Synthesis of Poly(vinylacetylene) Block Copolymers by Atom Transfer Radical Polymerization Junko Aimi, † Lynne A. McCullough, ‡ and Krzysztof Matyjaszewski* ,‡ Department of Chemistry and Biotechnology, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Department of Chemistry, Carnegie Mellon UniVersity, 4400 Fifth AVenue, Pittsburgh, PennsylVania 15213 ReceiVed September 30, 2008 ReVised Manuscript ReceiVed NoVember 5, 2008 The preparation of well-ordered nanostructured carbon materials has attracted interest due to their potential in a number of applications including energy storage devices, 1 sensors, 2 and electronics devices. 3 Recently, a novel low- cost route to prepare well-defined nanocarbon materials was developed based on the pyrolysis of block copolymers that contains a carbon precursor block such as polyacrylonitrile (PAN) and a sacrificial block (e.g., poly(n-butyl acrylate). 4-8 The resulting carbon structures rely on the self-organization of block copolymers to form well-ordered morphologies, 9 and fabrication conditions which can be controlled in the nanoscale by defining the relative compositions of each block. Carbons derived from PAN precursors, however, also contain up to 7% nitrogen, which results in defect sites that affect the electronic properties of the materials. This fact led us to explore other types of block copolymers that could result in the preparation of pure carbon structures. Poly(vinylacetylene) (PVA), a polymer that structurally resembles PAN without the inherent nitrogen group, may possess the potential to give similar nanostructured materials with high carbon yield without the presence of preformed n-type dopants. There have been a few reports on the preparation of PVA from simple conjugated enyne molecules by cationic polymerization, 10 anionic polymerization, 11-14 thermal polym- erization, 13 or conventional free radical polymerization. 15-17 The anionic polymerization of VA derivatives including 2-methyl derivatives of VA exhibit a living-like character. 18,19 Thermal treatment of PVA in the temperature range of 150-400 °C resulted in the formation of conjugated cross-linked materials with moderate electrical conductivity and thermal stability. 10 However, controlled polymerization of vinylacetylene remains challenging due to its conjugated character and side reactions related to the presence of pendant acetylene group and the acidic acetylene proton. The difficulty in polymerizing vinylacetylene has limited the investigation of PVA-containing materials as effective high yield carbon precursors for nanostructured carbon systems. In order to prepare well-defined block copolymers that contain PVA, we decided to investigate controlled/living radical po- lymerization (CRP) techniques. 20 Atom transfer radical polym- erization (ATRP), 21-26 one of the most widely used CRP methods, is a powerful tool for the preparation of well-defined polymers with predetermined molecular weight, low polydis- persity, and high degrees of functionality. Here we report the first CRP of vinylacetylene and synthesis of block copolymers containing poly(vinylacetylene) segments via ATRP. The active acetylenic hydrogen atom in vinylacetylene was substituted with the trimethylsilyl group in order to prevent 1,4-polymerization and Cu(I)-catalyzed coupling reactions. It was found that conducting the homopolymerization of trimethylsilyl-protected vinylacetylene (VATMS) at lower temperature prevented side reactions and gave a well-controlled polymer with low poly- dispersity. The more robust process, activators regenerated by electron transfer (ARGET) ATRP, 27,28 was also successfully utilized to polymerize VATMS. Based on these homopolym- erization studies, block extension of poly(methyl methacrylate) (PMMA) macroinitiators with VATMS was performed, provid- ing the first example of well-defined block copolymers with PVA segments. VATMS was prepared according to the reported method. 17 ATRP of VATMS was carried out using CuBr/4,4′-di(5- nonyl)-2,2′-bipyridine (dNbpy) 29 as the catalyst (Table 1, entries 1-4). Since PVATMS has relatively low polarity, a 50% v/v mixture of anisole or acetone was used as a solvent for polymerization of VATMS in order to dissolve the catalyst and polymer. 1 H NMR spectroscopy was used to follow monomer conversion by comparing integral ratios between monomer and solvent, while the molecular weight and the molecular weight distribution were estimated by gel permeation chromatography (GPC) with tetrahydrofuran as an eluent. GPC calibration using PMMA standards overestimates the molecular weight of PVAT- MS as a result of significantly different hydrodynamic volumes, as is the case with polyacrylonitrile, 30 giving molecular weights ∼2 times larger than those calculated by 1 H NMR. When the reaction was carried out at 90 °C, the polymeri- zation was less controlled, showing a low-molecular-weight shoulder in GPC traces with increasing polydispersity (Table 1, entry 1, Figure S1). Also, the first-order kinetic plot of the ATRP of VA-TMS showed curvature above 40% conversion. These results indicated that the radical concentration decreased during the reaction due to some side reactions. Several side reactions could be considered: (1) copolymerization with acetylenic, vinylic, and allenic polymers by 1,2-, 3,4-, and 1,4- addition, respectively; (2) an outer-sphere electron transfer (OSET) reaction 31 where growing radicals react with Cu(I) catalyst to produce carbanion which can abstract a proton; (3) an elimination reaction to produce HBr; (4) termination reactions due to plausible low values of the propagation rate constant. The 13 C NMR measurements revealed the absence of vinylic and allenic structures, indicating selective polymerization by 1,2-addition and negligible contribution of side reactions (1). However, when the reaction targeting low molecular weight polymer was followed by 1 H NMR (Table 1, entry 2), the content of bromine end-group functionality decreased during the polymerization, demonstrating in the loss of dormant chain ends (possible reactions 2, 3, and 4). In order to prevent, or significantly reduce, such side reactions, we investigated several techniques which increase control of the polymerization of VATMS. One strategy involved reducing radical concentration during ATRP to decrease ter- mination reactions and elimination reactions. For instance, lowering the reaction temperature can reduce radical concentra- tion during ATRP and thus reduce contribution of side reactions. Indeed, the reaction carried out at 60 °C improved the control of ATRP of VATMS (Table 1, entry 3), while the reaction at * Corresponding author: Tel +1-412-268-3209; e-mail km3b@ andrew.cmu.edu. † The University of Tokyo. ‡ Carnegie Mellon University. 9522 Macromolecules 2008, 41, 9522-9524 10.1021/ma8022074 CCC: $40.75  2008 American Chemical Society Published on Web 11/14/2008