The spiro-orthoester, cis-2,3-tetramethylene-1,4,6-trioxaspiro[4,4]nonane (cis-TTN) (1), underwent rapid cationic photopolymerization when exposed to UV light using diphenyliodonium salts as a photoinitiator. The polymer, poly[(trans-OCB)(x ')-(cis-OCB)(x '')-(CHO)(y) thus formed consisted of poly(trans-2-oxycyclohexyl butanoate) (trans-OCB)(x') (II), poly(cis-2-oxycyclohexyl butanoate) (cis-OCB)(x '') (III), and poly-(1,2-cyclohexene oxide) (CHO)(y) segments, and no expected pure poly(ether-ester), that is, poly(2-oxycyclohexyl butanoate), was isolated. The structure of the polymer was identified, and the mechanism of the reaction was deduced. The polymer thus formed exhibited expansion in volume during cationic photopolymerization when compared to that obtained by conventional cationic polymerization using a Lewis acid (e.g., BF3OEt2, CH3OSO2CF3, or SnCl4) as an initiator, which demonstrated volume shrinkage during polymerization. The volume expansion of the polymer during polymerization was due to (1) the lower content of the higher density (CHO), segment in the polymer chain and, more importantly, (2) the higher and optimal mole ratio of (trans-OCB)(x ') and (cis-OCB)(x '') segments that led the polymer in a more disordered, less dense, and higher volumetric state. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3680-3690, 2009