The synthesis of a novel Spiro orthoester containing monomer, 1,4,6-trioxaspiro[4.4]-2-nonylmethyl acrylate, is presented. This monomer was polymerized via a free-radical system to yield the homopolymer and a series of copolymers with phosphorus-containing comonomers. Diethyl vinyl phosphonate, allyldiphenylphosphine oxide, and diethyl(methacryoyoxymethyl)phosphonate were used in various feed ratios to produce copolymers with different phosphorous concentrations containing crosslinkable Spiro orthoester side-chain units. The crosslinking of the polymers was performed cationically with ytterbium triflate, and in all cases, the expansion of the polymer was observed. Moreover, the incorporation of phosphorus into the copolymers increased the limiting oxygen indices, regardless of the percentage of phosphorus used. (c) 2006 Wiley Periodicals, Inc.