Molecular weight and tacticity

Transformations in the Solid State. From a practical standpoint, the most important soHd-state transformation of PB involves the irreversible conversion of its metastable form II developed during melt crystallization into the stable form I. This transformation is affected by the polymer molecular weight and tacticity as well as by temperature, pressure, mechanical stress, and the presence of impurities and additives (38,39). At room temperature, half-times of the transformation range between 4 and 45 h with an average half-time of 22—25 h (39). The process can be significantly accelerated by annealing articles made of PB at temperatures below 90°C, by ultrasonic or y-ray irradiation, and by utilizing various additives. Conversion of... 

The recommended heat capacity data are currently being analyzed in terms of chemical structure, structure of the polymers in the glassy, crystalline and molten states. The data are further being analyzed to study the effect of branching, molecular weight and tacticity on the heat capacity of polymers. Comprehensive tabulation of heat capacities of various structural units are being prepared and will be available in the near future. 

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... 

Photoirradiated (TPP)AlMe was used to polymerize 6-[4 -(4"-methoxyphenoxy-carbonyl)phenoxy]hexyl methacrylate [90], 6-[4 -(4"-n-butoxyphenoxycarbonyl)phen-oxylhexyl methacrylate [91] and 6-[4 -(4"-cyanophenoxycarbonyl)phenoxy]hexyl methacrylate [92] in order to determine the effect of molecular weight and tacticity [(rr) =0.75]. Although high speed conditions [84-89] were not used, the polymerizations reached 76-92% conversion in 10 h to produce polymers with the expected molecular weight (DP <35) and pdi= 1.09-1.35. In contrast, that of 6-[4 -(4"-cyanophenyla-zo)phenoxy] hexyl methacrylate (Eq. 18)... 

PREFERENTIAL ADSORPTION. POLYMER MOLECULAR WEIGHT AND TACTICITY DEPENDENCE OF PREFERENTIAL ADSORPTION... 

Polymerization of trimethylsilyl methacrylate to alternatively form the isotactic, syndiotactic and atactic polymers has been described previously (22). The molecular weight and tacticity of these polymers could be determined after acidic workup of the polymerization reaction followed by isolation of the polymer and treatment with diazomethane to form poly(methyl methacrylate). The molecular weight and tacticity of these polymers are given in Table II. The values obtained represent dramatic differences in tacticity as a function of preparation method and thus provide a reasonable basis for the testing the effect of polymer microstructure on its subsequent behavior. 

Molecular weight and tacticity of poly(methyl methaciylate) formed by RAFT polymerization of methacryiic acid and salts. 

In the literature there is less information about template copol5unerization than about template homopolymerization. However, the process seems to be very interesting on account of the fact that the template can influence not only the molecular weight and tacticity of the daughter polymer but also the composition and distribution of units in the copolymer macromolecule. Two different schema of template copolymerization can be considered. First, the template can be a homopol5uner and one or both monomers can interact with the template. Second, the template can be a copolymer, or in the system two different templates exist. Literature in the field is not numerous. 

Wang, W., Zhang, Z., Zhu, J., Zhou, N., Zhu, X. 2009. Single Electron Transfer-Living Radical Polymerization of Methyl Methacrylate in Fluoroalcohol Dual Control Over Molecular Weight and Tacticity. f. Polvm. Sci. A Polvm. Chem. 47 6316-6327. 

The equations developed for contain no reference to the mode of polymerization. Although they are presented here in a chapter devoted to radical polymerization, they are characteristic of the polymer and not of its method of synthesis. The same applies to all polystyrenes of given molecular weight and tacticity, for example, regardless of whether they were polymerized by anionic, cationic, or free-radical initiation. 

In this early work, Exxon said the copolymers are more remarkable than the homopolymers. When ethylene is added, the initial chain-breaking reaction reverses and longer chains form again. The copolymers combine all three tailored molecular distributions — molecular weight, composition, and tacticity — whereas the broad MWD homopolymers display control of only molecular weight and tacticity distribution. These dual-metallocene PPs are such a recent development that Exxon appears not to have supplied them yet to customers. 

Needless to say, vinyl polymerization is one of the most important methods for polymer synthesis. A variety of carbon-carbon (C-C) main chain polymers have been prepared by the vinyl polymerization of monomers with diverse substituents, via radical, cationic, anionic, or coordination mechanism. Furthermore, with the technological achievement such as living and stereoselective (or stereospecific) polymerizations, fine-tuning of the polymer structure with respect to molecular weight and tacticity has been realized in a number of examples. In particular, polymers obtained with vinyl polymerization (vinyl polymer) as represented by polyethylene, polypropylene, polystyrene, and poly(methyl methacrylate) have contributed to the progress of modern society in various aspects as useful synthetic materials. 

Extensive morphological studies of polypropylene fracture surfaces have been carried out by Menges et al. [121, 136] and quite recently by Friedrich [183]. Using polypropylenes of different molecular weight and tacticity subjected to different thermal treatment Friedrich [183] obtained samples of different microstructure ... 

Miura Y, Satoh T, Narumi A, Nishizawa O, Okamoto Y, Kakuchi T. Atom transfer radical polymerization of methyl methacrylate in fluoroalcohol simifltaneous control of molecular weight and tacticity. Macromolecules. 2005 38 1041-1043. 

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