Polythioesters Prepared by Ring-Opening Polymerization of Cyclic Thioesters and Related Monomers

Abstract

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible polyesters with a wide range of applications; in particular, they currently stand as promising alternatives to conventional polyolefin-based "plastics". The introduction of sulfur atoms within the PHAs backbone can endow the resulting polythioesters (PTEs) with differentiated, sometimes enhanced thermal, optical and mechanical properties, thereby widening their versatility and use. Hence, PTEs have been gaining increasing attention over the past half-decade. This review highlights recent advances towards the synthesis of well-defined PTEs by ring-opening polymerization (ROP) of cyclic thioesters - namely thiolactones - as well as of S-carboxyanhydrides and thionolactones; it also covers the ring-opening copolymerization (ROCOP) of cyclic thioanhydrides or thiolactones with epoxides or episulfides. Most of the ROP reactions described are of anionic type, mediated by inorganic, organic or organometallic initiators/catalysts, along with a few enzymatic reactions as well. Emphasis is placed on the reactivity of the thio monomers, in relation to their ring-size ranging from 4- to 5-, 6- and 7-membered cycles, the nature of the catalyst/initiating systems implemented and their efficiency in terms of activity and control over the PTE molar mass, dispersity, topology, and microstructure.

Keywords: Degradability; Polythioester; Recyclability; Ring-Opening Copolymerization (ROCOP); Ring-Opening Polymerization (ROP); Thioester; Thiolactone.

Scheme 1

Chemical synthesis of poly(β‐propiothiolactone) (PβTE) from β‐propiothiolactone.

Scheme 2

ROP of NTs‐PTL towards optically active PTE. ^[8a]

Scheme 3

Proposed mechanism for the Me₂NH‐initiated ROP of NTs‐PTL. ^[8a]

Scheme 4

ROP of various α‐acylamino‐β‐thiolactones NTs^R‐PTLs. ^[8b]

Scheme 5

ROP of a N‐Boc‐cysteine derived β‐thiolactone (N ^Boc‐PenTL).

Scheme 6

Post‐polymerization functionalization of a poly(β‐thioester) obtained from the ROP of a cysteine‐derived β‐thiolactone.

Scheme 7

Synthesis and ROP of various N^R‐PenTL type β‐thiolactones.

Scheme 8

Facile chain‐end functionalization of various PTEs and subsequent depolymerization into N ^R‐PenTE.

Scheme 9

ROP of racemic β‐thiobutyrolactone (rac‐TBL) mediated by discrete yttrium catalysts towards stereoregular cyclic poly(3‐thiobutyrolactone) (P3TB).

Scheme 10

Proposed mechanism for the ROP of rac‐TBL promoted by complex Va with initiation, propagation, back‐biting, and ring‐expansion steps.

Scheme 11

Synthesis and ROP of N^R‐PTL and subsequent post‐polymerization functionalization of poly(N^RPTL).

Scheme 12

ROP of the bicyclic γ‐thiolactone ${}^{\left[221\right]}\mathrm{BTL}$ .

Scheme 13

ROCOP of cyclic thioanhydrides with episulfides towards diverse aliphatic polythioesters.

Scheme 14

ROCOP of phthalic thioanhydride (PTA) with propylene oxide (PO) towards semi‐aromatic poly(thioester‐alt‐ester)s. ^[25a]

Scheme 15

ROCOP of cyclic thioanhydrides with epoxides towards poly(thioester‐alt‐ester)s mediated by an acido‐basic cooperative catalyst. ^[25b]

Scheme 16

One‐step ROCOP of cyclic thioanhydride, epoxide and cyclic anhydride mixtures towards sequence‐controlled polyester‐b‐poly(ester‐alt‐thioester).

Scheme 17

ROCOP of N‐tosylaziridine (TAz) with phthalic thioanhydride (PTA) towards poly(thioester‐alt‐sulfonamide)s.

Scheme 18

Ring‐opening copolymerization of γ‐thiobutyrolactones with functional epoxides towards poly(ester‐alt‐thioether)s.

Scheme 19

A plausible alternative mechanism for ROCOP of γ‐thiobutyrolactone with an epoxide. ^[28a]

Scheme 20

ROCOP of episulfides with γ‐selenobutyrolactone (γ‐SeBL) towards alternating poly(thioester‐alt‐selenide)s.

Scheme 21

Early attempts at ROP of δ‐thiovalerolactone.

Scheme 22

(a) The synthesis of rac‐thiolactide and its ROP by a basic organocatalyst; (b) ROP of rac‐TLA initiated by a non‐nucleophilic amine.

Scheme 23

Ring‐opening copolymerization of thioglycolide and thiolactide.

Scheme 24

Synthetic route and ROP of dithiolactones by basic organocatalysts.

Scheme 25

ROP of nucleobase‐functionalized δ‐thiolactones.

Scheme 26

ROP of ϵ‐thiocaprolactone (TCL) initiated by inorganic bases.[ ³⁰ , ³⁵ ]

Scheme 27

ROP of ϵ‐thiocaprolactone (TCL) promoted by a basic organocatalyst and eventual addition of a thiourea.

Scheme 28

Proposed mechanisms for the ROP of TCL.

Scheme 29

One‐ and two‐step strategies for copolymerization of ϵ‐thiocaprolactone and ϵ‐caprolactone.

Scheme 30

Lipase‐catalyzed ROP of a large cyclic thioester.

Scheme 31

Typical synthesis of S‐carboxyanhydrides and their subsequent ROP using organocatalysts and a chain‐transfer agent (CTA).

Scheme 32

Plausible mechanistic pathway for the [PPN]OBz/BzOH‐mediated ROP of SCAs based on DFT calculations.

Scheme 33

Synthesis and isomerizing ROP of γ‐thionolactones.

Scheme 34

Generalization of the IROP strategy to related γ‐thionolactones.

Scheme 35

Possible mechanistic pathway for the isomerizing ROP of TnBL.