The formation of macrocycles during ring-opening polymerization

THE FORMATION OF MACROCYCLES DURING RING-OPENING POLYMERIZATION 

Cyclic fractions are formed either from monomers by some variant of propagation or, and this is the main source, from the growing chain. Examples of the former could be that already cited [314] or the observation of Weissermel and Nolken [334] that chloromethyloxirane reacts with triethyaluminium at low temperature to give a product which is a mixture of polymer and cyclic dimer 

Oligomers and macrocycles are generated from the growing chain by reaction of the active centre with a suitable place on its own chain, as shown in the following scheme in which X represents the heteroatoms O, N, P. ..). 

The occurrence of these reactions is always determined by thermodynamic factors. Oxirane has a large ring strain. Its polymerization around room temperature exhibits AGp 0. For 1,4-dioxane under the same conditions, AGp 0. In other words, polyoxirane will split off 1,4-dioxane because the Gibbs energy of its depolymerization is negative. Actually the polymer should depolymerize completely. That this is not the case, is caused by kinetic factors. Termination of depolymerization need not coincide with termination of polymerization. 

The ratio of back- and end-biting reactions depends on the reaction conditions, and may differ considerably in polymerizations of various monomers. In cationic polymerization, methyloxirane produces mainly a mixture of cyclic tetramers [335] and chloromethyloxirane yields both dimers and tetramers [336]. Under similar conditions, 1,3-dioxolane or 1,3-dioxepane yield a number of cyclic derivatives [337], the distribution of cyclics being in agreement with the Jacobson-Stockmayer theory [338], 

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