Poly main-chain degradation

When poly(BOMA-co-MMA)s are subjected to UV irradiation in benzene solution, in the absence of any monomer, extensive degradation takes place [55,75], thus confirming the occurrence of a photocleavage mechanism. Similar results have been found [55] by irradiation in monomer-free benzene solution of the copolymers of 1-phenyl-1,2-propanedione-2-0-methacryloyloxime with MMA [poly(POMA-co-MMA)], although a much higher efficiency of main chain degradation is obtained (Scheme 18 and Table 14). 

It has already been shown (e.g. Chapters 20 and 21) that the insertion of a p-phenylene into the main chain of a linear polymer increased the chain stiffness and raised the heat distortion temperature. In many instances it also improved the resistance to thermal degradation. One of the first polymers to exploit this concept commercially was poly(ethylene terephthalate) but it was developed more with the polycarbonates, polysulphone, poly(phenylene sulphides) and aromatic polyketones. 

The nucleophilic substitution on poly(vinyl chloroformate) with phenol under phase transfer catalysis conditions has been studied. The 13c-NMR spectra of partly modified polymers have been examined in detail in the region of the tertiary carbon atoms of the main chain. The results have shown that the substitution reaction proceeds without degradation of the polymer and selectively with the chloroformate functions belonging to the different triads, isotactic sequences being the most reactive ones. 

Simultaneously with Charlesby s findings, work along similar lines was carried out in G. E. s Research laboratories in Schenectady (22) and also in Research Institutes in the Soviet Union, although the latter only became known several years later (23). The results of this research demonstrated that in addition to polyethylene, many other polymers could be cross-linked by radiation. These include silicones, rubber, poly (vinyl chloride), polyacrylates and, to a lesser extent, polystyrene. In contrast, polymers such as polymethacrylates, polyisobutylene, polytetrafluoroethylene and cellulose underwent "degradation" by main-chain scission. These early findings were confirmed and extended to other compounds by numerous studies. 

The radiation degradation of poly(2-octyne) occurs only in the presence of oxygen. Its degradation products contain carbonyl and hydroxyl groups, and so dissolve in polar solvents (e.g., acetone). Such solubility change is essential to resist materials. The Gs value (number of main-chain scission per 100 eV of absorbed dose) of poly(2-octyne) is ca. 12. It is noteworthy that this value is higher than that of poly-(methyl methacrylate) (Gs ca. 2)118) which is being used as electron-beam resists. 

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