Towards highly functionalized and semi-rigid polyzwitterions, 2. Poly(zwitterionic isocyanides): synthesis and specific properties

Abstract

Selective quaternization of the tertiary amino site of N,N-dimethyl-N-(3-isocyanopropyl)amine by 1,3-propanesultone or 2,2-dicyanoketene-ethylene acetal quantitatively yields the zwitterionic isocyanides of the ammoniopropanesulfonate (SI, —N+(CH3)2—(CH2)3—SO3–) or ammonioethoxydicyanoethenolate (EI, —N+(CH3)2—(CH2)2—O—CO—C–(CN)2) type, respectively. Their polymerization initiated with NiCl2·6 H2O in methanol (SI) or in N,N-dimethylformamide (EI) solution leads with high yields (>70%) to the corresponding poly(isocyanides) in a rather low molecular weight range (DPw ˜ 80–600). PSI is soluble in a few organic protic solvents such as glycol or m-cresol and displays typical UCST behavior in pure water (Tc ≈ 35°C) with “salting in” effects in the presence of LiClO4. PEI is soluble in a few dipolar aprotic solvents such as DMF, NMP or DMSO with typical LCST behavior and “salting in” effects in the presence of LiCl; secondly, in fairly concentrated KSCN aqueous solutions (>1 M) with a typical UCST behavior strongly dependent on salt concentration (dT/d[KSCN] ≈ 252°C·L·mol–1). Analysis of the [η] – Mw data for PSI (0.05 M LiClO4 in H2O) and PEI (0.05 M LiCl in DMF) according to the Bohdanecky's approximation of the Yamakawa-Fujii theory for wormlike chains allows to estimate their persistence length to be about 60 A for a hydrodynamic diameter of about 15 A. For polymer volume fractions Φp > 0.30, the PSI/glycol gels display local nematic order as revealed by polarizing optical microscopy and small angle X-ray scattering (Bragg spacing d ∝ Φp–0.45). The two poly(isocyanides) are non-crystalline but slightly birefringent polymers showing only local order (one SAXS peak). PSI dissolves stoichiometric amounts of NaB(C6H5)4, LiClO4, NaI and NaSCN to yield amorphous and birefringent blends which display optimized long range order in the latter case: a hexagonal bidimentional lattice of zwitterionic cylinders spreading over a coherence length of about 300 A.