Kinetics of thermal depolymerization of trimethylsiloxy end-blocked polydimethylsiloxane and polydimethylsiloxane-N-phenylsilazane copolymer

The thermal degradation of Me3SiO end‐blocked polydimethylsiloxane (eb‐PDMS) and polydimethylsiloxane‐N‐phenylsilazane (eb‐PDMS–NPhSz) copolymer was studied. For both polymers relative degree of polymerization (DP/DP0) as a function of conversion (1 – W/W0) data were obtained. For eb‐PDMS the results were consistent with a mechanism involving a rate determining random siloxane bond cleavage initiation step followed by a rapid and complete depropagation of the active fragments evolving volatile cyclic oligomers. Rate constants (k) for initiation were obtained at four temperatures from plots of DP−1 vs. time. An Arrhenius activation energy of approximately 80 kcal/mol was determined and is consistent with a SiOSi scission transition state. The degradation of eb‐PDMS–NPhSz appears to follow the same depolymerization process evolving cyclic oligomers. Although DP/DP0 vs. C data suggest a random cleavage–complete depolymerization mechanism, an Arrhenius plot suggests a more complex degradation mechanism. The role of impurities as degradation catalysts is discussed.