Molecular weight distribution tacticity

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

Polymers manufactured via single site catalyst technologies, because of the unique chemical catalytic environment, exhibit a more controlled molecular weight distribution and tacticity than seen with Ziegler-Natta catalyst systems. 

How do metallocene catalysts define the molecular weight distribution and tacticity of polypropylene ... 

Macromolecular engineering is the ultimate goal of the polymer chemist when he has a monomer or a family of monomers at his disposal. Once each step of the polymerization process is carefully controlled, every molecular parameter of the polymer is predictable molecular weight, tacticity, molecular weight distribution, nature of the end groups, microstructure, and composition, and block... 

Subsequently, Takada et al.55) studied the molecular weight distribution of syndio-tactic polypropylenes produced with the soluble VC14/A1(C2H5)2C1 catalyst in heptane... 

The different organomagnesium species present may propagate at different rates and with different stereochemistries. Their persistence during the lifetime of the polymer chains is thought to be responsible for the multimodal molecular weight distributions and heterogeneity of tacticity observed in many systems [34]. Accordingly, one could view the polymerization process as an amplifier. ... 

Homogeneous catalysts, in which each catalytic center is identical, give polymers having very narrow molecular weight and tacticity distribution and, in the case of copolymers, random comonomer incorporation. The latter feature is particularly important in polymers such as LLDPE, while a uniform stereo-or regiodefect distribution in PP can give improved optical and barrier properties. However, utilization of... 

A particularly interesting result in Table V is seen for the solutions containing about 66% methylene chloride. At this point a sharp break occurred in several properties (1) the syndiotactic content decreased and the isotactic content increased, (2) the yield dropped drastically, (3) the molecular weight decreased, and (4) the molecular weight distribution broadened. These changes all point to a change in the reaction mechanism, probably from one of solvated ion pairs to one of predominantly contact ion pairs at that solvent concentration. Nevertheless, the tacticities of all the polymers formed in this series were found to fit on a Bovey plot, which indicates that they were formed by a Bernoullian process and no penultimate effect was present. 

Polymer Characterization. Molecular weight distributions (MWD s) were obtained from a Waters model 201 ALC/GPC using micro-styragel columns with pore sizes of 500, 10, 10, 10 and 10 A. Calibration was with polystyrene standards in THF solution. Fractionation of bimodal MWD polymers was achieved by use of a Knauer CPC using styragel columns of 10, 10, 10 and 10 A. Tacticity information was obtained from NMR spectra using a 270 MHz Bruker instrument. 

Fluorinated ligands have often been employed in olefin polymerization. Fluorine substituents on the aromatic ring in well-refined Kaminsky-type metallocence catalysts for olefin polymerization play an important role in controlling the activity of the catalysts, molecular weight, tacticity, and molecular weight distribution of the polymers [28]. The catalyst (67),... 

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