groups among them methyl and ethyl groups are preferred with methyl group being particularly preferred the molar ratio of the lower alkyl vinyl ether to maleic anhydride in the partially crosslinked polymer ranges from 40 60 to 60 40 with a range of from 45 55 to 55 45 being particularly preferred examples of the terminally unsaturated diene compound to be used as a crosslinking agent include c 6 18 compounds such as 1 5 hexadiene 1 7 octadiene 1 9 decadiene 1 11 dodecadiene 1 15 hexadecadiene divinylbenzene ethylene dimethacrylate methacrylic anhydride and diaryl phthalate among them terminally unsaturated alkadienes are preferred with 1 9 decadiene being particularly preferred the crosslinking degree of the partially crosslinked polymer by the above described crosslinking agent is preferred to be 1 to 5 especially 2 to 4 and its average molecular weight is preferably at least 1000 000 such a partially crosslinked polymer is available as a uniform suspension by polymerizing a lower alkyl vinyl ether and maleic anhydride in a proper solvent in the presence of a crosslinking agent a terminal unsaturated diene compound and a polymerization initiator at 60 to 80 u00b0 c as the solvent and polymerization initiator isopropyl acetate an the like and 2 2 u2032 azobis 2 methylbutanenitrile decanoyl peroxide and the like can be used respectively as the partially crosslinked polymer commercially available products can also be employed examples include u201c stabileze 06 u201d and u201c stabileze qm u201d each trade name product of isp inc that is a methyl vinyl ether maleic anhydride copolymer partially crosslinked with 1 9 decadiene as described above by the hydrolysis to convert the.