DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, there is provided a hard contact lens material made of a copolymer comprising at least two kinds of fumaric acid diesters at least one of which is a dialkyl fumarate as main components.
Representative examples of the above-mentioned dialkyl fumarate are those represented by the general formula (I): ##STR1## wherein R.
sub.
1 and R.
sub.
2 are the same or different, and each is a straight alkyl group, a branched alkyl group or a cyclic alkyl group having 1 to 13 carbon atoms.
Examples of the dialkyl fumarate are, for instance, a straight or branched dialkyl fumarate such as diethyl fumarate, dipropyl fumarate, diisopropyl fumarate, dibutyl fumarate, di-tert-butyl fumarate, dipentyl fumarate, dioctyl fumarate, di-tert-amyl fumarate, didodecyl fumarate, di(2-ethylhexyl) fumarate, and the like; cyclic alkyl fumarate such as dicyclohexyl fumarate, di(tert-butylcyclohexyl) fumarate, bis(trimethylcyclohexyl) fumarate, and the like.
Among the above-mentioned alkyl fumarates, di-tert-butyl fumarate (hereinafter referred to as DtBF) represented by the formula (II): ##STR2## diisopropyl fumarate, dicyclohexyl fumarate, and the like can be suitably used since these alkyl fumarates are excellent in polymerizability, and particularly, DtBF is preferably used since it is excellent in homopolymerizability and copolymerizability.
However, since homopolymers of the above-mentioned fumaric acid dialkyl esters are generally hard and brittle, it is preferable that these fumaric acid dialkyl esters are copolymerized with a monomer, for instance, diethyl fumarate, di-n-butyl fumarate, di-n-octyl fumarate, or the like to improve the mechanical strength (brittleness).
Examples of fumaric acid diesters other than the above-mentioned dialkyl fumarates are, for instance, di(fluoroalkyl) fumarate, di(silicon-containing alkyl) fumarate, and the like.
Representative examples of the above-mentioned di(fluoroalkyl) fumarate are those represented by the general formula (III): ##STR3## wherein A and B are the same or different, and each is --C
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LCD 
