Ambident nature of the polydimethylketene anion

By means of organometallic compounds, dimethylketene can be polymerized to products with varying structure. The influence of solvent and the cation of the initiator on the composition of the polymer were investigated by means of IR spectra. The variation of composition can be explained by the ambident nature of the polydimethylketene anion. Polyacetal structure formed by O‐acylation of the enolate anion is preferred in dipolar aprotic solvents especially in dilute solutions and propagation through the free ion mechanism was assumed. Polyketone structure formed by C‐acylation of the enolate anion is preferred in non‐polar solvents with lithium or magnesium type catalysts and propagation through chelated ion pair intermediate was assumed. Lithium or magnesium catalysts prepared from bulkier alkyl halide increase polyketone content, which shows clearly the existence of the associated counter ions. Polyester structure formed by alternative O‐ and C‐acylation is the major product in non‐polar solvents and another chelated complex was suggested.