Tertiary Mercaptans as Modifiers in Gr-S Polymerizations

Abstract

Tertiary octyl, decyl, dodecyl, and hexadecyl mercaptans were investigated as modifiers in GR-S polymerizations. -The rate of disappearance of these modifiers follow-ed a first-order rate equation during the first half of the polymerization, in accordance with the theory of chain-transfer reactions, During the latter part of the reaction, an increased rate of disappearance generally occurred. A mechanism is suggested.The rate of disappearance increased rvith decreasing chain-length of the mercaptan. The intrinsic viscosities and molecular weight distributions of the polymers obtained at different conversions rvere related to the activity of each mercaptan as a modifier. Values for the regulating index of each mercaptan were calculaled.The slow rate of disappearance of tertiary hexadecyl mercaptan leads to the formation of polymer that is relatively homogeneous in molecular weight, and this mercaptan is therefore a desirable modifier. Hor.ever, after 50/s polymerization, the regulator action shows a marked acceleration, leading to the formation of progressively lower molecular weight polymer and resulting in a spread in the molecular weight distribution.

Compositional Heterogeneity of Copolymers

“…The techniques used in this investigation were the same as previously described (8), except that the polymerization was carried out in 1-ounce bottles instead of the usual 4-ounce bottles. In this way, the complete contents of the bottle, amounting to 22.5 grams, were used for analytical determinations, thus avoiding sampling difficulties which are encountered especially at low conversions.…”

“…By using a mercaptan of low reactivity, the change in molecular weight with conversion may be kept at a minimum, leading to the formation of a polymer having a higher degree of homogeneity with regard to chain length than usually encountered. The higher mercaptans, such as tertiary hexadecyl mercaptan, have been found suitable for this purpose (8). This mercaptan is also known to be relatively free of side reactions, due to its low reactivity, and hence may be assumed to behave purely as a chain transfer agent-i.e., in such a way that one molecule of regulator will be consumed for every molecule of polymer formed.…”