Defect Structures in Polystyrene

There is a substantial literature on the thermal and photochemical degradation of PS and it is well established that polymer properties are sensitive to the manner in which a particular sample of PS is prepared. For example, it has been reported that PS prepared by anionic polymerization shows enhanced stability w ith respect to that prepared by a radical mechanism. I his has often been attributed to the presence of weak links in the latter polymers. However, the precise nature of the weak links remains the subject of some controversy. The situation is further confused by all PS prepared by radical mechanisms often being considered as a class without reference to the particular polymerization conditions employed in their preparation. In many cases the polymers are commercial samples with details of the method of preparation incomplete or unstated. 

BPO is commonly used as an initiator for S polymerizations and copolymerizations and it has been reported that its use can lead to yellowing and impaired stability in The initiation and termination pathways observed for 

S polymerization when BPO is used as initiator have been discussed in Sections 

NMR studies on polymers prepared with C-labeled BPO have shown that the primary benzoyloxy and phenyl end groups formed by tail addition to monomer are thermally stable under conditions where the polymer degrades. They persist to 50% weight loss at 300°C under nitrogen. Thus, these groups are unlikely to be directly responsible for the poor thermal stability of PS prepared with BPO as initiator. On the other hand, the secondary benzoate end groups, formed by head addition or transfer to initiator, appear extremely labile under these conditions. I heir half life at 300 C is 5 min. 

There have been many studies on the thermal and thermo-oxidative degradation of PMMA. It is well established that the polymer formed by radical polymerization can be substantially less stable than predicted by consideration of the idealized strueture and that the kinetics of polymer degradation are dependent on the conditions used for its preparation. There is still some controversy surrounding the details of thermal degradation mechanisms and, in particular, the initiation of degradation.  

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