Branching in high molecular weight polyhydroxyethers based on bisphenol A

Abstract

Abstract The polyaddition reaction of bisphenol A with the diglycidyl ether of bisphenol A is treated statistically on the basis of cascade theory to describe the branching process. The chain lengthening step is based on the reaction of the epoxide functional group with the phenolic hydroxyl group and leads to the formation of a 1,3‐diphenoxy‐2‐propanol link. The peculiar feature of this polyaddition reaction is the possibility of further addition of an epoxide to a secondary hydroxyl group to form branched molecules. This branching reaction does not, however, lead to a loss of hydroxyl groups since a new secondary hydroxyl group is created when an epoxide functional group reacts with a secondary hydroxyl group. Closed analytic expressions are derived for the weight‐average ( M w ) and number‐average ( M n ) molecular weights in terms of the mole ratio R = m b / m a and the extents of reaction of the functional groups α(phenolic hydroxyl), β(epoxide), and p (branching), i.e. the probability p that an epoxide group has reacted with a secondary hydroxyl group, where m b and m a are the mole fractions of the diglycidyl ether of bisphenol A and of bisphenol A, respectively. Consideration of a simple kinetic mechanism shows that α,β and ρ are related to the ratio b = k 2 / k 1 , where k 1 and k 2 are the rate constants of the chain lengthening step and the chain branching step, respectively. The equations derived for M w and M n as a function of a α allow the determination of the branching probability p (or the kinetic rate constant ratio b ). An equation for the gel point ( M w → ∞) is given which states a relationship between α c and p c where the subcript c refers to the critical point of gelation. Thus location of the gel point and measurement of α c , either directly or by extrapolation on samples taken prior to gelation, enables p to be measured.