Polyperoxide, as a new kind of thermally degradable polymer, is synthesized by the radical alternating copolymerization of a 1,3-diene monomer with oxygen. The introduction of polyperoxide units into any other polymer is useful for the fragmentation of polymers and a change in the physical properties of polymeric materials. In the present study, we demonstrate the synthesis and fragmentation of poly (L-lactic acid) (PLLA) modified with thermally degradable branching and cross-linking points, which consist of a repeating polyperoxide structure as the degradable junctions. We prepared several PLLA with a dienyl group at the alpha- and/or omega-chain ends by the anionic and metal-catalyzed polymerizations of L-lactide. Radical alternating copolymerization of the mono- and difunctional PLLA with oxygen was carried out to obtain branched polymers and gels. These branched and cross-linked PLLA materials readily degraded upon heating to yield linear PLLA chains.