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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.

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 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 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.

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.

How to make opposites compatible Polyethylene (PE) and isotactic polypropylene ( i PP) are the two most widely used commodity plastics and thus make up a large fraction of the waste stream. However, the two plastics will not mix together, which limits options for dealing with mixed waste and decreases the value of recycled products. Eagan et al. report the synthesis of multiblock copolymers of i PP and PE by using a selective polymer initiator (see the Perspective by Creton). The high-molecular-weig ht blocks could be used to reinforce the interface between i PP and PE and allow blending of the two polymers. Science , this issue p. 814 ; see also p. 797

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.

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.

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.

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 study of the crystallization behavior and mechanical properties of random isotactic butene–ethylene copolymers prepared with a metallocene catalyst is presented. The use of the metallocene catalysis ensures a fine control over the molecular structure with low concentration of rr stereodefects (0.8%), negligible amount of regiodefects, and random and uniform distribution of ethylene constitutional defects. This molecular characteristic has allowed evidencing the only effect of the presence of ethylene units on the polymorphic behavior and mechanical properties of isotactic polybutene (iPB). The presence of ethylene accelerates the transition of form II into form I at room temperature and at concentration of nearly 6 mol % favors the direct crystallization from the melt of the stable form I. The presence of ethylene also affects the mechanical behavior of iPB and produces increase of flexibility and ductility with increasing ethylene content. A significant modification of the properties of iPB is observed...

A strategy to tailor the properties of isotactic polypropylene, based on the combination of the crystallization of the mesophase and the control of the molecular structure and concentration of stereodefects via polymerization with metallocene catalysts, is presented. The mechanical properties can be modulated through the conventional slow crystallization from the melt in the α and γ forms, depending on the concentration of stereodefects, or the crystallization of the mesophase by melt quenching. 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 meso morphic form, but with remarkable differences in the values of the modulus and tensile strength.

A study of the thermodynamic stability and the related polymorphic transformations induced by thermal treatments of the mesomorphic form that crystallizes in stereodefective metallocene isotactic polypropylene (iPP) is presented. We show that the mesomorphic form of the more isotactic samples is stable at room temperature, whereas the mesomorphic form crystallizing in the more stereoirregular sample is unstable and crystallizes at room temperature in the crystalline α form. In any case, the mesomorphic form transforms during heating or by annealing at temperatures higher than 60–80 °C always in the α form, regardless of the stereoregularity, even in the case of stereoirregular samples generally crystallizing from the melt in the γ form. These data confirm the proposed model of structure of the mesomorphic form as small aggregates of chains in three-fold helical conformation packed with lateral correlations similar to the α form of iPP.

The crystal structure of syndiotactic polystyrene (sPS) co-crystals of δ -class is discussed with the aim of identifying molecular descriptors that allow predicting the lattice distortions induced by the inclusion of guest molecules inside the cavities of the nanoporous δ -form. Based on the consideration that monoclinic and triclinic clathrates and intercalates of sPS form as a result of the unique deformability of the cavities of the δ -form, the identifi ed parameters are the fractional occupancy factors of the undeformed cavity of the δ -form from guests along the a -axis, the c -axis, and the normal to the (010) planes of the crystals, and the relative orientation of the guests to the chain axes.

Random copolymers of syndiotactic polypropylene (sPP) with hexene and octene comonomeric units have been synthesized in a wide range of comonomer concentrations with a single-center syndiospecific Cs-symmetric metallocene catalyst. The effects of the presence of hexene and octene on the crystallization behavior of sPP and in particular on the stress-induced transformations have been analyzed. All copolymer samples crystallize from the melt in the stable helical form I of sPP. At very low concentrations of hexene or octene (around 1-2 mol %) the behavior of these copolymers is similar to that of sPP; that is, the stable helical form I of the melt-crystallized samples transforms into the trans-planar form III by stretching at high deformations. For hexene or octene concentrations in the range 2-5 mol % the stretching produces transformation of the helical form I into the trans-planar mesophase. Finally, for hexene or octene concentrations higher than 4-5 mol%the helical form I transforms by stretching at high deformation into a different mesomorphic form, characterized by chains in the ordered 2/1 helical conformation, and disorder in the lateral packing of the chains. This is a first evidence of the crystallization of a solid mesophase of sPP with chains in 2-fold helical conformation, never observed before in the sPP homopolymer.

Isotactic propylene−1-octene (iPPC8) and propylene−1-octadecene (iPPC18) copolymers have been synthesized with a highly isospecific C2-symmetric metallocene catalyst. iPPC8 copolymers crystallize in the α form of iPP for octene concentration up to 12−13 mol %, whereas iPPC18 copolymers crystallize in the α form for very low octadecene concentration and in a new mesomorphic form for octadecene contents higher than 2−3 mol %. In both copolymers crystals of α form transform into the new mesophase by stretching at high deformations. This mesophase is different from the quenched mesomorphic form of the iPP homopolymer. In fact, the diffraction patterns of the new mesophase of iPPC18 copolymers present a strong equatorial reflection at 2θ = 20°, which is absent in the diffraction pattern of the mesophase of iPP. The new mesophase is characterized by parallel chains in 3/1 helical conformation packed at average interchain distances of about 6 Å, defined by the self-organization of the flexible side groups, and high degree of disorder in the lateral packing of the chains

A study of the crystallization from the melt and of the polymorphic behavior of isotactic poly(1-butene) (iPB) prepared with metallocene catalysts is presented. Samples of isotactic polybutene of different stereoregularity, containing different concentrations of stereodefects (rr triads defects) and regiodefects (4,1 units) have been prepared with different C2- and C1-symmetric metallocene catalysts. The concentration of defects and the molecular mass are controlled through a rational choice of the catalyst structure and conditions of polymerization. Highly isotactic samples with concentration of rr stereodefects lower than 2 mol %, crystallize from the melt in the metastable form II. This is a common polymorphic behavior observed in iPB samples prepared with Ziegler-Natta catalysts and extensively reported in the literature. More stereodefective iPB samples, containing concentration of stereodefects higher than 2 mol %, crystallize form the melt as mixtures of form II and form I, the fraction of crystals of form I increases with increasing concentration of defects and decreasing cooling rate from the melt, and samples with concentration of rr defects of 3-4 mol%crystallize from the melt in the pure form I at low cooling rates. This is the first experimental observation of the crystallization of the stable trigonal form I of iPB from the melt at atmospheric pressure. This result may be of interest because the crystallization of the metastable form II from the melt and the spontaneous transformation at room temperature of form II into the stable form I has been considered for long time an unavoidable problem that stands in the way of the industrial development of the Ziegler-Natta iPB.

We report a new class of catalysts for the enantioselective copolymerization of CO2 and alicyclic meso epoxides. C1-Symmetric β-diiminate zinc catalysts were designed on the basis of mechanistic information and subsequently optimized using structure–activity relationships and iterative ligand design. The optimized catalysts yield highly isotactic poly(cyclohexene carbonate) and poly(cyclopentene carbonate) with units of up to 94% ee under mild conditions. A diblock copolymer of CO2/cyclohexene oxide and CO2/cyclopentene oxide was synthesized, demonstrating the controlled behaviour of the catalyst. Solid-state structures and solution-state dynamics of the catalyst were studied to explain trends in enantioselectivities and turnover frequencies. A strong correlation between the isotacticity of the polymer and its melting temperature was observed. X-ray powder diffraction was used to measure crystallinity and study the changes in morphology observed upon annealing.