Free-radical polymerization behavior of 2-triethoxymethyl-1,3-butadiene, a potential precursor for poly[2-(ethoxycarbonyl)-1,3-butadiene], whose carboxyl group was masked as an ortho ester, was investigated. When polymerization was carried out in bulk at 60°C for 50 h with using 1 mol % of 2,2′-azobisisobutyronitrile as an initiator, viscous polymer was obtained in 36% yield without forming gel. The resulting polymer containing an ortho ester function was soluble in common organic solvents, including methanol. It can readily be converted to carboxylic ethyl ester by adding small amount of dilute hydrochloric acid. The microstructure of the resulting polymer with ethyl ester group was predominantly a 1,4-E, cis-1,4 structure about the main chain.
IntroductionThere have been many reports on the free-radical polymerization of vinyl monomers. 1 Although this polymerization mechanism is applicable to a variety of monomers; however, polymerization of 1,3-butadiene derivatives containing functional groups is still quite limited. This was presumably due to the fact that functional groups directly attached to the polymerizable 1,3-butadienyl skeleton often affect the stability and the polymerizability of the dienes. As a matter of fact, nonfunctional alkyl-, phenyl-, and halogen-substituted butadienes were reported to give high polymers by freeradical polymerization. 2 On the other hand, some functional groups, especially those substituted at the 2 position of the butadienyl frame, decrease the stability of the monomer. To our knowledge, 1,3-butadiene derivatives having a carboxyl group and its ester at the 2 position have not been isolated whereas its isomer, ethyl 2,4-pentadienoate, was synthesized and polymerized under ordinary reaction condition. 3 2-Cyano-1,3-butadiene is reported to dimerize upon heating under polymerization conditions. 4 These reactions are presumably due to the high tendency of the reactants to form Diels-Alder dimers, which is caused by the electronwithdrawing character of the functional group. One of the methods to overcome this difficulty would be the transformation of such functional groups to the other group, which is inert during the polymerization. We chose here an ortho ester function, the trialkoxymethyl group, as a suitable masking group of carboxyl group. This protecting group is known to be stable under basic and neutral conditions whereas it can readily be hydrolyzed to carboxylic ester under mild acidic condition. 5 There have been a few examples where the ortho ester was applied for polymer syntheses. Padis and Hall have reported the free-radical polymerization of methacrylates containing bicyclic ortho ester group. 6 Ishizone et al. used the same protecting group for the anionic polymerization of styrene derivatives containing three hydroxyl groups in each of the monomer units. 7 The bicyclic ortho esters are reported to be more stable than acyclic ortho esters such as the triethoxymethyl group. 5 Because polydienes are potentially to be cross-linked under acidic conditions, the pr...
