Thiol-ene functionalisation of homo- and copolymers of butadiene with L-cysteine derivatives

In the present work the functionalisation of oligomers of butadiene (liquid 1,2-polybutadiene PBL) and of oligomers of a random copolymer butadiene/styrene (SBRL) was performed by the addition of either N-acetyl-L-cysteine, N-acetyl-L-cysteine methyl ester or L-cysteine methyl ester through the thiol-ene radical reaction. Functionalisation was carried out in solution and in presence of a free radical initiator (2,2’-azobisisobutyronitrile, 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 reaction mechanism. The 1H-NMR determination of the functionalisation extent brought out that the addition degrees of thiols were always high, regardless of the initial initial thiol / double bond ratio; the complete conversion of the double bonds, however, was never reached because of side-reactions. In order to better understand the occurrence of the latters, reactions with AIBN or with other free radical generating species were performed, in absence of thiol; the purpose of these experiments was to reproduce the presence of macroradicals similar to those generated during the thiol addition, and to investigate their reactivity towards other macromolecules present in the reaction environment. The reaction products of these runs were characterised, and some conclusions were drawn regarding the different type of side reactions occurring with the two polymeric substrates (PBL or SBRL). The high functionalisation degrees often caused a drastic change in the macroscopic properties of the polymers, ranging from improved solubility in hydrophilic solvents (for instance in alkaline aqueous solutions) to a remarkable increase of the glass transition temperature. Chiral properties were carried from the functionalising agents to the modified polymers; this phenomenon was evaluated by polarimetry of polymer solutions in organic solvents, and discussed with reference to the reciprocal proximity of the L-cysteine side chains. Circular dichroism measurements were also performed in order to investigate the possible presence of secondary / tertiary structures analogously to what reported in scientific literature for similar systems. In the last part of this work, a possible use of (N-acetyl-L-cysteine)-functionalised products was evaluated; the hydrophilically-modified polymers showed the ability to act as coupling agents between hydrophobic polymeric matrices and inorganic, hydrophilic fillers in composite materials. The incorporation of the functionalised polymers (PBL and SBRL having various functionalisation degrees) in compounds for ground tyres was performed, and the rheological / mechanical properties of the resulting compounds were evaluated; the formulation used are highly innovative compounds including an elevated silica content.