Branched-chain copolymeric polythioesters (PTE) were formed in good yield (similar to 87%) by chemoenzymatic reactions including thiyl radical-induced addition of 1,6- hexanedithiol to the > C = C < double bond of dimethyl 1,18-octadec-9-enedioate and transthioesterification of polyfunctional dimethyl 1,18-octadec-9-enedioate with bifunctional 1,6- hexanedithiol catalyzed by immobilized lipase from Rhizomucor miehei. The reactions were performed in vacuo at 80 C without a solvent. PTE was extracted from the reaction mixture using methyl-t-butylether and precipitated from i-hexane. The polymer structure of the i-hexane- insoluble PTE precipitate was elucidated by GPC/SEC showing an average molecular mass (M-w) of 1,857 Da corresponding to a molecular weight range of up to 24,000 Da and a maximum degree of polymerization of up to 50 monomer units. Chemical derivatization with TMSH demonstrated the formation of up to similar to 58 mol% of a branched-chain thio(S) ether, i.e., dimethyl S-9-(6-mercaptohexylthio)1,18-octadecanedioate, and small proportions (similar to 8 mol%) of a dimeric disulfide formed therefrom. The chemical structures of various low-molecular weight (< 900 Da) reaction products formed by transthioesterification, addition reaction or disulfide formation of the reactants or reaction intermediates, e.g., 1,18-octadec-9-enedioic acid methyl(O) ester 6'-S-mercaptohexyl thio(S) ester, dimethyl S-9-(6-mercaptohexylthio)-1,18-octadecanedioate, were elucidated by GC-MS. Similarly, dimethyl S-9-(6-S-methylthiohexylthio)-1,18-octadecanedioate and dimethyl 11,18,19,26-tetrathia-10,27-di-(7-carboxymethylheptyl) hexatriacontane-1,36-dioate were detected in the reaction mixtures after derivatization with trimethylsulfonium hydroxide.