N-Heterocyclic Olefin-Based (Co)polymerization of a Challenging Monomer: Homopolymerization of ω-Pentadecalactone and Its Copolymers with γ-Butyrolactone, δ-Valerolactone, and ε-Caprolactone

A setup consisting of N-heterocylic olefins (NHOs) and several simple Lewis acids (such as MgCl₂ or LiCl) was employed to homopolymerize ω-pentadecalactone (PDL) and to copolymerize it with five-, six-, and seven-membered lactones (γ-butyrolactone (GBL), δ-valerolactone (VL), and ε-caprolactone (CL)). Also, the copolymerization of GBL with VL and CL was investigated separately. This dual catalytic approach succeeded for the entropically driven high-temperature polymerization of PDL in course of fast, operationally simple polymerization procedures. PPDL could be generated in short reaction times to reach high conversion (85–97%), whereby the polymerization rates are significantly modulated by the metal halide cocatalyst (ranging from virtually 0 to >80% conversion after 15 min, 1% NHO loading). Application of mildly activating Lewis acids ensured that the frequently encountered excessive transesterification was reduced to yield relatively well-controlled polyester (Mₙ up to 40 kg/mol, ĐM = 1.5–1.8). The 1:1 copolymerization of PDL and GBL—unifying two lactones with thermodynamically opposite polymerization preferences—was observed to be strongly dependent on the applied Lewis pair, with yield (15–50%) and GBL content (5–22%, by ¹³C NMR) determined by the Lewis acid. Likewise, GBL/CL and GBL/VL copolymers displayed varying, catalyst-dependent compositions and were obtained as well-defined polyester (ĐM = 1.1–1.2) with intermediate molecular weight (2–8 kg/mol) if a suitable cocatalyst pair was chosen. One-pot 1:1 PDL/VL copolymerizations resulted in high or low PDL content, as well as virtually exclusive VL consumption, if Lewis pairs containing YCl₃, ZnI₂, or MgI₂ were employed, with the reaction temperature a convenient tool to further manipulate the polymer structure. Finally, PDL/CL copolymers were readily formed, reaching high or quantitative conversion (Mₙ = 10–30 kg/mol) whereby 50% PDL content and perfectly random polymer structures were accessible. For selected copolymers the thermal properties were elucidated by DSC measurements.