Copolymers of Styrene and Ethylene with a Series of Vinyl Esters: Impact of Vinyl Ester Molecular Weight On Thermal Mechanical and Rheological Properties

A number of new processes are being developed which produce bio-sourced fatty acids of various molecular weights. These fatty acids can easily be converted into vinyl esters using common processes. However, vinyl esters larger than vinyl propionate are not suitable for homopolymerization due to the low glass transition temperatures (T_g) of the homopolymers. Therefore, this work will focus on the copolymerization of vinyl esters of various molecular weights with both styrene and ethylene. Particular emphasis will be placed on the influence of vinyl ester molecular weight on the polymerization kinetics and thermomechanical characteristics of the resultant copolymers.

In this study copolymers of styrene (St) with a series of vinyl esters (VEs) including vinyl acetate (VAc), vinyl propionate (VPr), vinyl hexanoate (VHe), and vinyl decanoate (VDe) were prepared by bulk copolymerization initiated by azobisisobutyronitrile (AIBN) at 60 °C. Copolymers of ethylene with the VEs were synthesized by solution copolymerization initiated by AIBN also at 60 °C. The compositions and molecular weight distributions (MWD) of all the copolymers were determined by GPC. The impact of compositions and branch lengths on the thermal mechanical properties of copolymers including glass transition temperature (T_g), melting point (T_m), storage modulus (G°ä) and loss modulus (G°å) was determined by dynamic mechanical analysis. In addition, the rheological response (complex viscosity versus shear rate) of the copolymers of ethylene with VEs were studied.

Current results showed that the molecular weights (MWs) of the copolymers decreased as the mole fraction of VEs in the feed increased. Meanwhile, copolymers with similar VE incorporations had higher MWs for poly (St-co-VHe) and poly (St-co-VDe) than those of poly (St-co-VAc) and poly (St-co-VPr). It could be explained by the significantly higher reactivity ratios of both VHe and VDe than those of VAc and VPr. Incorporating VEs with different chain lengths from 1 mol% to 19 mol% reduced the T_g of PS by up to 32 °C. The T_g of poly (St-co-VEs) decreased as the mole fraction of VEs increased. Poly (St-co-VEs) with longer branches, such as poly (St-co-VHe) and poly (St-co-VDe), exhibited a larger drop in T_g than VAc and VPr copolymers at similar compositions. However, the branch effects on T_g were not significant for copolymers containing VEs less than 5%.