Molecular weight distribution attack

In addition, subsequent chain transfer reactions may occur on side chains and the larger the resulting polymer, the more likely will it be to be attacked. These features tend to cause a wide molecular weight distribution for these materials and it is sometimes difficult to check whether an effect is due inherently to a wide molecular weight distribution or simply due to long chain branching. 

The prediction of the MWD of emulsion polymers proved to be a relatively intractable problem even after the advent of the Harkins-Smith-Ewart theory. Perhaps the most successful early attack on the problem was that of Katz, Shinnar and Saidel (2). They considered only two microscopic events entry and bimolecular termination by combination. Their theory resulted in a set of partial integrodifferential equations, whose numerical solution provided the lower moments of the molecular weight distribution function. Other attempts to predict the MWD of emulsion polymers include those of Parts and Wat ter son (3 ), Sundberg and Eliassen (4), Min and Ray (5) and Gardon (6). 

In the absence of side reactions the number average degree of polymerization will be c/[M]/r/[I] if initiation is by nucleophilic attack on the monomer or 2 d[M]/c/[I] if initiation is by electron transfer followed by dimerization of the monomeric radical anions (r/fM] and d[I] are the reacted concentrations of monomer and initiator, respectively). If the rale of initiation is very rapid compared to the propagation rate and the initiator is mixed very rapidly and efficiently into the reaction mixture, then all macroions should start growing at almost the same time and should add monomer at equal rates. The active centers can be terminated deliberately and simultaneously since there are no spontaneous termination reactions under appropriate experimental conditions. Polymers made in such reactions have molecular weight distributions which approximate the Poisson... 

The molecular weight distribution of the original PDMS was rather broad (Mw/Mn = 1.3—1.4). Therefore, fractionation was performed in order to reduce the polydispersity of the product before conducting the subsequent steps of the synthesis. Despite the fact that the living PS chains were reported to attack the PDMS chains, no side reactions were detected in this study. 

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