CHAIN-LENGTH-DEPENDENT TERMINATION RATE-PROCESSES IN FREE-RADICAL POLYMERIZATIONS .3. STYRENE POLYMERIZATIONS WITH AND WITHOUT ADDED INERT DILUENT AS AN EXPERIMENTAL TEST OF MODEL

Experiments were performed to test a model for the kinetics of free-ra dical polymerization systems, including the dependence of the terminat ion rate coefficients on the lengths of both chains involved. The mode l has few adjustable parameters, the values of which are moreover conf ined within fairly narrow limits. The data comprised the rate of polym erization in a seeded emulsion polymerization of styrene, with and wit hout benzene as diluent, with initiation by persulfate and by gamma-ra diolysis. The latter can be switched off instantly, providing relaxati on data which are sensitive to termination kinetics. Data from a singl e relaxation at a fixed weight-fraction polymer (omega(p)) were fitted to fur the unknown parameters, of which the only significant one is t he probability p of reaction between two radicals upon encounter, inco rporating the effect of spin multiplicity; this must lie between 0.25 and 1. Modeling using the value so obtained then successfully fitted ( a) relaxation data at the same omega(p) but with 15 mol % benzene dilu ent, (b) relaxation data with and without diluent over the range 0.5 l ess than or equal to omega(p) less than or equal to 0.8, and (c) chemi cally initiated data over the same omega(p) range. This provides convi ncing evidence for the correctness of the termination model, which cal culates the termination rate coefficients between two chains from the Smoluchowski equation, incorporating p, with diffusion coefficients (a s a function of chain length and of omega(p)) obtained from a ''univer sal'' scaling law inferred from NMR data, and where the interaction di stance for termination is the van der Waals radius of a monomeric unit ; contributions from ''reaction diffusion'' (whereby a chain end moves by propagating) are also important at high conversion. The data also support a model for initiator efficiency in emulsion polymerization, t his model being based on competition between aqueous-phase propagation (to a sufficient degree of polymerization for surface activity) and t ermination.