Modeling of Free Radical Polymerization of Azobenzene-based Linear Polymers
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Modeling of Free Radical Polymerization of Azobenzene-based Linear Polymers D. Iqbal1, C. Melchert1, M. Behl1, A. Lendlein1, S. Beuermann2 * 1 Center for Biomaterial Development, Institute of Polymer Research, HZG Research Center Geesthacht, Kantstr. 55, 14513 Teltow, Germany 2 Institute of Chemistry, University of Potsdam, Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany
ABSTRACT Modeling of free radical polymerizations of the liquid-crystalline monomer 6-[4-(4heptyloxyphenylazo)phenoxy]hexylacrylate using the PREDICI software package is reported. The model accounts for all elemental reactions that were identified to be important for radical polymerizations of acrylate-type monomers. On the basis of butyl acrylate kinetic data a remarkable agreement between number average molar masses from modelling (Mn,sim) and from experiments (Mn,exp) is observed: Mn,sim = 17800 g·mol−1 and Mn,exp = 17400 g·mol−1. Similarly, dispersity values of 1.8 and 1.6 were determined via modelling and experiments, respectively. It is shown that the assumption of butyl acrylate kinetics provides a reasonable approximation even for acrylate-based monomers having mesogenic substituents. INTRODUCTION Azobenzene-based liquid-crystalline elastomers (LCE) are an interesting class of multifunctional polymers [1-5]. They combine the shape-changing capabilities of the LCE with light sensitivity of azobenzene moieties. The synthesis of linear polymers can serve as a model case for the prediction of the molecular chain length between two elastomer netpoints. To reduce the number of polymerizations, in particular if the monomer synthesis requires a number of reaction steps as in the case of azobenzene-based monomers, modeling of the reactions is highly desirable. For systems in which more than the classical reaction steps (initiation, propagation, termination) need to be considered the estimation of molecular weights on the basis of simple equations often fails. In these cases, the complete reaction scheme needs to be considered. For acrylate polymerizations it was shown that at least chain transfer to polymer and backbiting leading to the formation of midchain radicals (MCRs) as well as the reactions of MCRs must be accounted for [6]. With the simulation package PREDICI (CIT GmbH, Rastede, Germany) all relevant reaction steps may be accounted for and molar mass distributions are obtained. In this study, free radical polymerizations of 6-[4-(4-heptyloxyphenylazo)phenoxy]hexylacrylate are modelled with PREDICI on the basis of a kinetic model reported for butyl acrylate [6]. MATERIALS & METHODS 1-choloroheptane, 6-chloro-1-hexanol, acryloyl chloride, 4-nitrophenol, potassium hydroxide, sodium hydroxide, diethyl ether, dimethylformamide (DMF) (anhydrous), benzene (anhydrous), tetrahydrofuran (THF) (anhydrous), ethanol, ethyl acetate, chloroform,
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dichloromethane, methanol, hydrochloric acid, sulphuric acid, and hydrogen peroxide were purchased from ABCR Organics, Germany. Hydroquinone, magnesium sulphate, potassium carbonate, and azobisisobutyr
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