A copolymer is formed by emulsion polymerization and then hydrogenated to obtain a thermooxidatively stable composition. The copolymer is prepared from two monomer classes. The first monomeric class comprises a fluorodiene of the structure ##STR1## wherein substituent a is independently hydrogen or fluorine, R is hydrogen or a fluoro alkyl group containing from 1 to about 4 carbon atoms and containing at least three fluoro atoms, with the proviso that both R's are not hydrogen, R.sub.1 and R.sub.2 are independently fluorine, hydrogen or a fluoro alkyl group containing from 1 to about 4 carbon atoms and containing at least three fluorine atoms.The second monomeric class is (a) a hydrocarbon diene comprising a conjugated diene, a branched conjugated diene or mixtures thereof containing from 4 to about 8 carbon atoms or a mixture of (a) with (b) a monomer of the general formula CH.sub.2 .dbd.CR.sub.3 X wherein R.sub.3 is hydrogen or an alkyl group containing from 1 to 4 carbon atoms, and X is 2-pyridyl, 4-pyridyl, --COOR.sub.4, CONR.sub.5 R.sub.6 or --COOR.sub.7 OR.sub.4 wherein R.sub.4 is an alkyl group containing from 1 to about 4 carbon atoms, --CH.sub.2 CF.sub.3 or --CH.sub.2 CF.sub.2 CF.sub.2 H, R.sub.5 and R.sub.6 are alkyl groups containing from 1 to about 4 carbon atoms and R.sub.7 is an alkylene group containing from 1 to about 4 carbon atoms. The mole ratio of hydrocarbon diene (a): CH.sub.2 .dbd.CR.sub.3 X (b), when (b) is present, is from 1:7 to about 7:1 and the mole ratio of first monomer:second monomer is from about 4:3 to about 2:3. The copolymer thus obtained is hydrogenated in the presence of a transition metal catalyst and a trialkyl aluminum wherein the alkyl group contains from 1 to about 4 carbon atoms and further in the absence of boron trifluoride or boron trifluoride etherate. Additionally, a complexing agent for said transition metal catalyst is employed and said transition metal catalyst is deactivated after hydrogenation by the use of a second complexing agent, in the absence of air.