The mechanism of thermal degradation of homopolymers of styrene (St) and methacrylonitrile (MAN) and their copolymers was investigated theoretically and experimentally by the pyrolysis gas chromatography using a Curie-point pyrolyzer. Poly(St-co-MAN)s generate dimers and trimers as well as monomers by flash pyrolysis. Parameter β£ was proposed to account for the competition between the back-biting reaction and depolymerization. The back-biting parameter β£ is defined as the ratio of rate constants, β£ Ο k bb /k dp , where k bb is the rate constant for the back-biting reaction and k dp is that for depolymerization. The back-biting process is followed by β€-scission, where dimer and trimer are generated, and directly correlated with the COH bond dissociation energies in the polymer chain. Using the back-biting parameter β£, where 1/β£ is equal to the zip length n in depolymerization, the boundary effect for the difference of monomer yields from the homopolymers of St and MAN and their copolymers is well explained. The calculated values of boundary effect parameters, β€ St and β€ MAN , agreed well with the experimental results. It was found that thermal degradation mechanisms of homo-and copolymers of vinyl compounds can be analyzed comprehensively using the back-biting parameter β£ and the boundary effect parameter β€.