It is preferred that R.sup.1 is alkyl containing 1 to 20 carbon atoms, morepreferred that it contains 1 to 4 carbon atoms, and especially preferred that R.sup.1 is methyl.One such preferred dipolymer has about 60 to 100 methyl groups (excluding methyl groups which are esters) per 1000 methylene groups in the polymer, and contains, per 100 methyl branches, about 45 to about 65 ethyl branches, about