New isobutylene-rich elastomers bearing multiple pendant styrenic, acrylic, maleimidic, vinylic, and allylic functional groups have been prepared and examined in the context of peroxide-initiated crosslinking. Halide displacement from brominated poly(isobutylene-co-isoprene) (BIIR) by the requisite carboxylate nucleophiles in homogeneous toluene solutions provide the desired esters in quantitative yield without complications from dehydrohalogenation or premature crosslinking. Heating the resulting macromonomers with dicumyl peroxide to 160 C under solvent-free conditions gives thermoset derivatives, with reaction rates and yields depending markedly on functional group structure. In general, high cure extents can only be achieved using highly reactive pendant functional groups, owing to the competitive balance between crosslinking through C@C oligomerization, and degradation through b-scission of backbone macroradical intermediates. Independent control of crosslinking rates and cure extents is gained through the use of nitroxyl radical traps bearing acrylate functionality.
A series of new elastomeric ionomers
is prepared through halide
displacement from brominated poly(isobutylene-co-para-methylstyrene) (BIMS) by a range of N-substituted imidazoles.
Studies of the dynamics of solvent-borne and solvent-free reactions
demonstrate the influence of N-substituent structure on nucleophilicity,
with alkyl and allyl groups providing greater reactivity than vinyl
functionality. Ionomers bearing pendant styrenic, methacrylic, vinylic,
and allylic functionality are cross-linked by standard peroxide initiators
to give thermoset derivatives, with reaction rates and yields depending
markedly on functional group structure. The discovery of a benzylic N-allylimidazolium ionomer is particularly important, as
this material is amenable to solvent-free preparations, and provides
a unique combination of moderate cross-linking rates and high cure
extents, owing to a balance of radical addition and H-atom transfer
reactivity. Physical properties of the resulting thermosets are a
product of their hybrid ionic/covalent network, as the material response
to deformation is affected by a stable covalent network as well as
a labile network of aggregated ion pairs.
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