Core-first star synthesis precursors

The synthesis of core-first stars from multi-ionic precursors (XXVIII to XXXVII) may be complicated by the limited solubility of the multiple growing chains, giving rise to insoluble aggregates in organic solvents [38]. When using multifunctional precursors (XXXVIII to XLVI) in free-radical polymerization, the conditions must be identified that will avoid any of the growing arms undergoing irreversible terminations, that may result in a loss of control of the star functionality [46-48]. 

Taton D, Saule M, Logan J, Duran R, Hou S, Chaikof EL, Gnanou Y (2003) Polymerization of ethylene oxide with a calixarene-based precursor synthesis of eight-arm poly(ethylene oxide) stars by the core-first methodology. J Polym Sci A Polym Chem 41 1669-1676... 

Stars with high arm numbers are commonly prepared by the arm-first method. This procedure involves the synthesis of living precursor arms which are then used to initiate the polymerization of a small amount of a difunctional monomer, i.e., for linking. The difunctional monomer produces a crosslinked microgel (nodule), the core for the arms. The number of arms is a complex function of reaction variables. The arm-first method has been widely used in anionic [3-6,32-34], cationic [35-40], and group transfer polymerizations [41] to prepare star polymers having varying arm numbers and compositions. 

The anionic arm-first methods can also be applied to the synthesis of star block copolymers [59]. The procedure is identical except that living diblock copolymers (arising from sequential copolymerization of two appropriate monomers, added in the order of increasing nucleophilicity) are used as living precursor chains. The active sites subsequently initiate the polymerization of a small amount of a bis-unsaturated monomer (DVB in most cases) to generate the cores. If polystyrene and polyisoprene (or polybutadiene) are selected, the resulting star block copol)miers behave as thermoplastic elastomers because of their different glass transition temperatures. 

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