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Synthesis and characterization of hetero-armed star polymers

Hetero armed star molecules AB2, A2B, A2B2 can be made with low dispersity and controlled architecture by linking the corresponding living polymers formed from an organolithium initiator by means of an appropriate silicon chloride. The success of the syntheses has been confirmed by careful characterization. The polydiene arms in such stars can be reduced to the corresponding polyolefins by means of H2 in conjunction with palladium. The polyethylene-alt-propylene chains are soluble in alkanes but the polyethylene arms are insoluble and crystallize. Micelles are formed with a core of crystalline polyethylene and a brush formed by the polyethylene-alt-propylene arms emanating from both surfaces. The cores are planar discs and stacking of them leads to the formation of large paracrystals.

The dimensions of the micelle cores, coronas and inter-disc spacing have all been determined by neutron scattering. A thermodynamic description had been developed. The dependence of the dimensions upon star architecture and molecular weight has been established. Neutron scattering has shown, in model studies with simulated diesel fuel appropriately labelled with deuterium, that the wax crystallizes on the surfaces of the discs. The morphology of the polystyrene/polyisoprene stars resembles that of the corresponding AB linear diblock copolymers but there are some significant differences. Contributions to the rheological behaviour in the melt come from molecular relaxations, ordered mesophases and large scale domains. The modus operandi by which the micelles act as anti waxing agents has been established for the first time, showing that previous explanations were erroneous. This offers clues for the design of improved diesel oil additives. Preliminary tests indicate that the differences in the efficacy of the stars and the related linear diblocks are smaller than anticipated.

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