The present invention relates to polymeric compositions useful in the manufacture of biocompatible medical devices. More particularly, the present invention relates to certain cationic monomers capable of polymerization to form polymeric compositions having desirable physical characteristics useful in the manufacture of ophthalmic devices. Such properties include the ability to extract the polymerized medical devices with water. This avoids the use of organic solvents as is typical in the art. The polymer compositions comprise polymerized silicon-containing monomers end-capped with polymerizable cationic hydrophilic groups.