Synthesis of a novel biopolymer by means of Thiol-Ene Photopolymerization using diallyl sucrose and dithiotreitol as com
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Synthesis of a novel biopolymer by means of Thiol-Ene Photopolymerization using diallyl sucrose and dithiotreitol as comonomers Ricardo Acosta Ortiz, Antelmo Yasser Ruiz Martinez, Aida E García Valdez, María Lydia Berlanga Duarte Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna #140, C.P. 25253, Saltillo, Coahuila, México, email: [email protected], Tel, 52 844 4 38 98 44 ABSTRACT A novel biopolymer derived from diallyl sucrose (A2S) and dithiotreitol (DTT) was prepared by means of Thiol-Ene Photopolymerization. A2S was prepared by alkylating the sucrose with allyl bromide, using water as solvent. After purification by column chromatography, a fraction (F2A2S) with 94% diallyl sucrose (A2S), 4 % of triallyl sucrose (A3S) and 2 % of monoallyl sucrose (A1S) was obtained. This fraction was subsequently photopolymerized with Dithiothreitol (DTT) which is a difunctional thiol. Kinetics of photopolymerization were determined by means of Real-Time Infrared spectroscopy. It was found that the photocurable formulation with DTT and F2A2S, polymerized rapidly in the presence and absence of a photoinitiator, at low intensities of UV light. After bulk polymerization, a flexible material with high elastic modulus and a Tg of 30 ºC was obtained. Besides, the polymer displayed moderate water absorbance properties as a result of the presence of multiple hydroxyl groups. This property was pH dependent with maximum absorbance at pH=14. The polymer degraded rapidly under acidic conditions. INTRODUCTION Biopolymers derived from sucrose have found several applications either as chiral stationary phases for column chromatography [1], as plant growth regulators [2] and as surfactants [3], among others. Recently, a renewed interest in the development of sucrose polymers has arisen, because of the hydrophilic character of these kind of polymers that may result in the formation of hydrogels if the polymer is conveniently crosslinked. For example, Sachinvala [4] reported the preparation of sucrose hydrogels by preparing sucrose monoacrylate and polymerizing it in the presence of sucrose diacrylate as crosslinking agent. Our research group has also reported the preparation of sucrose biopolymers using diallyl sucrose (A2S) monomer and the tetrafunctional thiol pentaerythritol mercaptopropionate (PETKMP) as comonomers and photopolymerizing them by means of the Thiol-Ene mechanism [5]. To perform Thiol-Ene photopolymerizations is required that both comonomers, thiols and alkenes, must be bifunctional or mutifunctional. Scheme 1 shows the radical step growth mechanism for photoinitiated thiol-ene polymerization [6]. The primary radicals produced in the photolysis of the photoinitiator can abstract the hydrogen atom of the thiol group, producing a thiyl radical. This thiyl radical would then react with the double bond of the unsaturated monomer to generate a secondary free radical, which in turn can abstract the hydrogen atom of a second molecule of the thiol, generating a new thiyl radical to repeat the cycle. Termination occurs by th
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