Shape-directed assembly of a "macromolecular barb" into nanofibers: Stereospecific cyclopolymeriztion of isopropylidene diallylmalonate

(Figure Presented) Cyclopolymerization of isopropylidene diallylmalonate (1) in CH2Cl2 using as the initiator a crystallographically defined α-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 °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 CH₂Cl ₂. 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.