<p>Incorporation of backbone acetal groups is a potent strategy to create polymers that are cleavable or degradable under acidic conditions. We report here an in-depth study on the ring-closing-opening copolymerization of <i>o</i>-phthalaldehyde (OPA) and epoxide using Lewis pair type two-component organocatalysts for producing acetal-functionalized polyether and polyurethane. Notably, triethylborane as the Lewis acid, in comparison with tri(<i>n</i>-butyl)borane, more effectively enhances the polymerization activity by mitigating borane-induced reduction of the aldehyde group into extra initiating (borinic ester) species. Density functional theory (DFT) calculations present comparable energy barriers of OPA-epoxide cross-propagation and epoxide self-propagation, which is consistent with the experimental finding that an alternating-rich copolymer comprising mostly OPA-epoxide units but also epoxide-epoxide linkages is produced. In particular, when epoxide is added in a large excess, the product becomes a polyether containing acetal functionalities in the central part of the backbone and thus convertible into polyurethane with refined acid degradability.</p>