Bifunctional and Recyclable Polyesters by Chemoselective Ring-Opening Polymerization of a δ-Lactone Derived from CO2 and Butadiene

Recyclable polyesters from the direct utilization of carbon dioxide (CO2)
Published in Chemistry and Materials
Bifunctional and Recyclable Polyesters by Chemoselective Ring-Opening Polymerization of a δ-Lactone Derived from CO2 and Butadiene
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ChemRxiv
ChemRxiv ChemRxiv

Bifunctional and Recyclable Polyesters by Chemoselective Ring-Opening Polymerization of a δ-Lactone Derived from CO2 and Butadiene

When aiming at the direct use of CO2 for the preparation of advanced/value-added materials, the synthesis of CO2/olefin copolymers is very appealing but challenging. The δ-lactone 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVP), synthesized by telomerization of CO2 with 1,3-butadiene, is a promising intermediate. However, chemoselective ring-opening polymerization (ROP) of EVP is hampered by unfavorable thermodynamics and the competitive polymerization of highly reactive C=C double bonds. Herein, we report the first chemoselective ROP of EVP using a phosphazene/urea binary catalyst, affording exclusively a linear unsaturated polyester poly(EVP)ROP, with a molar mass (Mn) up to 6.5 kg·mol-1 and narrow distribution (Ð = 1.24), which can be fully recycled back to the pristine monomer, thus establishing a monomer-polymer-monomer closed-loop life cycle. Remarkably, poly(EVP)ROP features two pendent C=C double bonds per repeating unit, which show distinct reactivity and thus can be properly engaged in sequential functionalizations towards the synthesis of bifunctional polyesters. This methodology provides a facile access to bifunctional and recyclable polyesters from readily available feedstocks. In these polyesters, the carbon dioxide content reaches 33 mol% (29 wt%). The reasons for the remarkable chemoselectivity observed were investigated by Density-functional theory (DFT) calculations.

https://rdcu.be/dWmYg

The direct utilization of carbon dioxide (CO2) to produce valuable chemicals represents a most relevant and timely objective and chemists from Qingdao University of Science and Technology, Qingdao (China), Dalian Medical University, Dalian (China) and the University of Strasbourg (France) have been able to produce bifunctional and recyclable polyesters by chemoselective ring-opening polymerization of the δ-lactone 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVP), obtained by telomerization of butadiene and CO2. Although the selective ring-opening polymerization of EVP appears most attractive given the degradability and possible functionalization of the resulting aliphatic polyesters, all previous attempts have failed, owing to the stability of a six-membered disubstituted δ-lactone with low ring strain. Moreover, the conjugated double bonds could undergo Michael-addition reactions that would compete against or suppress ROP. 


This δ-lactone has now been ring-opened using a phosphazene/urea binary catalyst, to afford exclusively a linear unsaturated polyester with a molar mass (Mn) up to 16.1 kg·mol-1 and a narrow distribution (Ð < 1.6).


The resulting unsaturated polyesters have a high CO2 content (29 wt%) and feature two different C=C double bonds per repeating unit, which can be sequentially and selectively post-functionalized to synthesize bifunctional polyesters. Thus, benzyl mercaptan was used as a representative thiol and reacted with poly(EVP)ROP in DMF at 20 °C in the presence of DBU as the catalyst. The selective functionalization of the internal alkene was confirmed by NMR spectroscopy and GPC of the resulting polyester. Next, the terminal alkene was reacted with isobutyl mercaptan via photoinduced thiol-ene click reaction in the presence of 2,2-dimethoxy-2-phenylacetophenone to install the second functional group and yield a bifunctional polyester. This stepwise strategy allows the preparation of a wide range of bifunctional polymeric structures, which are not accessible by ROP of the hydrogenated derivatives of EVP.


Finally, thermolysis of poly(EVP)ROP obtained by chemoselective ring-opening polymerization of the δ-lactone EVP can be fully recycled back to the pristine monomer, thus establishing a monomer-polymer-monomer closed-loop life cycle. In these polyesters, the CO2 content reaches 33 mol% (29 wt%). The reasons for this remarkable chemoselectivity were investigated by DFT calculations.

The article can be downloaded in free Open Access from : https://rdcu.be/dWmYg

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