Poly(1-hexene-co-maleic anhydride) was synthesized by addition polymerization of 1:1 of 1-hexene and maleic anhydride using 1% benzoyl peroxide as initiator. The copolymer has been branched by different lengths of linear alkyl groups (C11-C22) via melt process esterification reaction with aliphatic alcohols using 1% para toluene sulfonic acid as catalyst. The prepared copolymers were purified and characterized by FTIR and NMR spectroscopy. All analyses indicate ring-opening reaction of anhydride group constituted backbone chains with aliphatic alcohols, which confirm the validity of the proposed chemical structures of copolymers. Molecular weight determination was confirmed by GPC and end group analysis (ASTM D1386-98) and found that 1-hexene monomer constitutes the largest portion in poly(1-hexene-co-maleic anhydride) backbone.