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In this paper, we report the full resonance assignments in the olefinic region of the 600 MHz ¹H NMR spectra of a typical ethene/1-octene copolymer (4.0 mol % of octene units) produced with a metallocene catalyst at high temperature, and of an ethene homopolymer prepared under similar conditions for comparison. The assignments were based on a thorough analysis of 1D (¹H, ¹H{¹H}, ¹³C) spectra and of 2D ¹H−¹H (COSY, TOCSY) and ¹H−¹³C (HSQC, DEPT-HSQC) maps. Thirteen different olefinic structures were identified, and their formation explained in terms of plausible mechanistic paths associated with the high polymerization temperature. The fraction of internal chain unsaturations traceable to allylic activation turned out to be comparable to that of terminal ones in both samples. Despite its complexity, the said spectral region is now amenable to deconvolution and full simulation in terms of known and recognizable patterns, and usable for fast (possibly on-line) measurements of chain unsaturation degree, as well as for catalyst “fingerprinting” studies.

Bis(phenoxyimine)Ti catalysts with ortho-F-substituted phenyl rings on the N can be “living” propene polymerization catalysts. On the basis of DFT calculations, it has been proposed that the “living” behavior originates from an unprecedented attractive interaction between the said ortho-F atoms and a β-H of the growing polymer chain, which would render the latter less prone to be transferred to the metal (or to the monomer). In this paper, we report on a thorough full-QM and combined QM/MM investigation of representative model catalysts, demonstrating that the key factor is instead the repulsive nonbonded contact of the F-substituted rings with the growing polymer chain and an incoming propene molecule, which destabilizes the sterically demanding six-center transition structure for chain transfer to the monomer. A conceptually similar substituent effect has been reported before for several metallocene and non-metallocene catalysts; in the present case, though, this is partly alleviated by a weak attractive interaction between the ortho-F and a close-in-space α-H of the growing chain.

Fundamental implications of occasional secondary (2,1-) insertions in propene polymerization with different MgCl2-supported Ziegler−Natta catalysts are discussed, with reference to the properties of the polymers obtained. The relatively narrow molecular weight distribution of polypropylene prepared using catalysts of type MgCl2/TiCl4/diether−AlR3 is ascribed to the fact that the active species in this system are relatively uniform, in the sense that significant 2,1-insertion takes place at both highly isospecific and weakly isospecific active species. In contrast, the isospecific species in the catalyst system MgCl2/TiCl4/diisobutyl phthalate−AlEt3−alkoxysilane undergo less 2,1-insertion and are therefore less responsive to chain transfer with hydrogen. The presence of (some) highly regiospecific active sites in such catalysts will therefore lead to the formation of a high molecular weight polymer fraction and, overall, a broad molecular weight distribution. The presence of high molecular weight chains leads to relatively slow molecular relaxation and a more rapid onset of crystallization of the polymer from the melt, as evidenced by rheological studies of polypropylenes prepared using different catalysts and having different molecular weight distributions.

DFT calculations are reported for all possible insertions of ethene and propene in rac-Me2SiInd2Zr−R&uprhbr;⁺ and Me2C(Cp)(Flu)Zr−R&uprhbr;⁺ (R = Et and iPr). The results confirm the basic stereoregulation mechanism of Corradini. In addition, they provide an ordering of the possible sources of stereoerrors:  chain misorientation is the main mechanism for unbranched (Et, nPr) chains, whereas for the β-branched iBu chain errors due to chain misorientation and chain−olefin syn orientation are equally likely.

The propene/ethene-[1-¹³C] copolymerization method has been applied to measure precisely the regioselectivity in propene polymerization of the “simple” MgCl2/TiCl4 catalyst (activated with Al(ⁱBu)3), which can be viewed as the ancestor of all present-day MgCl2-supported industrial catalysts containing Lewis bases as selective modifiers. It has been found that the average content of 2,1-regiomistakes in the polypropylene produced is fairly high (ca. 0.7 mol %), although&sbd;not unexpectedly, due to the multisite catalyst nature&sbd;these are distributed rather nonuniformly throughout the polymer. The difficulty in detecting the 2,1-units, despite their relatively high concentration, in the ¹³C NMR spectra of homopolymer samples at natural ¹³C abundance has been traced to their occurrence in a variety of stereochemical environments, which leads to splitting and broadening of the corresponding resonances. The fraction of “dormant” polypropylene chains with a 2,1-last-inserted unit was estimated to be in the 20−50% range. QM calculations of regioselectivity on classical Corradini-type models of epitactic octahedral catalytic Ti species turned out to be in basic agreement with the experiment.

Chemo-, regio- and enantioselectivities in the (co-)polymerization of propene and ethene have been measured (in some cases for the first time) for a number of prototypical metallocene catalysts (in combination with methylalumoxane), and compared with those calculated at the DFT level for gas-phase cationic systems. In this paper, the first part of the study, we discuss the achiral catalysts Cp2TiCl2, Cp2ZrCl2, and Me2SiCp2ZrCl2. All three catalysts were confirmed to be highly regioselective for propene, in favor of 1,2-insertion. The performance of the titanocene in this respect is truly remarkable (only two misinsertions in 10 000 at −15 °C), whereas that of the two zirconocenes turned out to be worse than generally assumed (two to three misinsertions in 1000), and&sbd;quite unexpectedly&sbd;nearly identical (both in experiment and calculations), despite the decidedly more open structure of the Si-bridged system. Understandably, in all cases ethene was found to insert faster than propene, but the difference in relative reactivity of the two monomers is particularly dramatic when the growing chain is secondary, which confirms the “dormant” character of the latter for propene. The fair correlation between observed and calculated selectivities suggests that, at least for the systems considered here, solvent and counterion effects, though undoubtedly important, are rather indiscriminate and that olefin insertion is indeed the rate-determining step.

A structural analysis of copolymers of syndiotactic polypropylene with small amounts of ¹³C-enriched ethylene, in the range 0.4−2.6 mol %, is reported. X-ray diffraction and solid-state ¹³C NMR CPMAS data indicate that the as-prepared copolymer samples are crystallized in the conformationally disordered modification of form II of sPP containing kink bands. The disorder corresponds to the presence of portions of chains in the trans-planar conformation in chains having a prevailing 2-fold helical conformation. A direct evidence of the partial inclusion of the ethylene units in the crystalline regions of the copolymers is provided.

In a previous letter, we proposed the synthesis and ¹³C NMR characterization of propene/ethene-[1-¹³C] copolymers at low ethene-[1-¹³C] content as a convenient tool for measuring the regioselectivity of coordinated propene polyinsertion, in case ¹³C NMR fails to detect the regiomistakes in propene homopolymers. In the present article, we introduce the foundations of the approach in more detail, and illustrate how its scope can be extended to the evaluation of catalyst “dormancy” due to regiochemical monomer inversion. The method was calibrated for two well-known C2-symmetric ansa-metallocene catalysts (namely, rac-Me2Si(1-indenyl)2ZrCl2 and rac-Me2Si(2-Me-4-phenyl-1-indenyl)2ZrCl2), by means of a cross-check with results of a ¹³C NMR characterization of samples of poly(propene-[3-¹³C]) prepared under corresponding experimental conditions. In particular, it was proved that the fraction of consecutive 2,1 insertions in propene homopolymerization promoted by the quoted catalysts, although nonzero, is low enough not to invalidate the propene/ethene-[1-¹³C] copolymerization approach.

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.

We have studied the balance between olefin insertion and β-hydrogen transfer to monomer for all “well-defined” aluminum polymerization catalysts reported to date. Consistently, the balance is predicted to be significantly worse than for Me2AlEt, implying that none of the proposed active species should give a high-molecular-mass polymer. A more systematic analysis of ligand effects allows a rationalization of these results and shows that small modifications to the proposed active species are unlikely to solve the problem. We conclude that olefin polymerization at a single aluminum center is rather unlikely. Alternative interpretations of the experimental data are discussed.

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.

In this paper, we report the results of a 150 MHz ¹³C NMR characterization of ethene-1-¹³C/propene copolymers at low (<5 mol %) ethene content prepared in the presence of the syndiotactic-selective ansa-metallocene catalyst (Me)(Ph)C(cyclopentadienyl)(9-fluorenyl)ZrCl2 (cocatalyst, MAO). In particular, from the fine structure of the resonances of the ethene-1-¹³C units we conclude that the enantioselectivity of 1,2-propene insertion is substantially lower and the probability of chain back-skip substantially higher after an ethene insertion than after a propene one. Moreover, we find that the regioirregular 2,1-propene units (whose concentration is higher than claimed in the literature) are also substantially stereoirregular.

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