Stereospecific polymerizations selectivity

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

Another important use of BC13 is as a Friedel-Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in other organic syntheses (see Friedel-Crafts reaction). Examples include conversion of cydophosphazenes to polymers (81,82) polymerization of olefins such as ethylene (75,83—88) graft polymerization of vinyl chloride and isobutylene (89) stereospecific polymerization of propylene (90) copolymerization of isobutylene and styrene (91,92), and other unsaturated aromatics with maleic anhydride (93) polymerization of norbomene (94), butadiene (95) preparation of electrically conducting epoxy resins (96), and polymers containing B and N (97) and selective demethylation of methoxy groups ortho to OH groups (98). 

On the basis of these results, the Lewis base has been assumed by several authors 50.67,69,81.100,113.115,117) selectively poison the less stereospecific polymerization centers, through a reversible complexation of the Lewis base to the coordinatively unsaturated active sites. 

Hence, it should be expected that, just as in photonics, in stereospecific polymerization and, consequently, in polymerization in general there act certain selection rules , i.e., some principles controlling the course of the process. 

Busico, V. Cipullo, R. Talarico, G Stevens, J. C. New high activity group 4 Ci-symmetric catalysts for isotactic-selective high temperature solution polymerization of propene, and copolymerization of propene with ethene. 2-Polymer microstructure and polymerization mechanism. Abstract of Papers, European Polymer Conference on Stereospecific Polymerization and Stereoregular polymers. Milano, Italy, June 8-12, 2003 pp 81-82. 

The study of the macromolecular microstructure gives information on the stereochemical mechanism of polymerization. The NMR analysis of homopolymers and copolymers of selectively deuterated propylenes allowed the determination of the mechanism of addition to the double bond in the stereospecific polymerizations. The causes of the steric control may be also evidenced by micro-structural analysis on suitable macromolecules. 

Enantiomorphic sites concept The stereospecific polymerization of racemic methyloxirane produces generally a polymer with some cristallinity which can be fractionated by selective solubility (using acetone for example) in a crystalline fraction and an amorphous one (an intermediate "semi-crystalline fraction can be also isolated). 

In this review, contributions of selected experimental and molecular modeling studies to the elucidation of even fine details relative to the stereospecificity of polymerization catalytic systems are outlined. 

Polymerization in which a tactic polymer is formed. However, polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific. For example, the polymerization of a chiral monomer, e.g., R)-propylene oxide ((i )-methyloxirane), with retention of configuration is not considered to be a stereospecific reaction however, selective polymerization, with retention, of one of the enantiomers present in a mixture of R)- and (S)-propylene oxide molecules is so classified. 

Alternatively, Soga and co-workers described how a series of (CsH4R)TiCl3 compounds in association with MAO could mediate 1,4-at-stereospecific living BD polymerization (e.g., R = Bu, —25°C, 1,4-m-selectivity = 93.5%, = 126000, PDI = 1.04). It was also revealed that an increase in the steric bulk of the substituent R results in an increase in both catalytic activity and 1,4-m-selectivity. Additionally, Shiono and co-workers found that stereospecific quasi-living BD polymerization could be catalyzed by a rather simple catalyst system composed of C0CI2 and MAO (0°C, l,4- j-selectivity = 98.2%, M = 361000, PDI = 1.3). ... 

In order to find a highly stereospedfic, homogeneous catalyst for the polymerization of propylene oxide, we selected the organometallic compound-primary amine catalyst system which exhibited excellent stereospecificity in the polymerization of acetaldehyde. 

On the basis of the data, it was concluded1 that the synthetic polyglucoses obtained by this method are linear, /3-d- (1— 4) -linked glucans which possess properties similar to those of cellulose. It is especially significant that this polymerization proceeded with apparent stereospecificity, to form only the /S-d anomeric linkage. The mild conditions under which the polycondensation occurs probably contribute to this selectivity of reaction. 

As the active metal-carbon bond assumes more covalent character, there will be a greater tendency for hemolytic cleavage and radical-type polymerizations. This becomes favorable when the alkyl is attached to a transition metal in one of its highest valence states or to a non-transition metal of Group IV or V. One can expect such catalysts to initiate polymerizations by both the conventional simple free radical and coordinated radical mechanisms. Stereospecificity generally suffers in these systems because both mechanisms are operative and because radical addition to a double bond is less selective for producing a head-to-tail polymer structure. 

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