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It was well known that the defects in the polymer chains could significantly influence the crystallization process of semi-crystalline polymers. In this work, the role of defect distribution in the melt memory effects was studied by comparatively investigating a pair of Ziegler-Natta and metallocene-made isotactic polypropylene (ZN-iPP and M-iPP) fraction specimens with the same total defect content which were obtained from two raw parent samples through the solvent extraction. The characterization results of melting behaviors, polymorphic structures, and the successive self-nucleation and annealing analysis demonstrated that these two fractions exhibited the different distribution of defects, i.e., the defect distribution in the M-iPP specimen was random while that in the ZN-iPP specimen was nonstatistical. The results of self-nucleation (SN) experiments indicated that the M-iPP exhibited much wider temperature range of Domain II than the ZN-iPP with similar total defect content, which indicated that the M-iPP showed a much stronger melt memory effect. Considering the effect of the microstructure on the crystallization behavior and the sequences containing defects could be excluded from the crystalline regions during the formation of crystal, we speculated that the random distributed defects in M-iPP could form a continuous interface between the crystalline regions and amorphous phase which could cause a heterogeneous melt and strengthen the melt memory effect.

The melt fracture of high-density polyethylenes (HDPEs) is studied primarily as a function of molecular weight and its distribution for broad molecular weight distribution metallocene and Ziegler–Natta catalyst resins. It is found that sharkskin and other melt fracture phenomena are very different for these two classes of polymers, although their rheological behaviors are nearly the same for many of these. Moreover, the metallocene HDPE shows significant slip at the die wall without exhibiting stick-slip transition. Important correlations are derived between the critical conditions for the onset of melt fracture and molecular characteristics.

The chain heterogeneity of a polyethylene resin can be assessed by simple dissolution techniques followed by compositional analyses. This heterogeneity influences common measurements of the randomness of the polymerization process. Specifically, the values for the product of reactivity ratios and the relative monomer dispersities will suggest a less random polymerization process than is actually occurring on a molecular scale. For the system studied, compositional analysis of the parent resins demonstrates the blockiness (non-random) character commonly reported for heterogeneous Ziegler–Natta polymerizations. Compositional analyses of the soluble and insoluble fractions show at least two chain populations with significantly different ethylene/1-hexene molar ratios, each showing a near random incorporation of comonomer. These data are consistent with at least two catalyst sites, each with different comonomer incorporation efficiency.

Most of polymerization reactions with Ziegler catalysts operate through the ‘non-living’ mechanism, whereas lanthanide catalysts show a living-mechanism character. The degree of ‘livingness’ in a polymerization reaction is generally investigated through kinetics and the product analysis rather than by direct analysis of a catalyst. We have developed a new insight for judging the ‘livingness’ in polymerization reaction in aspect of electronical properties of the active chain ends of Li, Ni and Nd catalysts for 1,3-butadiene polymerization using the frontier orbital analysis. The frontier orbitals of the Ni catalyst are different from those of the Li and Nd based catalysts. The HOMO of the Ni catalyst has mostly the d-orbital (Ni) character while the LUMO has the p-orbital (Cα and Cγ atoms) as well as the d-orbital (Ni) character. The π-butenyl coordination at the Ni catalyst is η1-type but those of the Nd and Li catalysts are η3. The atomic charge (+0.003) on the Ni atom and the atomic charge (−0.14) on Cα of the Ni catalyst are much lower than those of the Nd catalyst (Nd +1.09, Cα −0.56).

In this work, a 2 3 factorial experimental design for the evaluation of ethylene– 1-butene copolymerization was employed. The following reaction parameters were used as independent variables: catalyst type, Al/Ti molar ratio and 1-butene concentration. The copolymerization was carried out using hybrid Ziegler – Natta/Metallocene catalysts with different titanium molar ratios. The catalyst activity and polymer characteristics were statistically analyzed through response surface methods. It was found that the catalyst type has a significant effect on activity, melt flow index and 1-butene content. Copolymers presented crystallinity values ranging from 46 to 58% and melt temperature in the 128– 131 8C range. Copolymer comonomer content varied from 2 to 6% in weight. q 2002 Published by Elsevier Science Ltd.

The results of experiments with a supported Ziegler-Natta catalyst were interpreted by a two-site model of the active sites, in order to enhance the understanding of multiple types of catalyst active sites and in particular the role of the external donor. The applicability of the two-site model is discussed and the stochastic parameters of the model are correlated with both the experimental and molecular modelling data. The two-site model satisfactorily explains the major part of the active sites of this highly isospecific catalyst and is a suitable tool for analysing the effects of the donors on catalyst active sites. Nevertheless, it does not completely describe the behaviour of Mg-supported Ziegler-Natta Ti catalyst. There is probably more than one kind of active site producing the part of polypropylene that is soluble in boiling heptane. This study also supports the earlier suggestion that some of the changes in the population of active sites can be explained only through the formation of new sites, and more specifically through the existence of fluctuating sites that can be stabilized by external donors. The stability of these new sites depends on both the structural and electronic properties of the external donors.

The molecular structure of three isotactic polypropylenes polymerized using either slurry or bulk process and heterogeneous Ziegler-Natta catalysts without external donors was studied in this work. The influence of external donor addition was also examined. The samples were fractionated according to stereoregularity using a solvent gradient extraction method, and calorimetric properties, 13C nuclear magnetic resonance (n.m.r.) pentad tacticities and molecular weights of the fractions were determined. Statistical analysis was also applied to the 13C n.m.r. results. The samples polymerized with the novel catalyst system had the conventional stereostructure arising from a catalyst-site-controlled propagation mechanism. When compared with commercial polypropylenes, the amount of clustered defects was increased in the products made with the novel catalyst system. The addition of an external donor diminished the amount of highly atactic material, but the isotacticity of long-chain material was also increased. Qualitatively similar defects were detected in all samples, only the amount of defects varied depending on the catalyst structure and polymerization process used. Surprisingly, the bulk polymerization process seemed to favour the formation of non-isotactic material.

Microstructural analysis was carried out on five isotactic polypropylenes produced with different heterogeneous Ziegler-Natta catalyst systems and different polymerization processes. The samples were fractionated according to stereostructure using a direct extraction method. Pentad tacticities of unfractionated samples and selected fractions were determined by 13 C n.m.r. spectroscopy. I.r. isotacticities were also measured. Statistical analysis of the polymerization mechanism was carried out on the basis of the two-site model from the pentad tacticities, and average lengths of meso and racemic sequences were determined. In addition to atactic and isotactic materials produced at different catalytic sites, the polypropylene samples contained syndiotactic material as stereoblocks in isotactic chains. On the basis of the analysis carried out on the fractions, slight differences in the microstructure were found to exist between the samples produced by different processes and catalyst types. These differences have a significant effect on average sequence lengths.

We have recently published a kinetic mathematical model, based upon multiple active site distribution approach, for olefins polymerized with heterogeneous Ziegler-Natta catalysts1. Thismodel can be used to calculate not only production rates, but also both broad molecular weight and copolymer composition (temperature rising elution fractionation (TREF)) distributions. We now extend this model to account for both the stereo and the chemical sequence length distributions. We focus our analysis on α-olefin homopolymers and α-olefin-ethylene copolymers. The α-olefin must not be polymerizable by another mechanism (ionic or radical), so that styrene is not considered. Some simulations are carried out for typical productions of polypropylene and propylene-ethylene copolymer.

A conformational study of a possible model for chain end control in the homogeneous isotactic specific Ziegler-Natta polymerization is presented. The model site consists of a complex with two ligands (olefin and growing chain) besides two cyclopentadienyl ligands. The geometries corresponding to the located energy minima are very similar to those of the chiral model previously proposed by us for the site control in the homogeneous isospecific Ziegler-Natta polymerization. In this case the diastereoisomeric intermediates for isotactic and syndiotactic propagation are energetically similar but, according to our assumptions, the insertion path for the propene would be easier in the case of isotactic propagation.

Factors affecting the duration of the settling period in the polymerization of 4-methylpentene-1 by the catalyst system VCl 2 AIR 3 in benzene have been systematically investigated. Plots of log10 (100 — %Rp) versus time show that the so-called settling period can be divided into two distinct regions; an initial region which is not time dependent and a steady state region in which the formation of Co follows a strictly first order time dependence. The initial region is believed to arise from the removal of adsorbed AlR2Cl species, while the steady state region can be interpreted as describing a rearrangement reaction involving the production of initiated chains.

The paper discusses in detail the nature of the propagation step in Ziegler-Natta polymerization in terms of a one step or two step mechanism. A wide variety of evidence, both earlier literature results as well as current experimental data, is presented in support of a two-step mechanism comprising an initial monomer complexation at the active site followed by insertion into the growing transition metal-polymer bond. The nature of the rate determining step in the polymerization is also considered, and whereas in certain systems at low temperature insertion is clearly the main rate determinant, under other conditions both steps may together determine the overall rate.

A reaction model for Ziegler-Natta polymerization processes which is in accordance with the results of previous studies and which is able to explain the phenomena known in this process, at least qualitatively, has been formulated. It was assumed that complexation, as pointed out by Cossee and Henrici-Olive and Olive, is the fundamental process followed by a series of reactions at the active site. On this basis it was possible to develop a kinetic scheme applicable to homogeneous and heterogeneous catalysts. By making certain assumptions, simple equations which could be checked experimentally were derived.

By use of kinetic data of the polymerization of hexene-(1) and octene-(1) an attempt is made to determine the concentration of active sites. The complex kinetics of the supported Ziegler-Natta catalyst investigated, however, does not allow an explicit calculation of the concentration of active sites. With certain conditions met, the yield factor of active sites was determined. For the polymerization of hexene-(1) and octene-(1) the yield factors gave very low values. This is attributed to the fact that in the case of hexene-(1) and octene-(1) no disintegration of the catalyst particles was observed as is the case for ethylene polymerization.

A kinetic model has been derived based upon multiple catalyst sites of differing reactivities for the Ziegler-Natta copolymerization of olefins. The kinetic scheme accounts for the formation, initiation and deactivation of active sites, as well as for spontaneous transfer and for transfer reactions to hydrogen, monomer and organometallics. The model predicts the rate of polymerization, the copolymer composition and the molecular-weight distribution of the polymer produced. The model accounts for the observed broad copolymer composition and molecular-weight distributions. A method for the estimation of the model parameters via temperature-rising elution fractionation, gel permeation chromatography and nuclear magnetic resonance is discussed. Some simple calculations are performed to demonstrate that the observed broad copolymer composition and molecular-weight distributions can be predicted by this model.

The activating effect of triethylamine in the VCl 3 AlR 3 system has been explained in terms of a previously proposed kinetic scheme. Activation is observed at low amine concentrations and is thought to occur by removal of adsorbed aluminium alkyl species from the catalyst surface. Deactivation occurs at higher donor concentrations and is believed to be due to competitive adsorption of donor with monomer. These proposals are consistent with the rate, molecular weight and polymer tacticity findings of other workers in Ziegler-Natta systems where the donor is capable of interaction with both the transition metal and metal alkyl components.

A kinetic scheme is postulated for Ziegler-Natta polymerization where propagation is considered to occur between adsorbed monomer and an active centre which has been formed by interaction of the metal alkyl with the transition metal halide. Chain transfer is considered to occur with adsorbed metal alkyl and also with adsorbed monomer. Rate, molecular weight and active centre dependencies are derived which are in qualitative agreement with the results of previous studies in Ziegler-Natta polymerization.

The kinetics of the polymerization of 4-methylpentene-1 by the Ziegler-Natta system VCl 3 AlR 3 have been examined at 30°C and 40°C. The dependence of the overall rate of polymerization on the concentration of metal halide, aluminium alkyl concentration, monomer concentration, temperature, and nature of the aluminium alkyl has been established. The energy of activation of the overall polymerization process has been found to be 16·6kcal/mol in benzene and 13·7kcal/mol in heptane. The results have been examined in relation to a previously described kinetic scheme.

The X-ray diagrams of polypropylene specimens of widely varying properties have revealed the existence of three modifications besides the normal modification studied by Natta and co-workers. One of these modifications has been described in the literature. For this modification a modified tentative structure is proposed which is more consistent with the available data than the tentative structure suggested in the literature.

The number of active centres in the polymerization system VCl3/AIR3/4-MP-1 has been shown to lie in the range 2·3–6·1 × 10−4mol/mol VCl3 for a series of aluminium alkyls, and to vary with the nature of the aluminium alkyl used. Chain transfer with aluminium alkyl has been demonstrated. The propagation rate constant has been evaluated and found to be independent of the nature of the metal alkyl. The location of, and nature of, the active centre is discussed. The validity of the tritium labelling procedure is tested by comparison of calculated and experimentally determined molecular weights.

Molecular weight relationships in the polymerization of 4-methylpentene-1 by the Ziegler-Natta system VCl 3 AlR 3 have been examined at 30°C and 40°C. The dependence of the number-average degree of polymerization on the concentration of metal halide, aluminium alkyl concentration, monomer concentration, temperature, and nature of the aluminium alkyl has been established. The results have been examined in relation to a previously described kinetic scheme, and the rate constant for chain transfer with adsorbed monomer has been evaluated.