Mechanistic Study of Palladium-Catalyzed Hydroesterificative Copolymerization of Vinyl Benzyl Alcohol and CO

Abstract

The copolymerization of vinyl benzyl alcohol (VBA) and carbon monoxide (CO) to give a new polyester poly(VBA-CO) has been achieved via palladium-catalyzed hydroesterification. Reaction conditions involve moderate temperatures, moderate to low CO pressures, and low catalyst loadings to give a low molar mass (M n ∼ 3−4 kg/mol) polymer as a ∼2:1 mixture of linear to branched repeat units. The polymer molar mass increase is consistent with a step-growth polymerization mechanism, and ester yields of >97% are achieved within 24 h. However, increases in M n cease beyond 16 h. Control experiments indicate that the degree of polymerization is limited due to a combination of side reactions such as alcoholic end-group oxidation, hydroxycarbonylation, and alcohol acetylation, which lead to the degradation of monomeric and polymeric end groups. When a less promiscuous substrate is used such as 10-undecenol, higher molar masses (M n ∼ 16 kg/mol) are achieved. This method has the potential to be a mild route to new polyester architectures with appropriate mitigation of side reactions.

Synthesis of Poly(Ester-Ether) Polymers via Hydroesterificative Polymerization of α,ω-Enol Ethers

“…This was observed in the 10-undecenol hydroesterificative polymerizations as well, and may be due to different branched/linear domains in the polyester. 15,16 In comparison to poly-1, the melting temperature increased with increasing aliphatic chain length, from 53 °C for poly-1 up to 62 °C for poly-3. The degradation temperature also increased from 247 °C for poly-1 to 277 °C for poly-3.…”

“…For example, while an in situ generated catalyst from Pd­(OAc) 2 /dtbpx/MSA (dtbpx = L1; MSA = methanesulfonic acid) is highly active for alkene hydroesterification, reactions with undecenol result in significant formation of undecenal and acetal and limit polymer growth . Similarly, our group found that isomerized internal alkenes of undecenol are often unreactive toward hydroesterification and can contribute to significant functional group imbalance, limiting the molecular weight growth of these step-growth polymerizations …”

“…The first example of hydroesterificative polymerization was reported by the Mecking group in 2009, demonstrating that a Co 2 (CO) 8 /pyridine catalyst system could perform hydroesterificative head-to-tail polymerization of 10-undecenol under forcing conditions (Figure A) . More recently, two examples of 10-undecenol hydroesterification employing Pd catalysis under milder conditions have been reported. , …”

“…15 Similarly, our group found that isomerized internal alkenes of undecenol are often unreactive toward hydroesterification and can contribute to significant functional group imbalance, limiting the molecular weight growth of these step-growth polymerizations. 16 Based on these observations, we envisioned that modification of 10-undecenol through etherification would inhibit aldehyde formation, while also opening up this polymerization strategy to more structurally diverse monomers (Figure 1B). Further, introduction of ether linkages into a polymer can increase the biodegradability and decrease the crystallinity of the resulting polyester.…”

“…13 More recently, two examples of 10-undecenol hydroesterification employing Pd catalysis under milder conditions have been reported. 15,16 Despite these recent examples, this polymerization technique remains underexplored. An inherent challenge with practical application of hydroesterificative polymerizations is the step-growth nature of the polymerization, which ultimately requires extremely high-yielding and efficient catalysis.…”