Synthesis and characterization of new macromolecular architecture using living titanium(IV) initiators

Bimetallic initiators comprised of two titanium alkoxides connected by flexible links (such as trans- and cis-1,4-cyclohexane) were synthesized and used for the living polymerization of n-hexyl isocyanate, giving polymers with discrete functionalities in the center of the backbone. A comparison of the rates of polymerization at fixed concentrations of the bimetallic complex versus the analogous monometallic catalyst demonstrates that both titanium centers are active and propagate independently of one another. The effect of the flexible cyclohexyl group on solution properties of rodlike poly(n-hexyl isocyanate) (PHIC) was examined through gel permeation chromatography, static light-scattering, tandem gel permeation chromatography (GPC)/light-scattering, and solution viscometry. These experiments have shown that there is a 10-15% reduction in the chain dimensions of a "once-broken" PHIC versus the linear polymer. Additionally, a telechelic initiator containing a poly(dimethylsiloxane) (PDMS) segment between two titanium alkoxide endgroups was synthesized and used to produce PHIC-PDMS-PHIC triblock copolymers.; To prepare three-arm star polymers which contain rigid polyisocyanate arms, a trimetallic initiator comprised of three titanium alkoxides connected to a central cyclohexane core was synthesized. Kinetic and molecular weight experiments have confirmed that the trimetallic complex, like its monometallic analogue, initiates the living polymerization of n-hexyl isocyanate, and that the resultant three-arm stars are completely symmetrical. Tandem GPC/light scattering and solution viscometry studies have shown that there is a 50% reduction in the mean-square radius of gyration (g = {dollar}rmlangle RGsp2ranglesb{lcub}star{rcub}/langle RGsp2ranglesb{lcub}linear{rcub}=0.5pm0.05){dollar} of the three-arm star polyisocyanate from that of a linear polyisocyanate of the same molecular weight, while there is a 28% reduction in the intrinsic viscosity of the star polymer from that of its linear equivalent. Additionally, a 30% w/w solution of three-arm star polyisocyanate displayed a nematic, lyotropic liquid crystalline mesophase.