Free-Radical Frontal Polymerization with a Microencapsulated Initiator Brian McFarland, Sam Popwell, and John A. Pojman* Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi 39406-5034 Received April 21, 2004 Revised Manuscript Received July 20, 2004 Introduction. Frontal polymerization entails the conversion of a monomer into a polymer via a localized exothermic reaction zone that propagates through the coupling of thermal diffusion and Arrhenius reaction kinetics. Frontal polymerization was discovered in Rus- sia by Chechilo and Enikolopyan in 1972 using methyl methacrylate under high pressure. 1 The extensive work from Russia was reviewed by Davtyan et al. 2 Pojman and co-workers performed an extensive study of the macrokinetics and dynamics of frontal polym- erization. 3-7 They recently studied frontal copolymeri- zation. 8 Frontal polymerization has been used to pre- pare different materials, including thermochromic com- posites, 9 IPNs, 6 polymer-dispersed liquid crystal mater- ials, 10 functionally gradient materials, 11-13 large com- posites, 14 and hydrogels. 15 An overwhelming majority of work has been on free- radical systems, but other chemistries can be used. Begishev et al. studied frontal anionic polymerization of ǫ-caprolactam, 16,17 and epoxy chemistry has been used as well. 18-21 Mariani et al. demonstrated frontal ring- opening metathesis polymerization. 22 Fiori et al. pro- duced polyacrylate-poly(dicyclopentadiene) networks frontally, 23 and Pojman et al. studied epoxy-acrylate binary systems. 24 Polyurethanes have recently been prepared frontally. 25,26 Frontal atom transfer radical polymerization has been achieved 27 as well as frontal polymerization with thiol-ene systems. 28 Free-radical frontal polymerization has been dis- cussed in detail by Pojman et al. 5 and by Washington and Steinbock. 29 The velocity dependence on the initia- tor concentration has been studied for several sys- tems 3,30,31 and follows a power function dependence on the initiator concentration. A significant issue for applying frontal polymerization to real-world applications is the issue of pot life, i.e., how long can the initiator-monomer solution remain at room temperature before reacting homogeneously. We sought to address this issue by microencapsulating a free-radical initiator, cumene hydroperoxide, and dis- persing the capsules throughout a mixture of 1,6- hexanediol diacrylate (HDDA) and silica gel. Because the initiator was sequestered from the monomer, it could not initiate polymerization until the capsule burst open upon heating. Frontal polymerization was achievable with the CHP microcapsules and also when cobalt naphthenate was dissolved in the monomer, where it could react with the CHP in a redox reaction to produce free radicals. Experimental Section. 1,6-Hexanediol diacrylate (80%, technical grade) (HDDA) was obtained from UCB and used as received. Cumene hydroperoxide (88%) (CHP) and cobalt naphthenate in mineral spirits (8% cobalt) were obtained from Aldrich and used as received. Microcapsules loaded with a cumene hydroperoxide core were prepared using an interfacial polymerization method. The shell materials consisted of triethylene- tetramine (TETA, 60%, technical grade) obtained from Aldrich and Mondur MRS (a polymeric isocyanate based on 4,4′-diphenylmethane diisocyanate) obtained from Bayer Corp. and were used as received. Poly(vinyl alcohol) (87-89% hydrolyzed) (PVA) was obtained from Aldrich and used as received. A solution of the core material was made by dissolving 80 mL of CHP in 10 mL of Mondur MRS. The core solution was then emulsified in 250 mL of a 0.5% PVA solution with a stir motor equipped with a three-bladed propeller. The emulsion contained dispersed-core drop- lets with a size ranging from 100 to 275 μm, which was achieved by mixing at 230 rpm for 2 min. Once the desired droplet size range was achieved, a solution of 6 mL of TETA in 12 mL of deionized water was added, and the mixture was heated to 50 °C in a water bath. The mixture was allowed to react for 4 h at 50 °C with continuous mixing at 230 rpm. After 4 h, the microcap- sules were recovered by vacuum filtration and dried overnight with the aid of fumed silica gel (CAB-O-SIL, Cabot Corp.). The dried microcapsules were roughly spherical and had a size ranging from 150 to 300 μm. The microcapsules were composed of approximately 80 wt % CHP and were washed with heptane prior to use in order to remove any unencapsulated CHP from the outside of the shells. We performed all frontal polymerization experiments in glass test tubes, 16 × 125 mm (VWR #72690-022), on which a plastic cap (VWR #60826-290) could be securely screwed. Polymerization was initiated by heat- ing the top of the tube with a soldering iron. Fronts were performed using HDDA systems containing unencap- sulated CHP and encapsulated CHP. The front velocity was measured over a range of initiator concentrations; the CHP concentrations in the microcapsule systems were calculated using the approximate core weight percentage of the capsules and CHP density. To prevent the settling of the microcapsules, ultrafine silica gel (4% w/v) was added to the reaction medium. The same concentration of silica was also used in the unencapsu- lated CHP systems. The pot life was assessed by preparing tubes with the reactants and leaving them at ambient temperature and determining at what time they spontaneously polym- erized. For the microencapsulated system, several tubes were prepared, and their front velocities were deter- mined after several days. The tubes contained HDDA, 4% (w/v) silica, and 2% CHP. In one sample set the CHP was encapsulated, and in another sample set the CHP was unencapsulated. An addition of 0.04% (v/v) cobalt naphthenate was added to one tube from each sample set. Results and Discussion. The front position vs time data for all systems were linear, which indicates that constant velocity, self-sustaining fronts were achieved. Figure 1 shows images of a front in an encapsulated CHP system and of a front in an unencapsulated CHP system. In each of these systems the front is seen to * To whom correspondence should be addressed: e-mail john@pojman.com. 6670 Macromolecules 2004, 37, 6670-6672 10.1021/ma0492216 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/06/2004