Method of making a copolymer useful as viscosity index improving additive for hydraulic fluid

A method of making a viscosity index improving additive for a hydraulic fluid, comprising:

• heating a reaction mixture to a reaction temperature from about 75° C. to about 95° C., said reaction mixture comprising:
• from about 55 weight percent to about 99.5 weight percent of a first monomer selected form the group consisting of (C8-C15)alkyl meth)acrylates and mixtures thereof;
• from about 0.5 weight percent to about 45 weight percent of a second monomer selected from the group consisting of (C1-C7)alkyl (meth)acrylates, (C16-C24)alkyl (meth)acrylates and mixtures thereof;
• an effective amount of a polymerization initiator selected from the group consisting of 2,2′-azobis (2-methylbutanenitrile), 2,2′-azobis (2,4-dimethylpentanenitrile), and mixtures thereof;
• and
• a hydrocarbon diluent;
• and
• maintaining the reaction mixture at the reaction temperature of about 75° C. to about 95° C., for a period of time effective to allow copolymerization of the monomers.