Solvent-Free Free-Radical Frontal Polymerization: A New Approach to Quickly Synthesize Poly(N-vinylpyrrolidone) XIAOJING CAI, SU CHEN, LI CHEN State Key Laboratory of Material-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, No. 5 Xin Mofan Rd., Nanjing 210009, People’s Republic of China Received 30 October 2007; accepted 26 November 2007 DOI: 10.1002/pola.22552 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The first synthesis of poly(N-vinylpyrrolidone) without solvent by free- radical frontal polymerization at ambient pressure is reported. The appropriate amounts of two reactants N-vinyl-2-pyrrolidone (NVP) and initiator 2,2 0 -azobis-isobu- tyronitrile (AIBN) without solvent were mixed together at ambient temperature. Frontal polymerization was initiated by heating the wall of the tube with a soldering iron, and the resultant hot fronts were allowed to self-propagate throughout the reac- tion vessel. Once initiated, no further energy was required for polymerization to occur. To suppress the fingers of molten monomer, a small amount of nanosilica was added. The dependence of the front velocity and front temperature on the AIBN con- centration was thoroughly investigated. The as-prepared polymers were characterized by gel permeation chromatography (GPC) and thermogravimetric analysis (TGA). Results show that without postpolymerization solvent removal, waste production can be reduced. Solvent-free FP could be exploited as a means for preparation of PVP with the potential advantage of higher throughput than solvent-based methods. V V C 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2177–2185, 2008 Keywords: frontal polymerization; IR thermal image; kinetics (polym.); molecular- weight distribution; poly(N-vinylpyrrolidone); solvent-free; thermogravimetric analysis (TGA) INTRODUCTION Over the past decade, the study of frontal poly- merization (FP) is drawing significant research interest because it allows for synthesizing a wealth of uniform polymers and polymeric net- works with spatially controlled microstructures and morphologies in a rapid fashion. FP is a mode of converting a monomer into a polymer via a localized reaction zone that propagates through the coupling of thermal diffusion and tempera- ture-dependent reaction rate. Once ignited, conversion of reactants to polymers by the inter- nal reactive heat source needs no additional energy if the retained heat is sufficiently high to ignite polymerization after an initial initia- tion, and then a self-sustaining hot front propa- gates gradually through the reaction vessel when the rate of heat production exceeds the rate of heat loss. FP was first discovered by Chechilo and Eni- kolopyan in 1972, who studied methyl methacry- late polymerization in adiabatic conditions under high pressure (>3000 atm). 1 Subse- quently, this method was extended by Pojman and his coworkers to include numerous poly- mers. 2–7 They demonstrated the achievability of FP for a number of vinyl monomers and epoxy resins at ambient pressure. The overwhelming majority of FP work has been performed on free-radical polymerization system since it is usually highly exothermic, and the heat of the Correspondence to: S. Chen (E-mail: chensu@njut.edu.cn) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 2177–2185 (2008) V V C 2008 Wiley Periodicals, Inc. 2177