THE REACTIVITY-SELECTIVITY PRINCIPLE IN POLYMERIZATION - THE CASE OF POLYMERIZATION OF EPSILON-CAPROLACTONE

Anionic and pseudoanionic polymerizations of epsilon-caprolactone (CL) have been compared in terms of the principle relating reactivity with selectivity (reactivity-selectivity principle, RSP). RSP can be formulated as follows: there are two reagents X and Y; reagent X reacts faster with substrates A and B than reagent Y does (i.e. k(xa) > k(ya) and k(xb) > k(yb), where k(xa), k(ya), k(xb) and k(yb) are the respective rate constants); moreover, reactions with A are faster than reactions with B (i.e. k(xa) > k(xb) and k(ya) > k(yb)); finally, if RSP is operative the inequality k(ya)/k(yb) > k(xa)/k(xb) holds. Comparison of the anionic and pseudoanionic polymerizations of CL reveals that this system conforms to the RSP. Propagation on the ...-O-Na+ ion pairs proceeds much faster than on the ...-OAlR2 covalent species (k(p) +/- = 1,7 l/mol.s vs k(p)c = 3,0.10(-2) l/mol.s, THF solvent, 20-degrees-C). Propagation is accompanied by transesterification and cyclization. Formation of the cyclic dimer (with the rate constant k(b)(2)) was taken as a measure of an extent of these "side" reactions. Following selectivity coefficients beta = k(p)/k(b)(2) have been measured (in l/mol): 1,55.10(3) (...-O-Na+), 4,6.10(4) (...-OAl(C2H5)2), and 7,7.10(4) (...-OAl(i-C4H9)2). Thus, k(p)c/k(b)c(2) > k(p) +/-/k(b) +/- (2). Therefore formation of the linear, high molar mass polymer is more pronounced in the system with the less reactive active centers.