Degradable Cyclooctadiene/Acetal Copolymers: Versatile Precursors to 1,4-Hydroxytelechelic Polybutadiene and Hydroxytelechelic Polyethylene

In addition to environmental issues associated with polymers possessing easily cleavable links, the selective degradation of copolymers at one of the monomer units is a convenient route to telechelic polymers. Ring-opening metathesis polymerization (ROMP) initiated by (Cy₃P)₂Cl₂Ru=CHR (Cy = cyclohexyl; Ph = phenyl; R = CHCPh₂ or Ph) was used for the synthesis of cyclooctadiene (COD)/4,7-dihydro-1,3-dioxepin copolymers. 1,4-Hydroxytelechelic polybutadiene (HTPBD) is obtained upon fragmentation of a copolymer of COD and 4,7-dihydro-2-phenyl-1,3-dioxepin at the benzylidene acetal functionalities by stirring with acids (aqueous HCl/MeOH, aqueous H₂SO₄, or trifluoroacetic acid (TFAA)/MeOH in CH₂Cl₂). For the benzylidene acetal the copolymerization reaction may be run in neat monomer with no additional solvent. Reaction quenching, and polymer cleavage, precipitation, and purification may be effected in one step by stirring the viscous reaction product in MeOH containing 10% HCl. A two-step procedure using trifluoroacetic anhydride (TFAA)/acetic acid (AcOH) in CH₂Cl₂ followed by NaOMe/MeOH in THF was employed for the degradation of a COD/4,7-dihydro-1,3-dioxepin copolymer. Acetal incorporation and thus the molecular weights of the resulting telechelics are dependent upon the feed ratios. The percent methylene acetal incorporated into the COD/acetal copolymer is ~60% of the amount of acetal in the feed (e.g., for COD/acetal feed = 2:1, COD/acetal incorporation = ~4:1). For the less reactive benzylidene acetal, incorporation is ~40% of the amount of acetal in the feed. Polydispersities for 1,4-HTPBD prepared from the COD/benzylidene acetal copolymer are typically narrow (~1.1–1.3). It was also demonstrated that the COD/benzylidene acetal copolymer may be simultaneously hydrogenated and degraded in one step using p-toluenesulfonohydrazide to produce hydroxytelechelic polyethylene (HTPE).