Tacticity group transfer polymerization

Rh138 was almost the same (almost atactic, slightly syndiotactic) as the tacticity of those obtained with conventional radical initiators such as AIBN under similar conditions. The triad ratio of rr.mr.mm as determined by 13C NMR is usually 58 38 4 and does not change even with the use of chiral and/or bulky ligands.103116 These results may exclude a coordination mechanism and suggest a radical nature. However, the stereochemical structure alone is not strong evidence for the radical polymerization because, for example, group-transfer polymerization, basically via an anionic mechanism, results in a stereo structure of PMMA similar to those for free radical processes.263... 

Recently, several laboratories have reported stereochemical analysis using NMR spectroscopy of series of MMA oligomers (from unimer to pentamer) prepared by radical polymerization with tetraphenylethane initiators,266 by radical telomerization with thiophenol,267 and by group transfer polymerization.268 These polymerization systems are not stereospecific (rather syndio-tactic) and thus the resultant MMA oligomers consist of comparable amounts of some stereoisomers. Cacioli and co-workers267 prepared MMA oligomers by radical telomerization with cobalt (II) tetraphenyl porphyrin, and studied them using two-dimensional NMR. They isolated three of four possible stereoisomers of the pentamer ( = 3) ... 

The G-SmA-N-I transition temperatures of syndiotactic poly(6-[4 -(4"- -bu-toxyphenoxycarbonyl)phenoxyl)phenoxy]-hexyl methacrylate prepared by aluminum porphyrin initiated polymerizations also level off at approximately 25 repeat units [91]. Similarly, the glass and nematic-isotropic transition temperatures of poly[6-(4 -methoxy-4"- Z-methylstilbeneoxy)hexyl methacrylate] prepared by group transfer polymerization become independent of molecular weight at approximately 20 repeat units [48]. Both polymethacrylates reach the same transition temperatures as the corresponding polymers prepared by radical polymerizations, which have nearly identical tacticities. 

Tacticity measurements can be correlated with reaction mechanisms and physical properties. For example, the incorporation of an electron donor into the polymerization catalyst formulation has been found to increase isotacticity in a propylene-1-butene copolymer [123], and the distribution of propylene and 1-butene contents as a function of molecular weight varied, depending on donor type. External donors, such as dimethox-ysilane, decrease the butene content more than internal electron donors (in this case, di-n-butyl phthalate). Mechanisms of new polymerization reactions, such as the group-transfer copolymerization of methyl methacrylate and lauryl methacrylate, can be elucidated by comparing NMR-derived structural details [124]. The presence of unanticipated peaks in the spectrum of poly(ethylene-co-norbomene) suggest the occurrence of epimerization... 

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