Xanthate-Mediated Radical Polymerization of N-Vinylpyrrolidone in Fluoroalcohols for Simultaneous Control of Molecular Weight and Tacticity

Abstract

The simultaneous control of the tacticity and molecular weight of poly( N -vinylpyrrolidone) during radical polymerization is reported for the first time. For molecular weight control, xanthates of ( O -ethylxanthylmethyl)benzene and [1-( O -ethylxanthyl)ethyl]benzene were used as RAFT/MADIX chain transfer agents (CTAs) for the radical polymerization of N -vinylpyrrolidone (NVP). Both led to a controlled/living radical polymerization, and the latter showed higher chain transfer ability under the optimal conditions; the molecular weight distribution was 1.36 when the molecular weight was up to 26 700. The polymerization was studied between 20 and 120 °C and at various concentrations of CTA. All the polymerizations showed an induction period and rate retardation dependent on both the concentration of CTA and temperature. For tacticity control, the polymerization was carried out in fluoroalcohols via a conventional radical process without CTAs to give syndiotactic polymers. The polymer tacticity was dependent on the amount of the fluoroalcohol, and a more acidic and bulkier fluoroalcohol led to a higher syndiotacticity. Especially with (CF 3 ) 3 COH, the r dyad increased to 62.6% from 53.5% for the atactic poly(NVP) obtained in the usual solvents. The 1 H NMR analysis of the mixture of NVP and the fluoroalcohols indicated that a 1:1 hydrogen-bonding complex was formed, suggesting that the complex was responsible for the tacticity control of the polymer. When the CTA was used in the fluoroalcohols, the living and syndiospecific polymerization proceeded to enable the simultaneous control of the molecular weight and the tacticity.