Geometrical structures in solution and solid phase of poly(propargyl ester)s prepared by using a [Rh(norbornadiene)Cl]2-cocatalyst

Propargyl esters, HCCCH₂OCOC_nH_2n+1 (1: n = 3, 2: n = 5, 3: n = 9, and 4: n = 13), were polymerized using a Rh(nbd)Cl]₂ catalyst and cocatalyst in an appropriate solvent at 0 °C and 40 °C. Number average molecular weights, M_n of the resulting polymers, poly(1)–poly(4), were 8000–54,000, and its dispersities (M_w/M_n)s and the yields were estimated to be 1.6–3.6 and ca. ～99%, respectively. These polymers were soluble in CHCl₃, CH₂Cl₂, and THF, and insoluble in CH₃OH and DMF. The amines, e.g., 2,6-dimethylpyridine and triethylamine worked well as the cocatalysts for the polymerization to give the yields, 93% and 88%, respectively. Further N,N-diethylaniline gave the maximum in cis%, 74%, although pyridine, aniline, pyrrole or imidazole did not work effectively. Remarkable line broadening phenomenon, LBP observed in the ¹H NMR spectra of poly(1) and poly(2) prepared at 0 °C or 40 °C, and poly(3) prepared at 40 °C was correlated with the polymerization temperature and the radical concentration generated in the polymer. The LBP was interpreted by the dipole–dipole interaction between the protons in the polymer and unpaired electrons due to carbon radicals generated by the rotational scission of the cis CC bonds during the polymerization. Their UV–vis spectral shape of poly(1)–poly(4) depended on the polymerization temperature and/or the alkyl chain length in the polymer. The XRD powder patterns of poly(3) prepared at 0 °C, and poly(4)s prepared at 0 °C and 40 °C showed layer crystal structures with a slightly bent herringbone confirmed by molecular mechanics calculation. The λ_max values of poly(4) prepared at 40 °C in methanol showed 317 nm and 340 nm in the solution and on the alumina, respectively.