Polycarbonate polyester molecular weight effect

Polymerization in the melt is widely used commercially for the production of polyesters, polyamides, polycarbonates and other products. The reactions are controlled by the chemical kinetics, rather than by diffusion. Molecular weights and molecular weight distributions follow closely the statistical calculations indicated in the preceding section, at least for the three types of polymers mentioned above. There has been much speculation as to the effect of increasing viscosity on the rates of the reactions, without completely satisfactory explanations or experimental demonstrations yet available. Flory [7] showed that the rate of reaction between certain dicarboxylic acids and glycols was independent of viscosity for those materials, in the range studied. The viscosity range had a maximum of 0.3 poise, however, far below the hundreds of thousands of poises encountered in some polycondensations. 

Although solid-state polymerizations of polyamides and polyesters (which are crystalline polymers), have been known since 1939 and 1962 (13,14), until now, it has been considered impossible to produce polycarbonate by solid-state polymerization, because polycarbonates are amorphous polymers and become molten at the temperatures necessary to effect polymerization. The key technology in solid-state polymerization of polycarbonate is the crystallization of the amorphous piepolymer. It has been found that the low molecular weight amorphous prepolymer is easily crystallized, and the obtained crystallized prepolymer retains its solid-state when it is heated to the temperatures necessary for polymerization. 

Ma Z, et al. Biodegradable polyurethane ureas with variable polyester or polycarbonate soft segments effects of crystallinity, molecular weight, and composition on mechanical properties. Biomacromolecules 2011 12(9) 3265-74. 

Suitable trans-esterification catalysts are numerous and include a wide variety of bases and Lewis acids. Bases derived from alkaline and alkaline earth metals, such as, magnesium hydroxide and sodium stearate are known to cause transesterification of polyesters and polycarbonates in an extruder however, these basic catalysts can cause molecular weight loss during or post-extmsion, especially if the catalysts are not quenched downstream. Thus, there is a need for transesterification catalysts which are reactive enough to cause transesterfication in the extruder without the detrimental effect of causing molecular weight degradation or melt stability problems. 

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