1,1,3,3-Tetramethylguanidine-Promoted Ring-Opening Polymerization of N-Butyl N-Carboxyanhydride Using Alcohol Initiators

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© 2016 American Chemical Society. N-Butyl N-carboxyanhydride (Bu-NCA) was polymerized in THF to produce poly(N-butylglycine) using benzyl alcohol initiator and 1,1,3,3-tetramethylguanidine (TMG) promoter. Poly(N-butylglycine) species (PNBG) with controlled molecular weight (Mn = 2.9-20.5 kg mol-1) and narrow molecular weight distribution (PDI = 1.04-1.08) were obtained by controlling the initial monomer-to-initiator ratio and conversion. The reaction exhibits characteristics of a living polymerization, as evidenced by the linear increase of Mn with conversion and successful chain extension experiments. Kinetic studies revealed a first-order dependence of propagation rate on the monomer and benzyl alcohol concentration and zero-order dependence on the TMG concentration, consistent with a controlled polymerization where the TMG does not influence the chain propagation. It was further revealed that the polymerization activities and Mn control are strongly dependent on the structure of the alcohol initiators. Primary alcohols (e.g., methanol, ethanol, 2-methoxyethanol, n-propanol, and benzyl alcohol) can mediate the polymerization of Bu-NCA with the TMG present to produce PNBG polymers with good to moderate control of Mns and PDIs. By contrast, sterically hindered secondary or tertiary alcohols (e.g., isopropyl alcohol and tert-butyl alcohol) or a primary alcohol bearing strongly electron withdrawing substituents (e.g., 2,2,2-trifluoroethanol) or phenol either failed to initiate the polymerization or produced PNBGs whose molecular weights deviated to varying extents from the theoretical values based on single-site initiation with alcohols. 1H NMR analysis revealed hydrogen bonding interactions between TMG and various alcohols, thereby promoting the initiation by enhancing the nucleophilicity of the alcohols toward ring-opening addition of Bu-NCA monomer. Electron-rich primary alcohols can initiate the polymerization more efficiently than the electron-deficient primary alcohol or sterically hindered secondary or tertiary alcohols, resulting in enhanced control over polymer molecular weight in the former than the latter. The polymerization has been successfully extended toward the synthesis of heteroblock copolymers based on poly(ethylene glycol) (PEG) and PNBG by using a hydroxyl-ended PEG macroinitiator and TMG promoter.

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