Molecular weight distribution at high conversions

Molecular weights and molecular-weight distributions at high conversions are much less predictable and more dependent on the nature of the monomer. The two principal additional factors that come into play are chain transfer to polymer and the decrease of the rate coefficients for coupling and disproportionation with increasing polymer content, viscosity, and chain entanglement. 

Bulk (batch type) Minimum contamination. Simple equipment for making castings. Strongly exothermic. Broadened molecular weight distribution at high conversion. Complex if small particles required. 

Molecular-weight distribution. For low conversion or polymerization in dilute solution, approximate simple formulas can be derived. Complications arising at high conversion in bulk polymerization will be outlined later. 

When dwj fdi = 0, the curve has a maximum and / = I / In p. A series expansion of — 1 / In p gives p/(p — I) as the first term and this fraction approaches I /(p — I) as p approaches 1. This is the value of in linear step-growth polymerizations (Eq. 5-20). Thus, as a first approximation, the peak in the weight distribution of high conversion linear step-growth polymers is located at of the polymer if the synthesis was carried out under conditions where interchange reactions and molecular weight equilibration could occur. 

Throughout this section we have used mostly p and u to describe the distribution of molecular weights. It should be remembered that these quantities are defined in terms of various concentrations and therefore change as the reactions proceed. Accordingly, the results presented here are most simply applied at the start of the polymerization reaction when the initial concentrations of monomer and initiator can be used to evaluate p or u. The termination constants are known to decrease with the extent of conversion of monomer to polymer, and this effect also complicates the picture at high conversions. Note, also, that chain transfer has been excluded from consideration in this section, as elsewhere in the chapter. We shall consider chain transfer reactions in the next section. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

A porphinatoaluminum alkoxide is reported to be a superior initiator of c-caprolactone polymerization (44,45). A living polymer with a narrow molecular weight distribution (M /Mjj = 1.08) is ob-tmned under conditions of high conversion, in part because steric hindrance at the catalyst site reduces intra- and intermolecular transesterification. Treatment with alcohols does not quench the catalytic activity although methanol serves as a coinitiator in the presence of the aluminum species. The immortal nature of the system has been demonstrated by preparation of an AB block copolymer with ethylene oxide. The order of reactivity is e-lactone > p-lactone. 

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