A viscosity index improver containing a copolymer (A) whose essential constituent monomer is a monomer (a) having a number average molecular weight of 800 to 4,000 represented by the following formula (1), the copolymer (A) having a solubility parameter in the range of 9.00 to 9.40:

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wherein R.sup.1 is a hydrogen atom or a methyl group; X.sup.1 is a group represented by O, O(AO).sub.m, or NH, AO is a C2-C4 alkyleneoxy group, m is an integer of 1 to 10, each AO may be the same or different when m is 2 or more, and the (AO).sub.m moieties may be randomly bonded or block-bonded; R.sup.2 is a residue after removal of one hydrogen atom from a hydrocarbon polymer whose essential constituent monomer is butadiene in which the butadiene is present in a proportion of 50% by weight or more based on the weight of R.sup.2, or after removal of one hydrogen atom from a polymer formed by partial hydrogenation of the hydrocarbon polymer; and p represents a number of 0 or 1.

Selected comb polymers contain specified amounts of macromonomer and nitrogen-containing (meth)acrylates. Lubricant compositions can contain such comb polymers. The selected comb polymers can be used as solubilizers in lubricant compositions, especially in engine oil (EO) compositions.

Selected comb polymers contain specified amounts of macromonomer and nitrogen-containing (meth)acrylates. Lubricant compositions can contain such comb polymers. The selected comb polymers can be used as solubilizers in lubricant compositions, especially in engine oil (EO) compositions.

The present invention provides an engine oil composition including 70 to 95 percent by weight of a base oil and 0.01 to 15 percent by weight of a dispersant comb polymer, based on the overall weight of the engine oil composition. The dispersant comb polymer consists of 13.7% by weight of a macromonomer, which is an ester of methacrylic acid and a hydroxylated hydrogenated polybutadiene with Mn of 4750 g/mol; 51.5% by weight of n-butyl methacrylate; 17.3% by weight of LMA; 11.2% by weight of styrene; 0.2% by weight of methyl methacrylate; and 6.10% by weight of N,N-dimethylaminoethyl methacrylate. The modified dispersant inhibitor package contains 30 wt % or less of succinimide type dispersant based on the overall weight of the modified dispersant inhibitor additive package, and the engine oil composition has an SAE viscosity grade of 0W-X, wherein X is 30 or less.

A lubricant composition of (a) an oil of lubricating viscosity; (b) a poly(meth)acrylate ester polymer comprising 0.5 to 5 weight percent of amine nitrogen-containing dispersant monomer units and (c) an esterified copolymer with a backbone comprising units derived from (i) an -olefin monomer of at least about 6 carbon atoms, (ii) an ethylenically unsaturated carboxylic acid or derivative thereof, further containing nitrogen functionality in groups pendant from the copolymer backbone, provides a good combination of seal performance and oxidation resistance.

The present invention provides a process for preparing a polymer, employing the steps of: (1) contacting a free radical initiator; a chain transfer agent containing a thiocarbonyl thio group and a free radical leaving group; and a radically polymerisable monomer, to form a polymer chain; and (2) contacting the polymer chain of step (1) with at least one of a polyvalent coupling agent, a polymerisation inhibitor, a grafting acylating agent, an amine and an oil of lubricating viscosity. The invention further provides compositions and uses for the polymer.