Bridged Polysilsesquioxanes:Sol-Gel Chemistry

The sol-gel chemistry of organotrialkoxysilanes involves the hydrolysis and condensation of the alkoxysilane groups to afford siloxane bond linked networks. Bridged polysilsesquioxanes are prepared from hexa-functional monomers. This high degree of functionality permits network polymers capable of gelation to form quickly and at concentrations lower than observed with either organotrialkoxysilanes or tetralkoxysilanes. Solution Si-29 NMR and mass spectrometry are used to examine the pre-gelation sol-gel chemistry to help understand how the networks are built and how the final pore structure is created.

We have observed, however, that the flexibility and length of the organic bridging group lead to variations in the sol-gel reaction pathways that, in a few cases, slow or prevent gelation under our standard sol-gel conditions (0.4 M monomer in alcohol or tetrahydrofuran, 6 equivalents water, 10.8 mol% acid catalyst). In particular, 2,3 and 4 atom long bridging groups led to the formation of cyclic monomers and oligomers with sufficient stability and reduced functionalitity to significantly retard gelation rates. While these systems are an exception to the rule for bridged polysilsesquioxane sol-gel, they may provide a way to construct ladder polymers where the bridging group acts as a tether to restrict the stereochemical possibilities for the condensation products to those leading to the prerequisite cis-syndiotactic geometry.

Loy, Douglas A.; Carpenter, Joseph P.; Alam, Todd M.; Shaltout, Raef; Dorhout, Peter K.; Greaves, John; Small, James H.; Shea, Kenneth J. Cyclization Phenomena in the Sol-Gel Polymerization of a,w-Bis(triethoxysilyl)alkanes and Incorporation of the Cyclic Structures into Network Silsesquioxane Polymers. J. Am. Chem. Soc. (1999), 121(23), 5413-5425.

Contact: Doug Loy