Synthesis of new dental adhesive monomers and polymers

Abstract

New phosphonate, phosphonic acid, carboxylic acid, iminodiacetate containing adhesive monomers based on alkyl-hydroxymethyl acrylates and their derivatives were synthesized. A new phosphonate-containing cyclic monomer was obtained from the reaction of 2-(2-chlorocarbonyl-allyloxymethyl)-acryloyl chloride with diethyl hydroxymethyl phosphonate. This monomer showed high cyclization tendency during homopolymerization and copolymerization with 2-(2-tert-butoxycarbonylallyloxymethyl)- acrylic acid tert-butyl ester (TBEED) using initiators 2,2’- azobis(isobutyronitrile) (AIBN) at 75-77 ºC. The Tg of the copolymers decreased with increasing amounts of phosphonate-containing monomer whereas the char residues of the copolymers increased. The selective hydrolysis of the phosphonate monomer with trimethylsilyl bromide gave a water soluble phosphonic acid-containing monomer. This monomer could not be homopolymerized but copolymerized with TBEED and its dicarboxylic acid derivative, 2-(2-carboxy-allyloxymethyl)-acrylic acid. A series of 4,4-disubstituted 1,6-heptadiene monomers were synthesized from the reaction of ethyl- -bromomethyl acrylate and tert-butyl- -bromomethyl acrylate with triethylphosphonoacetate and tetraethylmethylenediphosphonate using sodium hydride followed by selective hydrolysis of the ester groups. Photopolymerizations of the monomers were investigated to understand the effect of the cyclic monomer structure on their polymerization reactivity. A strong effect of the substituents at 2, 4 and 6 positions of the monomers on polymerization rate was observed. The polymerizability of the monomers were successfully correlated with the 13C-NMR chemical shifts of the vinyl carbons. Conversion values were consistent with the Tg being a measure of the flexibility of a monomer. The monomers containing phosphonic acid groups were soluble in water and ethanol. The acidic nature of the aqueous solutions of these monomers is expected to give them etching properties, important for dental applications. The interaction of the acid monomers with hydroxyapatite was investigated using 13C NMR technique. It was found that phosphonic acid-containing monomers demineralized the calcium phosphate in the HAP in preference to the carboxylic acid. Four new methacrylate monomers containing phosphonic and/or carboxylic acids were synthesized from the reaction of tert-butyl a-bromomethyl acrylate with triethyl phosphite followed by selective hydrolysis of the phosphonate or tert-butyl ester groups using trimethylsilyl bromide and trifluoroacetic acid. The copolymerization of these monomers with 2-hydroxyethyl methacrylate (HEMA) was investigated by photodifferential scanning calorimetry. The reactivities of the monomers increased by decreasing steric hindrance and increasing hydrogen bonding capacity due to hydrolysis of phosphonate or tert-butyl ester groups. Furthermore, new adhesive monomers with diethyliminodiacetate as a potential chelating group were synthesized from the reaction of ethyl-bromomethyl acrylate, 2-chloromethyl-acryloyl chloride and 2-(2-carboxy-allyloxymethyl)-acrylic acid with diethyl iminodiacetate. The monomers were homopolymerized and copolymerized with TBEED and HEMA. Rate of copolymerizations and conversions decreased with an increase in the monomer concentrations. Low polymerizability of these monomers were explained with the steric effect of the N,N-disubstituted methacrylamides and the presence of allyl amine group leading to degradative chain transfer. The monomers were hydrolyzed by potassium hydroxide solution and the salt derivatives were copolymerized with acrylamide in water using 2,2’-azobis(N,N’-amidinopropane) dihydrochloride (V- 50). Polymer yields after dialysis were low and decreased with increasing monomer concentrations. The Ni+2 chelating ability of the monomers were shown using UV-visible spectroscopy. Thermal stability of the copolymer increased on complexation with Ni+2 ions.