Application of hyperbranched block copolymers as templates for the generation of nanoporous organosilicates

A general route to organic-inorganic hybrids with nanophase morphologies ha s been elaborated with the objective of ultimately generating nanoporosity in organosilicates. Hyperbranched block copolymers prepared by either the s equential or concurrent polymerization bf an ABC monomer (gamma-(epsilon -c aprolactone) 2-bromo-2-dimethylpropionate) with a BCD monomer (2-hydroxyeth yl methacrylate) were used as the macromolecular templates. The two monomer s, each polymerizing by different chemistries, for example ring-opening pol ymerization and atom transfer radical polymerization, bear initiating centr es that are targeted for the functionality located on the accompanying mono mer. Consequentially, a branched polymer is obtained which avoids the tradi tional multistep procedures. The branching density was altered simply by th e addition of the appropriate AB (epsilon -caprolactone) and/or CD (methyl methacrylate) comonomers. These polymers were readily soluble initially in the organosilicate prepolymer (methyl silsesquioxane), however, upon the on set of crosslinking, both the solubility parameters and molecular weight of the organosilicate (polymethylsilsesquioxane) change, causing the hyperbra nched polymer to phase separate by a nucleation and growth process. The org anic polymer was selectively removed by thermolysis, producing a nanoporous inorganic structure. The size and shape of the pores are identical to thos e of the initial hybrid morphology. A significant reduction in the dielectr ic constant was achieved by simply replacing a portion of the glass matrix with air, which has a dielectric constant of 1.0.