Cryogenic degradation of polystyrene in solution

An ultra-high molecular weight and narrow distribution polystyrene ($\text{M}{}_{\mathrm{<mtext>w</mtext>}}\text{= 7.3 × 10}{}^6$, M_w/M_n = 1.13) was dissolved in a wide range of solvents. Potential degradation by freezing was studied as a function of solvent type, concentration, cooling rate and number of freezing cycles. Cryogenic experiments were conducted in dioxane, tetrahydrofuran, benzene, dichloroethane, cyclohexanone, p-xylene, methyl methacrylate and styrene. The extent of degradation did not relate to a single solvent parameter, but there seemed to be a tendency towards a limited degradation in solvents with low melting points and/or solubility parameters greatly different from that of polystyrene. A low polymer concentration as well as a high cooling rate promoted chain scission, the latter parameter being the most important. In cyclohexanone and p-xylene, linear relationships were observed between the number of scission per molecule and the number of freezing cycles at high polymer concentrations and at high cooling rates. At lower concentrations and slower cooling, the relationships were non-linear suggesting a different degradation mechanism. The most extensive change in molecular weight distribution was observed on freezing in styrene. After 45 freezing cycles, an $\text{M}$_w of only 2.3 × 10⁶ was observed. The results indicate that chain scission occurred together with polymerization and combination reactions. Freezing of suitable solutions of ultra-high molecular weight polymers can thus be used as a new way of initiating polymerizations by cooling rather than heating.