The synthesis of functional copolyesters that contain pendant double bonds in their structures is reported by ring opening copolymerization of renewable monomer Tulipalin A (alpha-methylene-y-butyrolactone, MBL) and epsilon-caprolactone (CL) using either coordination insertion or monomer activated mechanisms. Aluminum tris(isopropoxide) (Al((OPr)-Pr-i)(3)) was successfully used as a cheap and commonly available catalyst for the coordination insertion ring-opening copolymerization, and functional copolyesters with dispersities below 1.2 and with contents of MBL units in the copolymer up to 25 mol % were prepared. In addition, linear or multiarm copolymers of MBL with CL were synthesized by cationic ring-opening copolymerization that was conducted using protic acid as a "metal-free" catalyst in combination with mono-or multihydroxyl alcohols as initiators. The molecular characteristics and composition of the functional copolyesters were determined by GPC, NMR, two-dimensional NMR, and MALDI-TOF spectroscopy. The effect of MBL content on thermal properties of the copolyesters was investigated using DCS and TGA analyses. The availability of resulting functional copolyesters containing pendant double bonds toward postfunctionalization was demonstrated by either thermal or photochemical thiol-ene reactions with benzothioxanthene fluorophore or N-acetylcysteine, respectively.