Thiol-Ene Radical Addition of L-Cysteine Derivatives to Low Molecular Weight Polybutadiene

The functionalisation of a low molecular weight, liquid 1,2-polybutadiene (PBL) was performed by the addition of either N-acetyl-L-Cysteine or its methyl ester through the thiol-ene radical reaction. The functionalisation runs were carried out in 1,4-dioxane solution and in the presence of a free radical initiator (2,2'-azobisisobutyrronitrile, AIBN). The initial feed ratio polymer/cysteine derivative/initiator was varied, in order to highlight the occurrence of side reactions and to understand their possible influence on the process. The (1)H NMR determination of the functionalisation degree brought out high addition degrees of thiols, regardless of the initial thiol/double bonds ratio. This caused a drastic change in the macroscopic properties of the PBL with improved solubility in hydrophilic solvents (for instance in alkaline aqueous solutions) and impressive increase of the glass transition temperature. The propagation of chiral properties from the functionalising agents to the modified polymers was evaluated by polarimetry and circular dichroism in solution.