A reappraisal of the photopolymerization kinetics of triethyleneglycol dimethacrylate initiated by camphorquinone-N,N-dimethyl-p-toluidine for dental purposes J. Nie + , L.A ˚ . Linde ´n + , J.F. Rabek* ++ , J.P. Fouassier ++ , F. Morlet-Savary ++ , F. Scigalski # , A. Wrzyszczynski # and E. Andrzejewska ## + Polymer Research Group, Department of Dental Biomaterials Science, Karolinska Institute (Royal Academy of Medicine), Box 4064, S-14104 Huddinge (Stockholm), Sweden ++ Laboratoire de Photochimie Generale CNRS n o 431, ENSCMU, 3 rue Alfred Werner, F-68200 Mulhouse, France # Faculty of Chemistry and Chemical Engineering, University of Technology and Agriculture, Seminaryjna 3, PL-85-326 Bygoszcz, Poland ## Institute of Chemical Technology and Engineering, Poznan University of Technology, Pl. Sklodowskiej-Curie 2, PL-60-965 Poznan, Poland This work describes the characteristics of triethyleneglycol dimethacrylate (TEGDM) polymerization when photoinitiated by camphorquinone (CQ) alone and in the presence of N,N-dimethyl-p-toluidine (DMT) in air and/or in N 2 . The rates of polymerization (R P ), double bond conversion (p), monomer conversion (p m ), temperature, and different concentrations of CQ and DMT were measured and analyzed. The second R p maximum was found for the polymerization of TEGDM in the presence of CQ (without DMT) in N 2 and in the presence of CQ + DMT in air. Formation of the second R p maximum has been explained by the different mobility of the initiating monomer radicals in the polymer matrix as a polymerization pro- ceeds. The 2,2,6,6-tetramethylpiperidine and 4-hydroxy-2,2,6,6-tetramethylpiperidinoxy radicals were used in order to eva- luate the role of amineoxy and amineperoxy radicals in the polymerization of TEGDM in air. Finally, photophysical studies allowed a more detailed evaluation of the role of excited states of CQ and DMT in the photoinitiation process. 1. Introduction Clinical photocuring of polymeric restorative resins in dentistry occurs under special conditions, which differ from any type of industrially applied curing. These special conditions are the following: 1. The whole procedure is performed in vivo, and is restricted by biophysiological demands such as the oral temperature, which should not exceed 50 8C (maximum 70 8C); necessity of application of visible light over 400 nm, which avoids photocancerogenic and photoaller- gic effects (caused by UV radiation) and the risk of tissue burning [1]; and the presence of air, water and saliva (the last two can be technically limited to some extent). 2. The monomers for dental restorative resins are not purified and are used as delivered. They contain about 0.01% of inhibitors (such as hydroquinone, methyl ether of hydroquinone, p-methoxyphenol, resorcinol, pyrogallol, benzoic acid, and thymol, however, the most common is 2,4-dimethyl-6-tert-butyl phenol (Topanol A, I.C.I.)) to prevent premature polymerization during storage. In addi- tion, some monomers can contain other impurities orig- inating from their synthesis and storage. 3. The photopolymerization reaction occurs in a compli- cated dental formulation system that, besides a mixture of different mono-, di- and tri-functional monomers and photoinitiator (and coinitiator), contains different addi- tives, such as stabilizers, pigments, reinforcing fillers (even up to 70 wt.-%), and coupling agents [2–6]. The concentration of the photoinitiator in the resin must be such that it will react at the proper wavelength of visible light and be present in sufficient quantities. Excessive initiator concentration has a detrimental effect on storage of the composite resin and may be responsible for biologi- cal hazards. 4. There are very strict toxic, neurotoxic, cancerogenic, mutagenic, and allergenic restrictions for the use of dental formulation components [7, 8]. Meth(acrylic) monomers released into saliva may cause certain reactions, such as redness, swelling and pain of the oral mucosa [9, 10]. In addition, these monomers can, after penetration of the skin, produce persistent parasthesia of the fingers in surgi- cal and dental personnel due to damage of peripheral mye- linized nerve fibres and the Ranvier nodes [11]. The amines present belong to hepatotoxins, which can cause activation of toxic substances in the liver, and which can act as hepatocarcinogens [8]. An amine such as N,N- dimethyl-p-toluidine (DMT) used as coinitiator is cancero- genic and mutagenic [12], but in spite of this it is widely used in dental compositions to accelerate the polymeriza- tion process. 5. The intra-oral photocuring must be carried out for highest possible monomer conversion since the unreacted monomer will diffuse out from the polymer matrix into the saliva in the oral cavity and will be swallowed by the patient. 6. The amount of the photoinitiating system should be limited to a concentration that is just sufficient to obtain an optimum of photocuring reaction with the highest possible monomer conversion. Unreacted photoinitiators and rest products of their photolysis (if they are not permanently bound to a polymer network) will also diffuse off from the polymer matrix into the saliva. 7. Unreacted monomers as well as photoinitiators (and their photolysis products) can be extracted by water (from saliva), which penetrates into a photocured resin in a tooth during use. Acta Polymer., 49, 145 – 161 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1998 0323-7648/98/0404-0145$17.50+.50/0 145 Fax: +46-8-6080891 Full Papers