Ring polymerization mechanism change

This is not a trivial problem, and has important implications for the mechanism of the reaction. However, the bulk of the evidence is for centrosymmetric rings, which would be in keeping with our experience in small-molecule systems. For the present purposes we assume this to be the case. On this basis DSP is one of a class of monomers of crystal structural type 100 that polymerize to polymers 101. Note that, as is typical of topochemical reactions, there are cases of polymorphism of the monomers, in which only those of structure 100 are reactive. Also small changes in the substitution of this molecule frequently result in changes in crystal structure and reactivity. 

Substituent Effects So reactive are the coordinatively unsaturated metallocenium cations that one would expect that introduction of substituents into the cyclopenta-dienyl rings would modulate catalytic activity. (11) If substituent effects could be understood, it should be possible to modify the catalyst system to produce polymers having predetermined properties. We suspect that the needed detailed understanding will be hard to achieve for two reasons. First, the polymerization involves a multi-step mechanism and substituents may affect different steps differently. Second, steric and electronic effects of substituents may act in concert or in opposition and they must therefore be disentangled. Table 1 shows the effect of structural changes... 

It is apparent that the bridged ring monomers in which the double bond is made more reactive by steric strain are comparable in reactivity with ethylene, whereas, as would be expected, the aliphatic dienes are more like a-olefins in reactivity. Baldwin and van Strate have critically discussed this work [244], drawing attention to the possibilities of E/P reactivity ratios being changed by the presence of the diene and of diene polymerization by cationic mechanisms. They identify as... 

A final crystallization mechanism concerns, as in the earlier described LiPOj case, crystallization coupled with polymerization. The example is the change of trioxane rings to poly(oxymethylene) chains within single crystals of the cyclic monomer trioxane ... 

Higher conversions in thiirane polymerizations, however, proceed with chain scission transfer mechanism under the influence of BF3 (C2H5)20 [192]. This is indicated by a change in the molecular weight distribution, a bimodal character. When the reaction is complete there is a marked decrease in the average molecular weight of the polymer. When thietane polymerizes with triethyl-oxonium tetrafluoroborate initiation in methylene chloride, the reaction terminates after only limited conversion [193]. This results from reactions between the reactive chain ends (cyclic sulfonium salts) and the sulfur atoms on the polymer backbone. In propylene sulfide polymerization, however, terminations are mainly due to formations of 12-membered ring sulfonium salts from intramolecular reactions [193]. 

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