Investigation of polyacetaldehyde structure by high resolution nuclear magnetic resonance

We examined the configurations of cationically produced amorphous polyacetaldehyde by high resolution NMR spectroscopy at high temperature in such solvents as dimethylformamide and aniline. Our results show that the polymer backbone is composed of heterotactic and isotactic triad sequences, the former predominating over the latter. Using a 60 Mcycle instrument with or without spin decoupling, we were unable to detect any syndiotactic triad sequences. In addition, the 60 Mcycle NMR spectrum of poly‐α‐deuteroacetaldehyde shows the presence of only the same two stereochemical triads as above. However, examination of polyacetaldehyde with a 100‐Mcycle instrument indicated that a small shoulder exists on the low field side of the heterotactic peak which is probably caused by a small syndiotactic triad content. We used these findings to propose a mechanism for the cationic polymerization of acetaldehyde. We suggest that acetaldehyde may exist in a dimeric form under polymerization conditions leading to a preferred incorporation of mesodimeric units into the propagating chain. Model compound studies involving α,α′‐dimethoxyethyl ether are consistent with this postulate.