Effect of Exogenous Carboxyl and Hydroxyl Groups on Pyrolysis Reaction of High Molecular Weight Poly(L-Lactide) under the Catalysis of Tin

Abstract

The effect of exogenous hydroxyl, carboxyl groups and/or Sn2+ on pyrolysis reactions of poly(L-lactide) (PLLA) was investigated by thermogravimetric analysis (TGA). The activation energy (Ea) of pyrolysis reactions was estimated by the Kissinger-Akahira-Sunose method. The kinetic models were also explored by the Malek method, and the random degradation behavior was determined by comparing the plots of In{-In[1 - (1 - w)0.5]} versus 1/T for experimental data from TGA with model reactions. The pyrolysis reaction rate of PLLA was affected slightly by exogenous hydroxyl and carboxyl groups at lower levels of Sn with 65–70 mg·kg-1 but increased appreciably in the presence of extraneous Sn2+, -COOH/Sn2+, or -OH/Sn2+. The Ea values for the pyrolysis reactions of the PLLAs that provided lactide were different under the catalysis of Sn2+ in different chemical environments because Sn2+ can form the new Sn-carboxylate and Sn-alkoxide with exogenous carboxyl and hydroxyl groups, which were different in steric hindrance for the formation of activated complex between Sn2+ and PLLA. Under the catalysis of Sn2+, a lactide molecule can be directly eliminated selectively at a random position of PLLA molecular chains, and the molecular chain of PLLA cannot change two PLLA fragments at the elimination site of lactide. However, it was regenerated into a new PLLA molecule with the molecular weight reduced by 144 g·mol-1.