alpha-Methylene-gamma-butyrolactone (MBL), a naturally occurring and biomass-sourced bifunctional monomer, contains both a highly reactive exocyclic C=C bond and a highly stable five-membered gamma-butyrolactone ring. Thus, all previous work led to exclusive vinyl-addition polymerization (VAP) product P(MBL)(VAP). Now, this work reverses this conventional chemoselectivity to enable the first ring-opening polymerization (ROP) of MBL, thereby producing exclusively unsaturated polyester, P(MBL)(ROP) with M-n up to 21.0 kg/mol. This elusive goal was achieved through uncovering the thermodynamic, catalytic, and processing conditions. A third reaction pathway has also been discovered, which is a crossover propagation between VAP and ROP processes; thus affording cross-linked polymer P(MBL)(CLP). The formation of the three types of polymers, P(MBL)(VAP); P(MBL)(CLP), and P(MBL)(ROP), can be readily controlled by adjusting the catalyst (La)/initiator (ROH) ratio, which is determined by the unique chemoselectivity of the La-X (X = OR, NR2, R) group. The resulting P(MBL)(ROP) is degradable and can be readily postfunctionalized into cross-linked or thiolated materials but, more remarkably, can also be fully recycled back to its monomer thermochernically. Computational studies provided the theoretical basis for, and a mechanistic understanding of; the three different polymerization processes and the origin of the chemoselectivity.