Synthesis and Characterization of Wormlike Three-Arm Poly(n-hexyl isocyanate) Star Polymers

We have previously reported the use of titanium alkoxide complexes as initiators for the living polymerization of isocyanates. In this paper, we present the synthesis of a trimetallic initiator, 2, comprised of three titanium alkoxides connected to a central organic core and the use of this catalyst to form three-arm star polymers which contain rigid polyisocyanate arms. Kinetic and molecular weight experiments confirm that the trimetallic complex 2, like its monometallic analogue, 6, initiates the living polymerization of n -hexyl isocyanate and that the three-arm stars obtained from 2 are completely symmetrical. The number-average molecular weight of polymer obtained from 2 increases linearly with the monomer-to-catalyst ratio used, which indicates a lack of chain transfer during polymerization. A comparison of the rates of polymerization of n -hexyl isocyanate by 2 and 6 at fixed initial monomer and titanium concentrations ( k 2 = (3.99 ± 0.2) × 10 -3 M min -1, k 6 = (3.84 ± 0.1) × 10 -3 M min -1 ) demonstrates that all three titanium centers in 2 are active and propagate independently of one another. The solution properties of these new three-arm star polymers were examined through tandem GPC/light scattering and solution viscometry experiments. Light scattering shows that there is a 55% reduction in the mean-square radius of gyration ( g = 〈 R G 2 〉 star /〈 R G 2 〉 linear = 0.45 ± 0.05) of the three-arm star polyisocyanate from that of a linear polyisocyanate of the same molecular weight. The experimental value of g approaches the rod limit of g that indicates that the arms of the three-arm star can be construed as stiff, wormlike chains. Additionally, there is a 28% reduction in the intrinsic viscosity of three-arm polyisocyanate star from that of its linear equivalent. Finally, a 30% w/w solution of three-arm star polyisocyanate displays a nematic, lyotropic liquid crystalline mesophase.