DICARBOXYLIC-ACIDS AND KETOACIDS FORMED IN DEGRADABLE POLYETHYLENES BY ZIP DEPOLYMERIZATION THROUGH A CYCLIC TRANSITION-STATE

The intermediate and final degradation products formed in six differen t low-density polyethylene (LDPE) films modified with either starch an d/or pro-oxidants or photosensitizers (Scott-Gilead formulation [SG]) were investigated. We propose that dicarboxylic acids and ketoacids, f ormed in the materials to varying degrees, are due to both secondary o xidation products and a zip depolymerization mechanism by backbiting t hrough a cyclic transition state. Hydrocarbons, ketones, carboxylic ac ids and dicarboxylic acids are formed during early stages of photo-oxi dation, and the ketones disappeared while several ketoacids appeared a nd the relative amount of dicarboxylic acids increased in the most sev erely degraded materials. During prolonged photo-oxidation, additional oxidation of ketones and monocarboxylic acids to dicarboxylic acids e xplains the high amount of dicarboxylic acids. In the thermooxidized s amples the amount of ketones and monocarboxylic acids remained high ev en in the most degraded samples. Mono-and dicarboxylic acids were form ed in several micrograms per 100 mg of polymer, while the ketones and ketoacids were formed in fewer micrograms per 100 mg of polymer. LDPE modified with the iron dimethyldithiocarbamate (SG1) was the most susc eptible material to photooxidation, while LDPE containing starch and p ro-oxidants (LDPE-MB) was the most susceptible material to thermo-oxid ation. Degraded LDPE-MB demonstrated less formation of degradation pro ducts; e.g., only in UV-initiated samples thermally degraded at 80 deg rees C for 5 weeks could degradation products be detected. Larger amou nts of ketones and ketoacids were formed in the SG materials than in t he starch-filled materials.