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216 JAMES A. MAY, JR., AND WILLIAM B. SMITH Polymer Studies by Gel Permeation Chromatography. 11. The Kinetic Parameters for Styrene Polymerizations1 by James A. May, Jr., and William B. Smith Department of Chemistry, Texas Christian University, Forth worth, Texas 76139 (Received July 9, 1967) Styrene monomer was polymerized to low conversion using a range of benzoyl peroxide concentrations. The resultant polymer samples were characterized by gel permeation chromatography (gpc). Calculations made from the molecular weight distributions allowed the determination of the following kinetic parameters: chain transfer to initiator (CI), chain transfer to monomer (CM), and the ratio kt/kP2, where kt and k, are the rate constants for termination and propagation, respectively. Introduction The molecular weight distribution of a given polymer is well known to be a function of its mechanism of forma- tion, and Flory2 has given distribution functions which can be related to the kinetic parameters for vinyl polymerization. Until recently, it has usually been easier to determine the appropriate rate terms and to calculate molecular weight distributions rather than to suffer the difficulties and uncertainties of an experi- mental measurement of a weight di~tribution.~ The advent of gel permeation chromatography4 offered a rapid and potentially accurate method for ascertaining molecular weight distributions. Recently, we have demonstrated that gpc results on samples of polystyrene and polymethyl methacrylate, prepared under known kinetic conditions, were consistent with calculated distributions.6 These findings clearly sug- gest the utility of the gpc technique as a mode of studying the details of vinyl polymerization. Con- sequently, we have initiated a more detailed study of the styrene system with the results reported below. Experimental Section Styrene monomer was purified by washing three times with 10% potassium hydroxide, followed by three washes with water. After drying over calcium chloride, the styrene was carefully distilled under reduced pres- sure, and only the middle cut (43" (16 mm)) was re- tained. The benzoyl peroxide, which was used, assayed 100% by iodometric titrations in 2-propanol. All reactions were carried out using the sealed-am- poule technique in an oil bath controlled at 60 f 0.1". In every case, a known weight of benzoyl peroxide was added to a known weight of monomer, with allowance being made for the subsequent expansion of the mono- mer upon heating in the bath. Duplicate portions (30 ml) of the monomer-initiator mixture were placed in glass ampoules and sealed under vacuum (5 X mm) after a series of three freeze-thaw degassing cycles. Ampoules were withdrawn from the bath at appropriate times and plunged into an ice bath. The polystyrene was precipitated by washing the contents of the am- poules with a small amount of toluene into 3 1. of cold methanol. The polystyrene was recovered by filtra- tion, and then it was dried to constant weight at 40" (15 mm). Evaporation of the filtrate at room temper-
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