Star polymer viscosity index improver for oil compositions

Abstract

The present invention includes star polymers which can be used as viscosity index improvers in oil compositions formulated for high performance engines. The star polymers have copolymer arms of hydrogenated polyisoprene and polystyrene with sufficient amounts of polystyrene to be finishable as a polymer crumb.

What is claimed is: 
     
       1. A process for improving the viscosity index (VI) of a base oil, comprising the steps of: sequentially polymerizing styrene, isoprene, and a polyalkenyl coupling agent to form a star polymer having the formula (A-B) n  -X wherein B is a hydrogenated block of polyisoprene having a number average molecular weight (MW 1 ) between 30,000 and 60,000, A is a block of polystyrene having a number average molecular weight (MW S ) from 3,000 to 4,000, X is a nucleus of the polymerized polyalkenyl coupling agent, and n is the number of block copolymer arms in the star polymer when coupled with 2.5 or more moles of the polyalkenyl coupling agent per mole of living block copolymer molecules;   recovering the star polymers as a solid crumb; and   blending a VI improving amount of the star polymer crumb with a base oil.   
     
     
       2. The process of claim 1, wherein the molecular weight of the polyisoprene block is 53,400. 
     
     
       3. The process of claim 2, wherein the molecular weight of the polyisoprene block is 3,100. 
     
     
       4. The process of claim 1, wherein n is the number of arms when coupled with 3 moles of divinylbenzene per mole of living block copolymer molecules. 
     
     
       5. The process of claim 1, wherein the combination of the base oil and the star polymer is a 10w-40 multigrade lubricating oil composition. 
     
     
       6. The process of claim 1 wherein the polyisoprene blocks are at least 98% hydrogenated.