Synthesis of a star-shaped poly (ethylene-co-propylene) copolymer as a viscosity index improver for lubricants

A saturated star-shaped poly ethylene-co-propylene) copolymer, (EP)(star), has been synthesized for use as a viscosity index improver in lubricants. Polyisoprene arms were first anionically synthesized using n-butyllithium as the initiator, followed by a linking reaction with divinylbenzene at the optimum temperature of 60degreesC. The resulting star-shaped polyisoprene, (I)(star), was then hydrogenated to eliminate the double bonds of the polyisoprene forming the poly(ethylene-co-propylene) structure. The degree of branching (number of arms on each molecule) increases with increase in the mole ratio of divinylbenzene to n-butyllithium. Increasing the arm length adversely affects the linking efficiency and a minimum amount of tetrahydrofuran (THF) at a THF:n-butyllithium molar ratio of 1.12 was needed in order to achieve a maximum linking efficiency of approximately 85%. The T-g of poly(ethylene-co-propylene) is about 10degreesC higher than that of the original polyisoprene. Compared with (I)(star), (EP)(star) has a thermal decomposition temperature that is 50degreesC higher but is independent of the arm length or the degree of branching. Viscosity measurement results for (EP)(star) reveal that intrinsic viscosity depends only on the arm length but not the degree of branching. Adding 1 wt % or (EP)(star) markedly increases the viscosity index of a LN base oil. The addition of 1 wt % of (EP)(star) increases the viscosity index (95 for base oil) up to a number between Ill and 145, with the exact number depending upon its arm length and degree of branching. With a fixed arm length, an (EP)(star) having a higher degree of branching increases the viscosity index more than one having a lower degree of branching, On the other hand, the viscosity index increases with increase in the arm length when the degree of branching is fixed. Adding 1 wt % of (EP)(star) also causes a change in the pour point of the lubricant with the pour point decreasing with increase in the degree of branching. (C) 2002 John Wiley Sons, Inc.