Studies in polymerization. I. A method for determining the velocity constants in polymerization reactions and its application to styrene

Abstract

A general method for determining the velocity constants of initiation, propagation, transfer and termination in polymerization reactions is described. The method depends essentially on the measurement of the rate of increase of viscosity for various rates of chain initiation. In the case of radical polymerizations photochemical initiation is most convenient. Measurement of the photochemical after-effect then gives four independent relations between the four velocity constants. The method has been tested by a detailed study of the thermal and photochemical polymerization of styrene. Velocity constants have been determined at 0 and 25° C, and the activation energies and frequency factors calculated. Results are given in tables 5 and 6. It is shown for the first time that chain transfer is important in the thermal polymerization of pure styrene, occurring 11 times per kinetic chain at 25° C. This result invalidates conclusions which previous workers have reached. Other rather unexpected results are that the initiation reaction has a much higher activation energy than has been generally supposed, and a normal frequency factor, while termination is activated. A detailed kinetic scheme is proposed which removes certain unjustifiable assumptions present in earlier ones. This is shown to be consistent with all the available evidence.

Ultrafast Conversion and Molecular Weight Control through Temperature Programming in Microwave-Induced Miniemulsion Polymerization

“…Chain growth is stopped only by transfer events which might occur to monomer, initiator, stabilizer, or at higher conversion also to polymer. For styrene at monomer conversion up to 40% (as obtained after the first microwave pulse) the growth of a single chain is practically only stopped by transfer to monomer, which has however no effect on conversion. , …”

“…The fact that polymerization takes place predominantly after  and not during  the actual temperature pulse can be illustrated by a model calculation assuming a miniemulsion droplet with a diameter of 100 nm contains almost 3 × 10 6 styrene molecules, which would lead to a potential molecular weight of 300 × 10 6 g/mol. The limiting size of a polymer molecule is determined by the so-called transfer limit, which is the ratio of the temperature-dependent rate constants of propagation, k p , and transfer to monomer, k tM (e.g., see ref for k tM at 70 °C and refs and for k tM at 0 °C). As chain transfer to monomer has a higher activation free energy than propagation, lower temperatures favor higher molecular weights.…”

“…Accordingly, polystyrene molecules with molecular weight of more than 5 × 10 6 Da can only be produced with radical polymerization at temperatures lower than room temperature; hence, the ultrahigh molecular weights in the experiments cannot have been formed during the hot phase. Particularly, for 0 °C a transfer limit of M n of about 10 7 Da is estimated. , As experimental data for 0 °C are not available, estimated values were used assuming that the activation free energies of the kinetic processes obtained at higher temperatures can be applied.…”

Modeling Lignin Polymerization. I. Simulation Model of Dehydrogenation Polymers

“…It does not affect the results, as only the relative reaction constants are of importance (data not shown). To a first approximation, we assumed all binding probabilities to be the same for each reactant combination, doubling the propensities for reactions of oligomers with two bindable ends (Bamford and Dewar, 1947). Also, the bond probabilities for monomer-oligomer reactions are unbiased.…”