Metal-free anionic polymerization of n-hexyl isocyanate catalyzed by phosphazene bases

Abstract

Metal-free anionic polymerization of n-hexyl isocyanate (HIC) catalyzed by phosphazene bases in THF at −98 °C under 10⁻⁶ Torr was attempted to obtain poly(n-hexyl isocyanate) (PHIC) peptide mimics with a high purity. tert-Butylimino-tris(dimethylamino)phosphorene (t-BuP₁), 1-tert-butyl-2,2,4,4,4-pentakis(dimethylamino)-2λ⁵,4λ⁵-catenadi(phosphazene) (t-BuP₂), and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)-phosphoranylidenamino]-2λ⁵,4λ⁵-catenadi(phosphazene) (t-BuP₄) with the characteristic orders of basicity and bulkiness (t-BuP₁ < t-BuP₂ < t-BuP₄) were used as catalysts to activate N-phenethyl-3-phenylpropanamide ((PhEt)₂OCNH, PEPPA). Only t-BuP₂ and t-BuP₄ generated the initiating amidate anions paired with each phosphazenium cation, which produced PHICs with MWs higher than the theoretical ones and characteristic ranges of dispersities. A moderately low dispersity was achieved by t-BuP₂. The anionic polymerization of HIC catalyzed by t-BuP₂ with molar feed ratios of the monomer to the initiator ([HIC]₀/[PEPPA]₀) within the range 207–801 produced PHICs with high yields (96.9–98.7%) and high MWs (M_n = 44.1–174 kDa, Đ = 1.37–1.64) that were linearly controlled by the feed ratios, even though no polymers were obtained by t-BuP₁, and the products whose production was catalyzed by t-BuP₄ had unpredictable MWs and high dispersities (Đ = 1.82–2.20).