Vinyl isobutyl ether, stereoregular polymerization

The cationic polymerization of vinyl isobutyl ether at —40°C produces stereoregular polymers (structure 5.21). The carbocations of vinyl alkyl ethers are stabilized by the delocalization of p valence electrons in the oxygen atom, and thus these monomers are readily polymerized by cationic initiators. Poly(vinyl isobutyl ether) has a low Tg because of the steric hindrance offered by the isobutyl group. It is used as an adhesive and an impregnating resin. 

The observation in 1949 (4) that isobutyl vinyl ether (IBVE) can be polymerized with stereoregularity ushered in the stereochemical study of polymers, eventually leading to the development of stereoregular polypropylene. In fact, vinyl ethers were key monomers in the early polymer Hterature. Eor example, ethyl vinyl ether (EVE) was first polymerized in the presence of iodine in 1878 and the overall polymerization was systematically studied during the 1920s (5). There has been much academic interest in living cationic polymerization of vinyl ethers and in the unusual compatibiUty of poly(MVE) with polystyrene. 

Ionic polymerizations yield highly stereoregular polymers when control is exercised over monomer placement. The earliest stereospecific vinyl polymerizations were observed in preparation of poly isobutyl vinyl ether) with a BFa-ether complex catalyst at -70 °C. An isotactic polymer formed. °The same catalyst was employed later to yield other stereospecific poly(vinyl ether)s. " The amount of steric placement increases with a decrease in the reaction temperature, and, conversely, decreases with an increase in the temperature. ... 

The open transition state—-hybridized carbocations— has low stereoselectivity, thus mostly atactic polymers are formed (94). At the same time, reportedly the very first synthetic stereoregular polymer was an isotactic poly(isobutyl vinyl ether) prepared by carbocationic polymerization initiated with BF3/0(C2115)2 at -78°C (95). Other examples of stereoregular crystalline polymers synthesized by homogeneous and heterogeneous carbocationic polymerization are polymers of vinyl ether derivatives, and 2,5-dimethyl styrene or a-methylstyrene (95-100). Stereoregular vinyl ether polymers have also been synthesized using... 

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