Curing Kinetics of the Synthesis of Poly(2-hydroxyethyl methacrylate) (PHEMA) with Ethylene Glycol Dimethacrylate (EGDMA) as a Crosslinking Agent CHEN-WEI HUANG, YI-MING SUN, WEI-FUNG HUANG Department of Chemical Engineering, Yuan-Ze Institute of Technology, Chung-Li, Taiwan 320, Republic of China Received 19 August 1996; accepted 3 December 1996 ABSTRACT: An experimental study was carried out to investigate the effect of ethylene glycol dimethacrylate ( EGDMA, as a crosslinking agent ) content on the curing kinetics of the polymerization of 2-hydroxyethyl methacrylate ( HEMA ) , using differential scan- ning calorimetry ( DSC ) and Fourier transform infrared spectroscopy ( FTIR ) . EGDMA may cause a crosslinking-facilitated gel effect which reduces the termination rate of living free radicals and enhances the overall reaction rate, but it may also induce a diffusional resistance for the reactants so that some free monomers are trapped and pendant vinyl groups are prohibited from reaction by the crosslinked structure. At higher content of EGDMA, the later effect becomes predominant, and the reaction rate and the final conversion are limited. The exothermic peak of the curing reaction tends to carry a shoulder and then split into two peaks as the amount of EGDMA is increased, possibly due to a later reaction of the trapped monomers and pendant vinyls. The heat of reaction measured by DSC in the scanning mode is 61.2 kJ /mol C|C. The activation energy ( E ) of the curing reaction ranges from 56.5 to 78.3 kJ/mol C|C depending on the EGDMA content and the type of operation. The diffusion-limited reaction rate and the different thermal history experienced in the nonisothermal and isothermal curing can result in variations of the results in the activation energy measurement.  1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1873 – 1889, 1997 Keywords: free-radical polymerization; kinetics; hydrogel; thermoset; crosslinking INTRODUCTION The hydrogels in a matrix form have been used as soft contact lenses, 1,6 artificial organ, 2,6 im- Hydrogels are crosslinked hydrophilic polymer plants, 3–4,6 and devices for controlled drug re- materials that can absorb a significant amount of lease. 5,7 On the other hand, the hydrogels in a water while maintaining a distinct three-dimen- spherical microparticle form ( microspheres or sional structure ( insoluble ) . The preparation of beads ) are used in hemoperfusion, 8 enzyme im- poly ( 2-hydroxyethyl methacrylate ) (PHEMA) mobilization, 9,10 endovascular occlusion, 11 and hydrogels is a subject of great interest, especially also in controlled drug release. 5,12–13 They have for biomedical and pharmaceutical applications. 1–6 the major advantages of good biocompatibility, moderate degree of swelling in water, and biologi- cal inertness. Correspondence to: Y.-M. Sun Several techniques have been used to prepare Contract grant sponsor: National Science Council, Republic these hydrogels: such as bulk polymerization ( for of China; contract grant number: NSC-83-0425-B-155-001- M08 matrix of various shapes ) , 5,7 suspension polymer- Contract grant sponsor: Standard Chemical and Pharma- ization 14,15 ( for microspheres ) , and solution poly- ceutical Co.; contract grant number: NSC-83-0425-B-155-001- merization for a linear polymer then followed by M08  1997 John Wiley & Sons, Inc. CCC 0887-624X/97/101873-17 a crosslinking reaction. 16 Many comonomers can 1873 / 8G42$$0239 05-30-97 07:23:20 polca W: Poly Chem