Advancing the Synthesis of Isocyanate-Free Poly(oxazolidones)s: Scope and Limitations

Abstract

Poly­(oxazolidone) is an emerging class of polyurethanes (PUs) that is easily accessible by an isocyanate-free pathway via the step-growth copolymerization of CO2-based monomers (bis­(α-alkylidene cyclic carbonate)­s) with primary diamines at room temperature. Here, we explore the scope and limitation of this process by investigating the influence of the diamine and the reaction conditions on the structure and macromolecular parameters of the polymer. Less hindered diamines (aliphatic and benzylic) provide selectively poly­(hydroxyoxazolidone)­s, whereas the bulkier ones (cycloaliphatic) furnish polymer chains bearing two types of linkages, oxo-urethane and hydroxyoxazolidone ones. The increase of the reaction temperature or the addition of DBU as a catalyst enables to accelerate the polymerizations. The quantitative polymer dehydration is also achieved by refluxing in acetic acid, providing a new class of unsaturated poly­(oxazolidone)­s composed of α-alkylidene oxazolidone linkages (for hindered polymers) or a mixture of α- and β-alkylidene oxazolidone linkages (for the less hindered ones). These unsaturated poly­(oxazolidone)­s present a high glass transition temperature (90 °C ≤ T g ≤ 130 °C) and a remarkable thermal stability (T d > 360 °C), rendering these polymers attractive for applications requiring high temperatures. This work is therefore opening an avenue to novel functional isocyanate-free PUs, with the pendant hydroxyl or olefin groups that are expected to be easily derivatized.

Exovinylene Cyclic Carbonates: Multifaceted CO2‐Based Building Blocks for Modern Chemistry and Polymer Science

“…Then, the latest undergoes a spontaneous cyclization by reaction between the secondary NH of the carbamate group and the adjacent ketone (Scheme 10b). With heptyl‐, benzyl‐, or cyclohexylamine, the whole transformation is complete within few hours at room temperature (r.T.) without any catalyst (Scheme 11, 30 a – c ) [91, 92] . When heated at reflux in glacial acetic acid, the hydroxy‐oxazolidone undergoes quantitative dehydration into the corresponding α‐alkylidene oxazolidone ( 31 a – c ) [91] .…”

“…With heptyl‐, benzyl‐, or cyclohexylamine, the whole transformation is complete within few hours at room temperature (r.T.) without any catalyst (Scheme 11, 30 a – c ) [91, 92] . When heated at reflux in glacial acetic acid, the hydroxy‐oxazolidone undergoes quantitative dehydration into the corresponding α‐alkylidene oxazolidone ( 31 a – c ) [91] . With secondary amines, the catalyst‐free aminolysis of αCCs at r.T. furnishes exclusively oxo‐carbamates.…”

“…Section 5.3) [93] . Optionally, with less reactive primary or secondary amines, both transformations can be accelerated by addition of a superbase (DBU) [91] …”

“…Capitalizing on the catalyst findings for the CO 2 transformation (Section 3) and the transformation of αCCs (Section 4), bis‐α‐alkylidene cyclic carbonates (bisαCCs) have been synthesized either by carboxylative cyclization of CO 2 with bispropargylic alcohols or via Heck reaction of αCCs with 1,4‐diiodobenzene (Scheme 21). For the first methodology, bispropargylic alcohols with terminal or internal alkyne functionality have been synthesized by the Grignard reaction of diketones with ethynylmagnesium bromide (path a) [91, 92, 109] or via the ring‐opening of bisepoxides by butynediol (an industrial waste) (path b) [142] . These templates have been further transformed into the corresponding bisαCCs M1–M4 with yields of 72–95 % by catalytic CO 2 fixation ( p =20–100 bar, T =25–40 °C) using binary catalysts (CuI/nBu 4 NPhO or AgOAc/Dave Phos).…”

“…polymers characterized by cyclic carbamate repeating linkages, display unique thermal and chemical features making them valuable options for the design of new high‐performances materials [149, 150] . Commonly prepared through the thermal copolymerization of epoxides with isocyanates, [151, 152] they are now accessible by the r.T. polyaddition of bisαCCs ( M1 , M2, M5 ) to primary diamines under catalyst‐free conditions (Scheme 22a) [91, 92] . The resulting polymers, showing M n up to 79 000 g mol −1 , differ slightly from conventional polyoxazolidones owing to the presence of additional hydroxyl moieties and undergo facile post‐dehydration in acetic acid at reflux [91] .…”