Origin of characteristic properties of natural rubber - Effect of fatty acids on crystallization of cis-1,4-polyisoprene

Natural rubber (NR) contains linked fatty acids, composed of mainly saturated fatty acids, in conjunction with a mixture of free saturated and unsaturated fatty acids. The crystallization of synthetic cis-1,4-polyisoprene (IR) is accelerated by the addition of 1 wt % mixture of saturated fatty acid as a nucleating agent and unsaturated fatty acid as a plasticizer, As an NR model, IR was esterified with stearic acid selectively at the 3,4-isoprene units after introduction of hydroxyl group by hydroboration. The linked stearoyl group stimulated the crystallization of IR at -25 degrees C, while linked fatty acids other than stearoyl group showed no stimulating effect on the crystallization. The addition of methyl linoleate to the stearoyl-esterified IR gave the highest rate of crystallization at -25 degrees C. A rapid crystallization of NR is presumed to originate from the mixed saturated and unsaturated fatty acids composition present and the presence of saturated fatty acids linked to NR.