AN EFFECT OF POLYMERIZATION TEMPERATURE ON THE HUGGINS CONSTANT k′ FOR A DIENE POLYMER

Abstract

San~ples of poly(butadiene-co-styrer~e) were polymerized in redos systems a t 5" and -18" C., the recipes being such as to yield samples similar in composition, in percentage conversion, and in intrinsic vistex to those prepared at.50°, 40°, 30°, and 15" C. for an earlier study. The two sa~nples were carefully fractionated and the viscosity f~mctions [ q ] , 0, and k' were determined for each of the fractions. k' \\-as shown to be the same for all the fractions of a given sarnplean indication that there is little or no branching a t these low teniperatures.Comparison with the earlier results reveals, however, that for unbranched species k' increases as the temperature of polymerization decreases, from 0.327 + 0.008 a t 50" to 0.370 + 0.005 a t -IS0 C. This increase in h' is attributed to an increase in the trans-1,4-content of the polymer. INTRODUCTIONIn earlier papers, evidence was presented that the Huggins slope constant k' (8) can be a useful measure of branching in high polymers (10, 5, 3, 4). When polymers containing branched species were carefully fractionated, the values of k' obtained with the fractions decreased regularly with the intrinsic viscosity of the fractions, finally reaching, with the lower fractions, a constant value h' characteristic of linear species of the same polymer (5, 3, 4). When samples of poly(butadiene-co-styrene), prepared a t several different temperatures but alike in other respects, were studied in this way, the extent of branching was found to decrease with decreasing polymerization temperature. In other words, as the temperature of polymerization was lowered, more of the fractions were found to have a k ' value equal to the base value 4' common to unbranched species.I n fact, when the polymerization temperature was 15"C., all of the fractions had the same k'. I t was noted, however, that this constant value of k' a t 15°C.was a little higher than the base value a t 30°C. The increase was slight but too definite to be ignored. Accordingly, the investigation has been extended to still lower temperatures. This paper reports evidence, so obtained, that the effect is real and significant. E X P E R I M E N T A LThe apparatus and procedure have already been described in detail (4). With the exception of the polymers, the materials (solvents, etc.) for this investigation were the same as those used earlier (4). The two polymer samples were prepared in much the same way as before, the only difference being the temperature of polymerization-5°C. and -18°C.-and the slight modifications (6) in the recipe that were required t o make the intrinsic vistex, the percentage Afanltscript received Afarch 6 , 1953.