Polyisoprene Asymmetric stars

FIGURE 1.13 Concentration dependence of the zero-shear viscosity of 1,4-polyisoprene linear (N250K, A/ = 256,900g/mol), three-arm symmetric star (SIOIK, aim = 101,000 g/mol), and three-arm asymmetric star (A2B73K, = 73,000g/mol,... 

With the exception of a few commercial polymers such as polyisobutylene, polybutadiene and styrene-butadiene block copolymers, living polymers are prepared in small quantities under stringent conditions. Larger amounts can only be prepared by repeating the synthesis many times, and this is a costly and time-consuming process. In the case of hydrogenated polybutadiene, to prepare samples that resemble polyethylene, the need for a secondary reaction step renders the preparation even more costly. This has so far limited the extent to which it has been possible to use these materials to test models. Cell et al. [ 18] prepared asymmetric stars with structures similar to ethylene-propylene copolymers by hydrogenation of star-branched polyisoprene. The reactions to produce these materials took up to three weeks, and... 

Asymmetric polyisoprene (PI) three-arm stars with variable length of the third arm were synthesized using the same method [8]. The reaction of the living PI chains with excess methyltrichlorosilane was performed at 5 °C. This low temperature was selected in an effort to minimize the coupled byproduct. Nevertheless the reduced steric hindrance of the PILi chain end in association with the low molecular weight of the polydienes used (Mn=5500 and 1100) led to the formation of an appreciable amount of the coupled byproduct, which was later separated by fractionation, with the excess of the last coupled arm, using a sol-vent-precipitant system. Pure products were finally isolated as evidenced by the molecular characterization techniques used (SEC, MO, SLS). 

Hadjichristidis group later worked on 3-miktoarm star terpolymers of polyisoprene, polystyrene, and poly(methyl methacrylate), where they also investigated an asymmetric system. They found a cylindrical morphology with a nonconstant mean curvature i.e., the cylinders were deformed to rhombohedral structures [222,223], In symmetric systems they found threefold morphologies [224], similar to the results of Hashimoto s group [221] (Fig. 16a,b). 

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