Synthesis and Characterization of Well-Defined, Tadpole-Shaped Polystyrene with a Single Atom Junction Point

An efficient method for synthesis of well-defined, well-characterized, tadpole-shaped polystyrene with a single atom junction point that is optimal for the study of dynamics has been developed using anionic polymerization, silicon chloride linking chemistry, and metathesis ring closure. The difunctional macromolecular linking agent, methyldichlorosilylpolystyrene, was formed by reacting sec-butyllithium-initiated poly(styryl)lithium with excess (30×) methyltrichlorosilane to eliminate formation of linear dimer and three-arm star polystyrene. The asymmetric, three-arm, star precursor was formed by linking excess α-4-pentenylpoly(styryl)lithium (α-PSLi) with the macromolecular linking agent, and the excess α-PSLi functionalized with ethylene oxide before termination with methanol to facilitate chromatographic separation. Cyclization of the three-arm, star precursor to form tadpole-shaped polystyrene was effected in methylene chloride at high dilution using the Grubbs first generation catalyst, bis(tricyclohexylphosphine)benzylidene ruthenium(IV) chloride. The tadpole product was uniquely characterized by MALDI-MS using peaks that appeared characteristically 28 m/z units lower than those of the corresponding asymmetric, three-arm, star precursor, which corresponds to the loss of an ethylene unit. MD simulations find a smaller hydrodynamic volume for the tadpole-shaped PS as compared to the three-arm star precursor, in quantitative agreement with GPC results. Incorporating one cycle in the molecule, while leaving one chain end, leads to an increase in T_g of only 2.7 °C, much smaller than the increase of 13.6 °C seen when going from the linear chain to cyclic analog with no ends at all. The results are consistent with self-plasticization by free chain ends.