An effective method to prepare sucrose polymers by Thiol-Ene photopolymerization
Introduction
Thiol-Ene photopolymerizations have gained considerable importance in the technological and scientific ambits of radiation curing because of the inherent advantages of these systems. For instance, these reactions proceed with high regioselectivity under mild conditions, even in the presence of oxygen, reaching high conversions (Killops et al., 2008, Service, 2008). These characteristics make them very attractive for certain applications where other systems like radical or cationic photopolymerizations are less suitable. One of the areas where this kind of polymerization may have impact is in the development of biomaterials. Nowadays, the research for novel biocompatible materials for medical applications has acquired increasing importance (Opsteen, Ayres, & Van Hest, 2008). There are reports where the Thiol-Ene photopolymerizations have been used to prepare materials with potential applications as restorative dental resins (Carioscia, 2005; Lu, Carioscia, Stansbury, and Bowman, 2005) and scaffolds for biomaterials (Rydholm, Reddy, Bowman, & Anseth, 2004). The polymers intended to be applied in these applications need to fulfill certain characteristics such as non-toxicity, cell-adhesiveness and controlled biodegradability (Baroli, 2006).
Thiol-Ene photopolymerizations require that the monomers involved must have two or more functional groups. In (Scheme 1), the radical-step-growth mechanism for the photoinitiated Thiol-Ene polymerization is shown (Cramer and Bowman, 2001, Jacobine, 1993). The primary radicals produced in the photolysis of the photoinitiator can abstract the hydrogen atom of the thiol group, producing a thiyl radical (Eq. (1)). This thiyl radical would then react with the double bond of the unsaturated monomer to generate a secondary free radical (Eq. (2)), which in turn can abstract the hydrogen atom of a second molecule of the thiol, generating a new thiyl radical to repeat the cycle (Eq. (3)). Termination occurs by the reaction of two radical species (Eqs. (4)–(6)).
On the other hand, polymers derived from sucrose and other carbohydrates have attracted increasing attention due to the multiple applications where they are involved (Gruber and Knaus, 2000, Spain et al., 2007). Among others the following applications can be mentioned: preparation of stationary phases for chromatography (Liu & Dordick, 1999), biocatalytic (Novick & Dordick, 1998) and biosensible sensible hydrogels (Miyata, Uragami, & Nakamae, 2002) and controlled release of drugs (Yun, Goetz, Yellen, & Chen, 2004).
The preparation of sucrose polymers via Thiol-Ene photopolymerization is of interest to our research group in using this technique to prepare different biocompatible materials. This paper deals with the preparation of the diallyl sucrose monomer and its photopolymerization with multifunctional thiols. The kinetics as well as the mechanical and thermal properties of the obtained polymers were determined.
Section snippets
Materials and equipment
Sucrose, allyl bromide, sodium hydroxide, tetrabutyl ammonium perchlorate, pentaerythritol tetrakis (3-mercaptopropionate) [PETKMP], trimethylol propane tris (3-mercaptopropionate) [TMPTMP], 2,2-dimethoxy-2-phenylacetophenone (DMPA), benzophenone (BP), were all reagent grade and purchased from Aldrich Co (Milwaukee). Routine infrared spectra and photopolymerization kinetics were performed on a Magna Nicolet 550 Infrared spectrometer (Middleton, WI). NMR spectra were obtained using a 300 MHz Jeol
Synthesis of monomer A2S
Scheme 2 shows the methodology of synthesis of the A2S monomer. The method used was straightforward using water as solvent and a phase transfer catalyst. After purification by column chromatography, the degree of substitution of the sucrose was determined by 1H NMR considering the integral of the peak at 5.8 ppm that is not overlapped by any other peak and represents one vinylic proton The integral of this peak is compared against the integral of the peaks in the range 3.3–4.3 ppm that includes
Conclusions
Novel sucrose polymers were prepared using the Thiol-Ene photopolymerization technique. It was demonstrated the high reactivity of the photocurable system that comprises the allyl sucrose monomer A2S with multifunctional thiols such as PETKMP and TMPTMP when they were irradiated with UV light. The obtained polymers displayed high uniformity of crosslinking as well as high thermal stability. These materials may have potential applications as biomaterials. The method described herein can be very
Acknowledgments
The authors thank the Mexican National Council of Science and Technology (CONACYT) for funding this project (49209). Assistance in running NMR, DSC, DMA and SEM samples by Judith N. Cabello, Guadalupe Mendez and Blanca Huerta is gratefully acknowledged.
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