Periodic Vinyl Copolymers Containing γ-Butyrolactone via ADMET Polymerization of Designed Diene Monomers with Built-in Sequence

The synthesis of a new family of periodic copolymers containing gamma-butyrolactones by acyclic diene metathesis polymerization (ADMET) and their thermal properties are presented. Two symmetric diene monomers, M6 and M8, were designed. Both monomers contain two gamma-butyrolactone units, but they are different in the length of methylene spacers between cyclic structures and terminal alkenes. The monomers have been prepared by a two-step approach; first, the atom transfer radical addition (ATRA) of diethyl meso-2,5-diiodohexanedioate with either 1,5-hexadiene or 1,7-octadiene was conducted to yield intermediates containing two gamma-iodo ester sequences; subsequently, the specific sequence was transformed into gamma-butyrolactone unit via intramolecular cyclization upon heating. The two monomers were polymerized using two Grubbs catalysts (Grubbs I and Grubbs II) to produce four polymers with moderate to high molecular weights (P6-1, P6-2, P8-1, and P8-2) and hydrogenation of which gave the final saturated polymers. The expected periodic copolymers have been obtained and were characterized with a variety of methods, indicating that the gamma-butyrolactone units could endure the polymerization and hydrogenation. Polymers catalyzed by Grubbs II catalyst suffer from chain heterogeneity due to severe olefin isomerization. Thermal properties of the polymers were investigated via thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA measurements show that these polymers are stable up to 350 degrees C. DSC results demonstrate that the glass transition and melting behaviors of the polymers are not only affected by the rigidity of gamma-butyrolactone units in polyethylene chains but also dependent on the methylene spacer length and chain homogeneity. Copolymerization of M6 or M8 with 1,9-decadiene resulted in random copolymers with lower gamma-butyrolactone content and less regular chain structure. These copolymers exhibit lower T-g or T-m compared with the periodic copolymers.