The effect of depropagation on copolymer composition. I. General theory for one depropagating monomer

Abstract

Listen

Translate article

AbstractThe following mechanism is postulated for the copolymerization of α‐methylstyrene (or similar 1,1‐disubstituted ethylene), M2, at temperatures near its ceiling temperature but far below the ceiling temperature of the comonomer, M1. Either M1 or M2 may combine with a polymer chain ending in either m or m. When either monomer adds to a chain ending in m, the new active chain is stable to depropagation, as is a new active chain formed by either monomer adding to a chain ending in m1m. However, whenever either monomer adds to a chain ending in m1(m2), where n ≥ 2, the newly formed active chain will have a finite tendency to depropagate and to “unzip” until the terminal radical is m1(m2), which itself is stable to depropagation. However, a sequence of m1(m2) becomes trapped, or stabilized, if it reacts with a molecule of M1 and then with a molecule of either M1 or M2 before the first unit of monomer M1 can split off. A steady‐state kinetic analysis of this mechanism leads to a copolymer composition equation which, although unwieldy, allows the following general predications. (1) Temperature will have a marked effect on the shape of the copolymer composition curve in the region of the ceiling temperature of M2. (2) Dilution of the monomers by an inert solvent will have a marked effect on the shape of the copolymer composition curve in the region of the ceiling temperature of M2. (3) The limiting mole fraction of M2 in a copolymer prepared at high temperature will be two‐thirds. Two other mechanisms of copolymerization involving the depropagation reaction of one monomer are analyzed to obtain copolymer composition equations. In one of these, it is assumed that M1 will never split off an active chain. This results in the same qualitative predictions as above but slightly different quantitative predictions. In the other case studied, it is assumed that M1 will never split off an active chain and that when a chain ends in m1(m2), it can unzip all the way to m1m. This leads to the same qualitative predictions as before but the limiting mole fraction of M2 in a copolymer prepared at high temperature is one‐half rather than two‐thirds.