AbstractWe have investigated the free radical polymerization of phenyl‐substituted 2‐methylene‐1,3‐dioxanes. It was shown that there is a basic tendency of such ketenacetals to undergo exclusively a vinyl polymerization forming high‐molecular polyacetals without any detectable ring opening. The polymerization with the desirable ring opening reaction cannot be controlled by factors like resonance stabilized chain ends or steric hindrance in the growing step.From densitometric measurements we got new information about the shrinkage behaviour. It was demonstrated that bulky substituents attached to the 2‐methylene‐1,3‐dioxane system result in a significant low shrinkage in homopolymerization and the shrinkage of common comonomers can be decreased by c