Spiroorthoesters (SOEs), cis-2,3-tetramethylene-1,4,6-trioxaspiro[4,5]decane (I) and cis-2,3-tetramethylene-1,4,6-trioxaspiro[4,6]undecane (II), with different cyclic ether ring sizes were synthesized, and their stereostructure and steric energy were determined. With steric-hindrance-sensitized 9-phenyl-9,10-dihydro-anthracen-10-ylium cation as an initiator, I and II underwent regiospecific polymerization to yield trans form of stereoregular poly(ether esters)poly(trans-2-oxycyclohexyl pentanoate) (-[trans-2-OCHP](n)-) (III) and poly(trans-2-oxycyclohexyl hexanoate) (-[trans-2-OCHH](n)-) (V), respectively. With SnCl(4) as another initiator, I and II underwent regiospecific polymerization through different mechanisms to afford cis form poly(cis-2-oxycyclohexyl pentanoate) (-[cis-2-OCHP](n)-) (IV) and trans form (-[trans-2-OCHH](n)-) (VI) stereoregular poly(ether esters). The polymerization mechanisms of SOEs proceeded in the regiospecific manner were determined by the relationship among the sterostructures of SOEs and its subsequently formed polymers, the steric energy of monomers, and the free energy difference in the transition state of reaction. Owing to the conversion of cis substitution at C-2 and C-3 in I or II to the trans form during polymerization, polymers III, V, and VI exhibited a higher volume of expansion during polymerization than IV, which showed high volume shrinkage. Group contributions of divalent trans- and cis-1.2-cyclohexyl groups were derived and confirmed by measuring the densities of the corresponding stereoregular polymers. (c) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 720-728, 2012