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.