NOVELTY - A soluble polyfunctional copolymer is polyfunctional copolymer obtained by copolymerizing monofunctional aromatic compound (A), bifunctional compound (B) and chain transfer agent (C). At least one portion of monofunctional aromatic compound (A) is monofunctional aromatic compound (A1) (I) containing one or more sulfur atom. The polyfunctional copolymer has weight average molecular weight of 2000-1000000, and is soluble in toluene, xylene, tetrahydrofuran, dichloromethane, dichloroethane, or chloroform.    USE - Soluble polyfunctional copolymer is used for resin composition for curable resin composition for resin cured material used as optical lens and prism (all claimed), optical pick-up of imaging camera, sensor, spectacles, recording medium, display apparatus, illuminating device, solar cell panel, reflection preventing material, copier and printing apparatus.    ADVANTAGE - The soluble polyfunctional copolymer has high molecular weight and reactivity, excellent solubility in solvent and monomer, and low shrinkage rate and birefringence. The curable resin composition using the soluble polyfunctional copolymer has excellent moldability and metal mold transferability, and provides resin cured material having excellent optical characteristics and light transmittance, and high refractive index.    DETAILED DESCRIPTION - A soluble polyfunctional copolymer is polyfunctional copolymer obtained by copolymerizing monofunctional aromatic compound (A) containing one unsaturated double bond, bifunctional compound (B) containing two unsaturated double bonds, and chain transfer agent (C). The polyfunctional copolymer contains a reactive group derived from bifunctional compound (B) in a side chain, and structural unit derived from chain transfer agent (C) at the terminal. At least one portion of monofunctional aromatic compound (A) is monofunctional aromatic compound (A1) of formula (I) containing one or more sulfur atom. The polyfunctional copolymer has weight average molecular weight of 2000-1000000, and is soluble in toluene, xylene, tetrahydrofuran, dichloromethane, dichloroethane, or chloroform.    R2-R6=H, Cl, Br, 1-4C alkyl, 1-4C alkyloxy, 1-4C alkylthio, 6-12C aryl, 6-12C aryloxy, 7-11C aralkyl or -S-Ar1;    X1=direct bond, O, S, or 1-4C alkylene, 1-4C oxyalkylene, 1-4C thioalkylene, 1-4C alkyleneoxy or 1-4C alkylenethio (all optionally substituted by substituent (a));    substituent (a)=Cl, Br, 1-4C alkyloxy, 1-4C alkylthio, 6-10C aryl, 6-10C aryloxy or 6-10C arylthio;    R1=acryloyl, methacryloyl, vinyl, 8-16C vinyl aryl or 9-17C vinyl aryl acryloyl (all optionally substituted by Cl, Br, 1-4C alkyl, 1-4C alkyloxy or 1-4C alkylthio), phenyl or phenylthio;and    Ar1=6-14-membered aryl or 5-14-membered complex aryl (both optionally substituted by Cl, Br, 1-4C alkyl, 1-4C alkyloxy or 1-4C alkylthio), phenyl or phenylthio.    An INDEPENDENT CLAIM is included for curable resin composition, which is formed by mixing monomer containing 1 or more polymerizable functional group, and polymerization initiator with the soluble polyfunctional copolymer.