The unusually large film thickness at which confinement effects manifest themselves in surface fluctuations of unentangled four-arm star polymers has been defined using film thicknesses from 10R g to 107R g . For 15k four-arm star polystyrene (SPS), confinement appears at a thickness between 112 nm (40R g ) and 72 nm (26R g ), which is remarkably larger than the thicknesses at which confinement appears for unentangled 6k linear (<15 nm, <7R g ) and 6k and 14k cyclic (24 and 22 nm, respectively) polystyrenes. Data for 15k star films can be rationalized using a two-layer model with a 17 nm (6R g ) thick highly viscous layer at the substrate, which is significantly thicker than the 1R g thick "irreversibly adsorbed" layer. For a 29 nm (10R g ) thick film, more striking confinement occurs due to the combined influence of both interfaces. These results underscore the extraordinary role long-chain branching plays in dictating surface fluctuations of thin films.

An efficient method for synthesis of welldefined, 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, ω-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.