Shape-Directed Assembly of a Macromolecular Barb into Nanofibers: Stereospecific Cyclopolymerization of Isopropylidene Diallylmalonate

Cyclopolymerization of isopropylidene diallylmalonate (1) in CH2Cl2 using as the initiator a crystallographically defined alpha-diimine Pd(II) complex with a trans-azobenzene strap (trans-3) proceeds stereospecifically to give a cycloolefinic polymer that is rich in threo-disyndiotactic sequences (st(rich)-2; syndiotactic tetrad content = 60% at 10 to 0 degrees C). Polymer 2, when perfectly threo-disyndiotactic, adopts a barb-shaped geometry with its cyclic malonate pendants sticking out alternately up and down along the rigid main chain. Under appropriate conditions, st(rich)-2 regularly self-assembles, not only in the solid state but also in solution, into nanofibers that eventually give rise to physical gelation of halogenated solvents such as CH2Cl2. In sharp contrast, a reference polymer 2 that is rich in threo-diisotactic sequences (it(rich)-2). which likely adopts a helical geometry, has poor self-assembly capability in solution and neither forms nanofibers nor induces physical gelation of CH2Cl2.