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Phantom chain simulations can be utilized for studying the molecular arrangements in systems of chain molecules filled with randomly distributed solid nanoparticles, provided that the interaction energy between chains and filler is modified in order to counterbalance the intrinsic tendency of the chain segments to avoid the filler surfaces. In fact, all properties of systems containing phantom chains with suitably modified interactions of this kind are found to be in excellent agreement with those previously obtained for dense systems having the same composition. This makes possible to study systems that cannot be simulated at full density (i.e., systems with long chains and/or with large particles and small filling density). Simulations performed for long phantom chains show that the simple rules recently proposed to predict the mutual arrangements of particles and chains in the dense systems simulated up to now can be extended to very long chains. However, these rules should be modified in order to be valid also for large particles and small filling density.

A microstructural method for the determination of the reaction order with respect to the monomer in coordinated 1-alkene polymerizations is proposed. The method is applicable whenever enantioselective monomer polyinsertion competes with an intramolecular process introducing stereodefects of known and recognizable nature in the growing polymer chains at a rate independent of monomer concentration, [M] (e.g., chain or site epimerization for C2-symmetric and Cs-symmetric ansa-metallocene catalysts, respectively). Indeed, in such cases, the stereoregularity of the polymer obtained at a given temperature results from the balance between the rates of the two competing processes, and is obviously a function of [M], because so is the rate of polyinsertion, 〈vp〉. Therefore, from the way polymer stereoregularity changes with [M], the corresponding change of 〈vp〉 can be inferred. Compared with the conventional macroscopic approach based on measurements of monomer consumption, the microstructural route is more reproducible, and allows one to discriminate between “authentic” and apparent higher-order polyinsertion rate laws. In this paper, in particular, we show that the “trigger” hypothesis invoked to explain the observed pseudo-second-order kinetics of propene polymerization promoted by C2-symmetric ansa-metallocene catalysts is inconsistent with the experimental results and that an alternative explanation assuming first-order kinetics at catalytic species which interconvert between a propagating and a “resting” state is more plausible.

The results of realistic computer simulations of dense polymer melts filled with solid nanoparticles are compared with results obtained for similar systems near planar solid surfaces and with those of Monte Carlo calculations performed for single chains in the presence of spherical solid obstacles. The polymer units at the interface with the filler particles are arranged in densely packed and ordered shells analogous to those found near planar solid surfaces. The polymer chains, reduced in size compared to the unfilled melt, are constituted of sequences of surface segments, totally contained in the interface shell of a given particle, and of bridge segments, connecting different particles. Each chain visits the interface shell of several filler particles, and each particle is in contact with many different polymer chains, such that the filler particles behave as highly functional physical cross-links.

We recently reported the assignment of the methyl and methylene regions in the 150 MHz ¹³C NMR spectra of regioregular polypropylenes with different tacticities. In this article, we complete the assignment by extending it to the methine region. Despite a comparatively low resolution, the methine resonance provides additional information that can be of use in the configurational analysis of these polymers. Moreover, its chemical shift spreading can be predicted quantitatively in terms of semiempirical methods based on the γ-gauche effect, with an appropriate choice of the parameter scheme.

The stereosequence distribution of the “atactic” and “isotactic” fractions of a polypropylene sample made with a MgCl2-supported catalyst was determined by means of high-resolution ¹³C NMR and analyzed in terms of statistical models of increasing sophistication. Two-site models, including the one normally used for the interpretation of “routine” ¹³C NMR data at pentad level, were shown to be inconsistent with the much finer high-resolution data. A good agreement between experimental and calculated distributions could be obtained only in terms of a three-site model, describing each fraction as a mixture of highly isotactic, weakly isotactic (“isotactoid”) and syndiotactic sequences. According to such model, the two fractions comprise the same three building blocks (the configuration of the three different types of stereosequences being almost invariant) and differ merely in their relative amounts (which indicates a stereoblock nature). The correlations with the physical properties of the materials and the implications on the nature of the catalytic species are also briefly discussed.

Model compounds in which flexible polymethylenic sequences of variable length alternate with -NH2-ZnX2- groups were characterized both in the crystalline and in the molten state, in order to evaluate the effect of regularly distributed dipoles on the melting behavior of linear flexible chains. Microphase segregation of the polar groups was observed resulting in the formation of smectic mesophases or of smectic-like fluids. Possible analogies with the structural organization of (fx224-1) dipole-containing polymers and ionomers are briefly discussed.

A molecular mechanics analysis on model metallocene complexes, as possible intermediates for the propene polymerization, in which the two coordination positions available for the monomer and the growing chain are diastereotopic, is presented. The energy difference between the corresponding diastereoisomeric preinsertion intermediates appears to be relevant for the model complexes based on the meso-ethylenebis(4,5,6,7-tetrahydro-1-indenyl) ligand (1), the isopropylidene[(3-methyl(η⁵-cyclopentadienyl))(η⁵-9-fluorenyl)] ligand (2), and the rac-ethylene[1-(η⁵-9-fluorenyl)-1-phenyl-2-(η⁵-1-indenyl)] ligand (3), while it appears to be small for those based on the rac-ethylene[1-(η⁵-cyclopentadienyl)-1-phenyl-2-(η⁵-1-indenyl)] ligand (4). It is suggested that these energy differences can be related to an increased probability of a back-skip of the growing chain toward the outward coordination position after the monomer insertion and prior to the coordination of a new olefin molecule. The kinetic competition between the back-skip of the chain outwards and the monomer coordination is able to rationalize the kinds of defects in prevailingly hemi-isotactic chains obtained with systems based on 2, as well as the increased stereoregularity at decreasing monomer concentration for systems based on 3 (not shown by systems based on 4). The molecular mechanics analysis shows that the nonbonded interactions are also able to account for the influence of the ethylene bridge conformation on the stereospecificity of the propene polymerization in ansa-zirconocene complexes.

Off-lattice Monte Carlo simulations were performed on linear and 12-arm regular star polymers with up to 472 repeat units in the excluded-volume regime. For both topologies, the overall size scales asymptotically with molecular weight through a Flory exponent, ν = 0.588, but for finite molecular weights, the apparent exponent is slightly different in the case of the radius of gyration and of the root-mean-square end-to-end distance and definitely smaller in the case of the hydrodynamic radius. The interatomic distances within the star follow a 2-fold behavior, according to whether the two atoms belong to the same arm or to different arms. This result is related with the peculiar behavior of the scalar products among the bond vectors. If these belong to the same arm and are remote from the branch point, the scalar products follow the same pattern as in the linear chain. If however the bonds are close to the branch point, the scalar products are much larger and essentially constant, indicating a substantial arm stretching. Conversely, if the two bonds are on different arms, the scalar products across the branch point drop at once by more than 1 order of magnitude. Therefore, the initial arm directions are essentially noncorrelated, and the interarm repulsions are best accommodated with a minimum entropy loss.

Evidence for long range, anisotropic diffusion of the long chain n-alkylammonium cations in layer compounds (n-CnH2n+1NH3)2MCl4 is presented. Annealing mechanical mixtures of such compounds with M = Mn, Zn; n = 12, 18 results in the formation of mixed crystals of composition (n-C12H25NH3)x12 (n-C18H37NH3)2-2x12MCl4 in which bilayers of n-alkylammonium groups with differing chain lengths, positionally and conformationally disordered, alternate with crystalline metal halide layers (modification β').When the ionic layers are polymeric (M = Mn), solid solutions are obtained on cooling from modification β' in which the polymethylenic chains of the alkylammonium groups, while maintaining positional disorder, are partly cocrystallized in spite of the high difference in chain length (modification α'). Demixing into the pure components, on the other hand, is observed on cooling for nonmacroanionic systems (M = Zn).Possible structural models of the mixed compounds in modifications α', β' are described, and the stability of such modifications relative to the unmixed components evaluated on the basis of a simple thermodynamical treatment.

Apart from the liquid crystalline phases of a number of derivatives of long chain mono- and dicarboxylic acids, few evidences have been reported so far on the existence of (non lyotropic) mesophases of aliphatic compounds.1

¹³C NMR spectroscopy is the main source of information on the stereochemistry of Ziegler−Natta and related transition metal catalyzed propene polymerizations. In simple cases, like those of polypropylenes formed under pure enantiomorphic-site or chain-end control, the origin of the stereoselectivity can be easily recognized from the steric pentad distribution obtained from routine ¹³C NMR spectra. On the other hand, the variety of innovative polymers that can now be prepared with “high-yield” heterogeneous and metallocene-based homogeneous catalysts under hybrid, multiple, or oscillating stereocontrol represent very complex systems, which are beyond the possibilities of configurational analysis by routine ¹³C NMR. In such cases, high-field ¹³C NMR can be highly advantageous. Indeed, in this paper we show that from the methyl and methylene regions of 150 MHz ¹³C NMR spectra of polypropylenes of various tacticities, the stereosequence distribution can be determined at a much finer level of detail, so as to obtain an adequate experimental basis for the investigation of the many complicated mechanisms of stereocontrol presently encountered in Ziegler−Natta catalysis.

Off-lattice Monte Carlo simulations were performed on linear and 12-arm star polymers in a good-solvent and in the Θ state to investigate both the molecular shape and the size distribution. We find that in a good solvent the star polymer is more spherical than a random walk, the more so the smaller its size is; moreover, a star of finite chain length in the Θ state has a shape more similar to that in good solvent than to that in a random walk because of the excluded-volume restrictions that are particularly relevant in the core region. Both results are at variance with what is found in linear chains. Furthermore, the size distribution of the star is much narrower in a good solvent than in a random walk, approaching somewhat a δ function. This effect is also seen in the strong deformation of the density profile away from the center of mass.

The stereoregularity of polypropylene produced with C2-symmetric group 4 ansa-metallocene catalysts results from the interplay of two competing reactions, namely isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end; therefore, for this class of homogeneous catalysts, at variance with the “classical” heterogeneous Ziegler−Natta ones, enantioselectivity and stereoselectivity are not (necessarily) coincident. In this paper, possible methods for the separate determination of these two parameters are introduced and applied to propene polymerization in the presence of the prototypical catalyst rac-ethylene−bis(4,5,6,7-tetrahydro-1-indenyl)ZrCl2. The results prove that the relatively poor stereoselectivity of this catalyst above room temperature is consequent primarily to chain epimerization; monomer insertion indeed is highly enantioselective up to at least 80 °C. Preliminary evidence for the existence of more than one epimerization mechanism is also presented; this complicates the measurements of enantioselectivity based on ¹³C NMR characterizations of d-labeled poly(propene)s.

The molecular arrangements of liquid tridecane near solid surfaces and in narrow slits have been studied by atomistic Monte Carlo computer simulations. A single surface perturbs the liquid over a distance of 1.5 nm, inducing the formation of progressively less dense and ordered layers of thickness 0.4 nm. Chains with atoms in the first layer tend to run parallel to the surface. The tendency to form layer structures is maintained or destroyed in narrow slits, depending on the slit thickness. For instance, it is slighly increased in slits of thickness 1.2 nm, while it is practically destroyed when the slit thickness is reduced to 1.0 nm. When added in small amounts, shorter-chain components are preferentially adsorbed at the solid surface, making the first layer of liquid in contact with these surfaces much less ordered.

Simulations of realistically dense polymer melts filled with randomly distributed solid nanoparticles and calculations performed with various methods for single chains in the presence of filler do not show the large increase of chain dimensions with respect to the unfilled polymer predicted by previous RIS calculations when the chains are larger than the particles. It is concluded that the increased dimensions found in recent SANS experiments for poly(dimethylsiloxane) filled with relatively small polysilicate particles cannot be explained on the basis of simple excluded-volume arguments. It is also shown that simulation studies of polymers in the presence of solid obstacles can only be performed by simulating systems with realistic densities.

Stepwise melting behavior for a series of primary long chain n-alkylammonium chlorides was pointed out. A plastic phase, a smectic mesophase and a “clear” liquid possibly unisotropic were observed. The same trend seems to be common to other hydrocarbon molecules with ionic endgroups, including alkali metal n-alkanoates.