elasticity is also assisted by the enthalpic gain achieved when the sample is relaxed. When the tension is removed, both the enthalpic factor, due to the structural transition in the crystalline region, and an entropic factor, due to the conformational transition of the chains in the entangled amorphous phase, contribute to the elastic recovery of the fiber. A possible mechanism for the martensitic crystal-crystal phase transition of sPP is suggested.

A structural characterization of copolymers of syndiotactic polypropylene containing ethylene comonomeric units (PPET) in a wide range of concentrations (0.4-60 mol %) is presented. The copolymer samples are crystalline up to an ethylene content of 18-20 mol %. Ethylene units are partially included in the crystals of both as-prepared and melt-crystallized samples. As-prepared PPET samples basically crystallize in conformationally disordered modifications of form II of syndiotactic polypropylene (sPP) containing kink bands. The amount of kink bands defects, represented mainly by T(6)G(2) sequences, increases with the ethylene content. These modifications are metastable and transform by crystallization from the melt into the most stable form I or form H, depending on the ethylene concentration. PPET samples having low ethylene content (up to 6-7 mol %) crystallize from the melt into the antichiral form 1, even though disordered modifications of form I are always obtained. For higher ethylene contents, mixtures of crystals of forms I and H are obtained by crystallization from the melt. The fraction of form II increases with increasing the crystallization temperature and the ethylene content. The form II obtained by melt crystallizations does not present kink-band disorder, all chains being in the more stable (T(2)G(2))(n) helical conformation. This is the first example of crystallization of the isochiral form II of sPP from the melt at atmospheric pressure.

The crystallization behavior and the mechanical properties of random isotactic butene–octene copolymers (iPBC8) prepared with a metallocene catalyst are presented. Samples with low octene concentrations, up to nearly 6 mol%, crystallize from the melt in form II. In these samples octene units stabilize the tetragonal form II, decelerating or even preventing the transformation of form II into the trigonal form I. Octene concentrations higher than 7 mol% prevent crystallization of the samples from the melt and the obtained amorphous samples crystallize by aging at room temperature in mixtures of form II and form I'. The sample with the highest octene concentration of 11.5 mol% crystallizes surprisingly from the amorphous phase directly in the pure form I'. In these samples, form I' crystallized from the amorphous phase transforms into form II by stretching. Form II transforms back into form I' upon releasing the tension. Therefore, in samples where form I' is favored in powder samples, form II is favored in stretched fibers. The transformation of form I into form II by stretching and the back transformation of form II into form I after releasing the tension is unusual for isotactic polybutene (iPB) and rather unexpected. The presence of octene comonomer units also affects the mechanical behavior of iPB and produces increase of flexibility and ductility with increasing comonomer content, and samples with octene concentration higher than 6–7 mol% show unexpected remarkable elastomeric properties. The formation of crystals of form II or form I' during aging allows development of elastomeric properties and enhancement of mechanical strength. The elastic properties are associated to the reversible crystal–crystal phase transition between form II and form I occurring during stretching and when the tension is removed.

The structure and thermal properties of a new class of isotactic copolymers of ethylene (E)/4-methyl-1,3-pentadiene (4MPD) with 4MPD content in the range 40–73 mol % and of isotactic poly-1,2-(4-methyl-1,3-pentadiene) (iP4MPD) homopolymer have been analyzed. The copolymers have been synthesized using 1,4-dithiabutanediyl-linked bis(phenolate) titanium complexes as catalyst precursors, activated by methylaluminoxane. Solution 13C NMR analysis reveals the tendency of the catalyst system to produce copolymers with alternating 4MPD/E constitution. All copolymers are crystalline and show melting temperatures in the range 90–100 °C. The samples with nearly equimolar concentration of units crystallize in a new crystalline form because of the alternating constitution. Structural analysis indicates that in this new form the alternating 4MPD-E sequences assume a nearly trans-planar conformation with periodicity c = 5.13 Å and are arranged in a monoclinic unit cell with parameters a = 5.70 Å, b (unique axis) = 14.95 Å, c (chain axis) = 5.13 Å, and β = 114.4°, according to the space group symmetry Pn.

A detailed analysis of the structure and the thermal behavior of samples of isotactic polypropylene (iPP) prepared with a highly regiospecific metallocene catalysts is presented. The samples present variable amounts of stereodefects (isolated rr triads) and provide the first example of metallocene-made iPP not including regioirregularities. They crystallize from the melt in mixtures of the alpha and gamma forms. The content of the gamma form increases with increasing the crystallization temperature, reaches a maximum value, and then decreases for a further increase of the temperature. The crystallization of the gamma form is favored by the presence of stereodefects. The thermal analysis has indicated that the crystals of gamma form melt at lower temperature than the crystals of alpha form obtained at the same crystallization temperature. A comparison between the experimental X-ray diffraction profiles of the melt-crystallized samples and the calculated profiles for models of structure has shown that the gamma form obtained in these samples always presents structural disorder, which produces local situation of packing typical of the alpha form. The amount of structural disorder decreases with increasing the crystallization temperature and the degree of stereoregularity of the samples. The presence of disorder in the crystals of gamma form accounts for their lower melting temperature. This analysis indicates that metallocene isotactic polypropylene crystallizes in a continuum of disordered modifications intermediate between the alpha and gamma forms, and the amount of disorder present in the crystals depends on the crystallization conditions and on the stereoregularity of the sample.

An analysis of the elastic properties of syndiotactic polypropylene at different temperatures is reported. Fibers stretched at low temperatures are in the trans-planar form III, which transforms into the helical form II upon the release of the tension, whereas fibers stretched at high temperatures are in the stable helical form I. The mechanical analysis of these fibers has shown that only at room temperatures the specimens show good elastic properties. The elastic recovery of the samples is in part lost also at room temperature in annealed fibers, which are in the stable form I, in both the stretched and unstrained states. When the structural transition from the trans-planar form III into the helical form occurs upon removing the tension, a nearly total elastic recovery is observed. Only a partial elastic recovery is instead observed when this transition is absent. The relative amount of crystalline phase in unoriented films and in stretched and stress-relaxed fibers is rather constant (approximate to40-43%) and does not depend on the stretching temperature in the range 25-80 degreesC. These data indicate that the elastic behavior of syndiotactic polypropylene is in part linked to the enthalpy gain achieved when the tension is removed, due to the metastability, in the unstrained state, of the trans-planar form, which transforms into the stable helical form. On the other hand, also an entropic factor must play a role, due to the entangled amorphous chains which experience a reversible conformational transition between the disordered (coil) and extended conformations during the stretching and relaxing process.

Themorphologies and structures of single crystals of syndiotactic poly(propene-co-1-butene) (PPBU) with 1-butene contents of 2.6, 4.2, 9.9, 16.2, and 47.9 mol % are studied by transmission electron microscopy and electron diffraction. The electron diffraction results show that the 1-butene units are included in the crystalline phase of the sPP homopolymer. A small amount of 1-butene (

The homogeneous non-catalytic hydrogenation of cis-1,4 poly(isoprene), isotactic cis-1,4 poly(1,3-pentadiene) and syndiotactic cis-1,4 poly(1,3-pentadiene) with diimide, formed by thermal decomposition of para-toluenesulfonylhydrazide, is examined. Perfectly alternating ethylene/propylene copolymers having different tacticity (i.e., isotactic and syndiotactic), which are difficult to synthesize by stereospecific copolymerization of the corresponding monomers, are obtained. Both isotactic and syndiotactic alternating ethylene/propylene copolymers are amorphous, with very low glass transition temperatures.

The crystallization of the trans-planar mesomorphic form of syndiotactic polypropylene by quenching the melt at 0 degreesC is investigated as a function of the stereoregularity of the samples. The formation of the trans-planar mesomorphic form at 0 degreesC is followed as a function of the permanence times at 0 degreesC by X-ray diffraction and FTIR spectroscopy. Samples kept at 0 degreesC for short time rapidly crystallize into the helical form I at room temperature, whereas longer permanence times at 0 degreesC increase the stability of the trans-planar mesomorphic phase which remains stable and inhibits the normal crystallization of the sample into the helical form at room temperature. The stereoregularity of the polymer sample strongly influences the rate of formation of the mesomorphic form at 0 degreesC, Higher the syndiotacticity, easier the formation of trans-planar mesomorphic form. Very short permanence times (few hours) at 0 degreesC are enough to form and stabilize the mesomorphic form for highly syndiotactic samples, whereas for stereo-irregular samples the amount of mesomorphic form observed when the sample is removed from the 0 degreesC bath and heated to room temperature, remains always very low, even for very long permanence times (months) at 0 degreesC and the crystallization into the normal helical form is not inhibited. (C) 2001 Elsevier Science Ltd. All rights reserved.