Fig. 1. The degradation reactions of MM-PAMS. (a) Geometric structures of di-radical MM-PAMS. R1 and R3 are degradation paths that produce the α-methylstyrene (AMS) monomer. R2 indicates the H atom transfer reactions. R4 is the isomerization process. R5 represents the H atom transfer reactions after R4. R2-1/R2-2 and R5-1/R5-2 are chain scission reactions after the corresponding H transfer reaction. The locations of arrows denote the bonds breaking on the corresponding degradation path. (b) Potential energy curves of these reactions for di-radical structure. ‘Int’ represents the intermediate. The energy of the reactant is taken as the zero. All energies were corrected with ZPE. The light blue and beige areas in the figure represent the degradation reactions related to the C-unsaturated and CH₂-unsaturated ends, respectively. (c) Geometric structures of saturated MM-PAMS. R6 and R7 are degradation reactions that produce the AMS monomer after H transfer of two ends. (d) Potential energy curves of two reactions for saturated structure. (e) Selected frontier molecular orbitals (MOs) of reactants in R1–R4 and R6–R7 reactions. Isovalue equals 0.035.

Fig. 2. Geometric images before and after reactions. Here, the green arrows indicate the site of broken C–C bonds, and the green circles and red dash circles, respectively, denote the H atom and benzene ring involved in reactions.

Fig. 3. Dynamic simulations of the degradation processes of di-radical MM-PAMS at 550 K. These figures represent the bond length changes with respect to time for breaking sites on the backbone chain. Typical conformations during degradation are also drawn. C1–C2, C3–C4 and so on are the positions of broken bonds during degradation.

Fig. 4. Reaction rate constants and ratios of degradation reactions as functions of temperature. (a) Reaction rates of degradation reactions related to the C-unsaturated end. (b) Reaction rates of degradation reactions related to the CH₂-unsaturated end. All values were calculated by TST/Eckart using the KiSThelP program^[33]. In both (a) and (b), values of these reaction rates corresponding to 350 K, 450 K and 550 K are also marked. (c) Reaction rate ratios of R1 and R5. The equations in the lower right corner of each graph represent the calculation formulas of rate constant and ratio, respectively.

Fig. 5. Degradation experiment on MM-PAMS. (a) The TG and derivative thermogravimetric (DTG) curves of MM-PAMS. The black dotted line is the TG curve, which has been shown in our previous work^[17]. The red line represents the DTG curve. The grey arrows represent the locations of three peaks in the DTG curve. (b) The thermal degradation kinetics of MM-PAMS. Black dots represent values obtained through experiment. The solid line is the fit for the function = (see the text for a detailed description).