WO2019116006A1

WO2019116006A1 - Polymerisation of cyclic esters and cyclic amides

Info

Publication number: WO2019116006A1
Application number: PCT/GB2018/053539
Authority: WO
Inventors: Dermot O'hare; Jean-Charles BUFFET; Zoe R. TURNER
Original assignee: Scg Chemicals Co., Ltd.
Priority date: 2017-12-13
Filing date: 2018-12-06
Publication date: 2019-06-20
Also published as: GB201720778D0

Abstract

A catalytic process for the polymerisation of cyclic ester and cyclic amides is described, which uses, as catalysts, Rare-earth complexes comprising permethylpentalene or (hydro)permethylpentalene ligands. The complexes are notably more active in lactide polymerisation that zirconium-containing analogues.

Description

POLYMERISATION OF CYCLIC ESTERS AND CYCLIC AMIDES

INTRODUCTION

[0001] The present invention relates to a catalytic process for the ring opening polymerisation (ROP) of a cyclic ester or a cyclic amide. More particularly, the catalytic process employs, as catalysts, rare earth metal complexes having permethylpentalene or (hydro)permethylpentalene ligands.

BACKGROUND OF THE INVENTION

[0002] Poly(lactic acids) (PLAs) have been studied intensively over the past few decades due to the promise they have shown as potential alternatives to petroleum-based polymers for uses a plastics, fibres and coatings. Moreover, since PLAs are both biodegradable and biocompatible, they are of equal value to the field of medicine, wherein their versatile physical properties make them suitable for in vivo applications (e.g. as media for controlled drug delivery devices).

[0003] Lactic acid forms PLA upon polycondensation. However, the fact that this reaction is in equilibrium, and the difficulties in completely removing water, makes it difficult to obtain PLAs of high molecular weight. With this in mind, ring opening polymerisation (ROP) of lactides is the most efficient route to PLAs with controlled molecular weights and narrow molecular weight distributions.

[0004] Metal complexes useful for initiating ring opening polymerisation of lactides are known.

[0005] Wenshan Ren et al, Inorganic Chemistry Communications, 30, (2013), 26-28 report that benzyl thorium metallocenes [n⁵-1 ,3-(Me3C)2CsH3]2 Th(CH2Ph)2 (1) and [n⁵-1 ,2,4-(Me3C)3CsH2]2 Th(CH2Ph)2 (2) can initiate the ring opening polymerisation of racemic-lactide (rac-LA) under mild conditions. Complete conversion of 500 equiv of lactide occurs within 5h at 40°C in dichloromethane at [rac-LA]=1.0 mol L ¹ , and the molecular weight distribution is very narrow (ca.1.15) over the entire monomer-to-initiator range, indicating a single-site catalyst system.

[0006] Yalan Ning et al, Organometallics 2008, 27, 5632-5640 report four neutral zirconocene bis(ester enolate) and non-zirconocene bis(alkoxy) complexes employed for ring-opening polymerisations and chain transfer polymerisations of L-lactide ( -LA) and e- cap ro lactone (e- CL). [0007] In spite of the above, due to the high value that industry places on such materials, there remains a need for catalysts/initiators capable of effectively polymerising cyclic esters (such as lactides) and cyclic amides.

[0008] The present invention was devised with the foregoing in mind.

SUMMARY OF THE INVENTION

[0009] According to a first aspect of the present invention there is provided a process for the ring opening polymerisation (ROP) of a cyclic ester or a cyclic amide, the process comprising the step of contacting a compound having a structure according to formula (I) defined herein with one or more cyclic esters or cyclic amides.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

[0010] The term“alkyl” as used herein refers to a straight or branched chain alkyl moieties, typically having 1 , 2, 3, 4, 5 or 6 carbon atoms. This term includes reference to groups such as methyl, ethyl, propyl (n-propyl or isopropyl), butyl (n-butyl, sec-butyl or tert-butyl), pentyl, hexyl and the like. In particular, an alkyl may have 1 , 2, 3 or 4 carbon atoms.

[0011] The term“alkenyl” as used herein refers to straight or branched chain alkenyl moieties, typically having 1 , 2, 3, 4, 5 or 6 carbon atoms. The term includes reference to alkenyl moieties containing 1 , 2 or 3 carbon-carbon double bonds (C=C). This term includes reference to groups such as ethenyl (vinyl), propenyl (allyl), butenyl, pentenyl and hexenyl, as well as both the cis and trans isomers thereof.

[0012] The term“alkynyl” as used herein refers to straight or branched chain alkynyl moieties, typically having 1 , 2, 3, 4, 5 or 6 carbon atoms. The term includes reference to alkynyl moieties containing 1 , 2 or 3 carbon-carbon triple bonds (CºC). This term includes reference to groups such as ethynyl, propynyl, butynyl, pentynyl and hexynyl.

[0013] The term“alkoxy” as used herein refers to -O-alkyl, wherein alkyl is straight or branched chain and comprises 1 , 2, 3, 4, 5 or 6 carbon atoms. In one class of embodiments, alkoxy has 1 , 2, 3 or 4 carbon atoms. This term includes reference to groups such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, tert-butoxy, pentoxy, hexoxy and the like.

[0014] The term "aryl" as used herein refers to an aromatic ring system comprising 6, 7, 8, 9 or 10 ring carbon atoms. Aryl is often phenyl but may be a polycyclic ring system, having two or more rings, at least one of which is aromatic. This term includes reference to groups such as phenyl, naphthyl and the like.

[0015] The term “aryloxy” as used herein refers to -O-aryl, wherein aryl has any of the definitions discussed herein. Also encompassed by this term are aryloxy groups in having an alkylene chain situated between the O and aryl groups..

[0016] The term "halogen" or“halo” as used herein refers to F, Cl, Br or I. In a particular, halogen may be F or Cl, of which Cl is more common.

[0017] The term“substituted” as used herein in reference to a moiety means that one or more, especially up to 5, more especially 1 , 2 or 3, of the hydrogen atoms in said moiety are replaced independently of each other by the corresponding number of the described substituents. The term“optionally substituted” as used herein means substituted or unsubstituted.

[0018] It will, of course, be understood that substituents are only at positions where they are chemically possible, the person skilled in the art being able to decide (either experimentally or theoretically) without inappropriate effort whether a particular substitution is possible. For example, amino or hydroxy groups with free hydrogen may be unstable if bound to carbon atoms with unsaturated (e.g. olefinic) bonds. Additionally, it will of course be understood that the substituents described herein may themselves be substituted by any substituent, subject to the aforementioned restriction to appropriate substitutions as recognised by the skilled person.

[0019] The terms “cyclic esters” and “cyclic amides” as used herein refer to heterocycles containing at least one ester or amide moiety. It will be understood that lactides, lactones and lactams are encompassed by these terms.

Processes of the invention

[0020] The first aspect of the invention provides a process for the ring opening polymerisation (ROP) of a cyclic ester or a cyclic amide, the process comprising the step of contacting a compound having a structure according to formula (I) shown below with one or more cyclic esters or cyclic amides:

wherein

M¹ and M² are each independently a group 3 metal or a lanthanide,

X¹ is selected from halo, (1 -6C)alkoxy, aryl(1 -3C)alkoxy and aryloxy, wherein the (1 -6C)alkoxy, aryl(1 -3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1 -5C)alkyl, (2-5C)alkenyl, (2-5C)alkynyl and (1-5C)alkoxy,

X² is absent, or is selected from halo, (1 -6C)alkoxy, aryl(1 -3C)alkoxy and aryloxy, wherein the (1 -6C)alkoxy, aryl(1 -3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-5C)alkyl, (2- 5C)alkenyl, (2-5C)alkynyl and (1-5C)alkoxy,

or X² is a neutral ligand (e.g. a complexed solvent), and

Li and l_2 are each independently selected from permethylpentalene and (hydro)permethylpentalene.

[0021] When compared with existing permethylpentalene-based catalysts for the polymerisation of polar monomers, the compounds of formula (I) are noticeably more active. In particular, the rare earth metal complexes of formula (I) exhibit a stark increase in catalytic activity over zirconium-based analogues.

[0022] M¹ and M² may be independently selected from any group 3 metal or any lanthanide. It will be understood that the compounds of formula (I) are neutral (i.e. they carry no net charge). Therefore, M¹ and M² must each have a complete coordination sphere of charge balancing ligands X¹ , X², L¹ and L².

[0023] In an embodiment, M¹ and M² are each independently selected from scandium, yttrium, lanthanum, cerium and lutetium. Suitably, M¹ and M² are each independently selected from yttrium and lutetium.

[0024] In an embodiment, M¹ and M² are the same. Suitably, M¹ and M² are both yttrium or lutetium. [0025] In an embodiment, X¹ is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy.

[0026] It will be understood that X¹ may associated with M¹ and M² via electrostatic interactions, or by a mixture of covalent and electrostatic interactions, all of which are shown herein, for simplicity, as solid bonds. When X¹ is halo, it associates with M¹ and M² as depicted below:

When X¹ is an oxygen-containing ligand, it associates with M¹ and M² as depicted below:

[0027] In an embodiment, X¹ is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy.

[0028] In an embodiment, X¹ is selected from chloro, bromo, (1-4C)alkoxy, benzyloxy and phenoxy, wherein the (1-4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy. [0029] In an embodiment, X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from (1-4C)alkyl and (1-4C)alkoxy.

[0030] In an embodiment, X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from methyl, isopropyl and tertbutyl.

[0031] In an embodiment, X¹ v selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with two substituents selected from methyl, isopropyl and tertbutyl.

[0032] In an embodiment, X¹ is selected from chloro, benzyloxy, -0-2,6-dimethyl-phenoxy, -O- 2,6-diisopropyl-phenoxy and -0-2,4-ditertbutyl-phenoxy.

[0033] In an embodiment, X² is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy, or X² is a complexed solvent or another neutral ligand capable of completing the coordination sphere around M¹/M².

[0034] In an embodiment, X² is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy, or X² is a complexed solvent.

[0035] In an embodiment, X² is selected from chloro, bromo, (1-4C)alkoxy, benzyloxy and phenoxy, wherein the (1-4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy, or X² is a complexed solvent.

[0036] In an embodiment, X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from (1-4C)alkyl and (1-4C)alkoxy, or X² is a complexed solvent.

[0037] In an embodiment, X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from methyl, isopropyl and tertbutyl, or X² is a complexed solvent.

[0038] In an embodiment, X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with two substituents selected from methyl, isopropyl and tertbutyl, or X² is a complexed solvent. [0039] In an embodiment, X² is selected from chloro, benzyloxy, -0-2,6-dimethyl-phenoxy, -O- 2,6-diisopropyl-phenoxy and -0-2,4-ditertbutyl-phenoxy, or X² is a complexed solvent.

[0040] In those instances where X² is a solvent or another neutral ligand capable of completing the coordination sphere around M¹/M², it is suitably selected from an ether or pyridine. It will be understood that the solvent bonding is likely to proceed via an electrostatic interaction (rather than a covalent interaction) between the heteroatom of the solvent and MVM²) via its heteroatom). More suitably, when X² is a complexed solvent, it is tetrahydrofuran.

[0041] It will be appreciated that in the compounds of formula (I), any of those definitions recited in respect of X¹ may be taken in combination with any of those definitions recited in respect of X².

[0042] U and L² are independently selected from permethylpentalene and (hydro)permethylpentalene.

[0043] Permethylpentalene (also denoted herein as Pn*) will be understood to refer to the following moiety:

When L¹ is permethylpentalene, it typically coordinates to M¹ via 2 h⁵ bonds. Similarly, when L² is permethylpentalene, it typically coordinates to M² via 2 h⁵ bonds. When U/L² is permethylpentalene, X² is suitably a complexed solvent or another neutral ligand capable of completing the coordination sphere around MVM². Alternatively, When U/L² is permethylpentalene, X² is absent.

[0044] (Hydro)permethylpentalene (also denoted herein as Pn*(H)) will be understood to refer to the following moiety:

When L¹ is (hydro)permethylpentalene, it typically coordinates to M¹ via 1 h⁵ bond. Similarly, when L² is (hydro)permethylpentalene, it typically coordinates to M² via 1 h⁵ bond.

[0045] It will be understood that in the compounds of formula (I), M¹, M², X¹ , X², L¹ and L² may have any of the definitions recited hereinbefore.

[0046] In an embodiment, the compound of formula (I) has a structure according to formula (la) shown below:

wherein M¹ , M², X¹ and X² have any of those definitions recited hereinbefore.

[0047] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

M¹ and M² are each independently selected from scandium, yttrium, lanthanum, cerium and lutetium;

X¹ is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy; and

X² is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0048] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

M¹ and M² are each independently selected from yttrium and lutetium; X¹ is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy; and

[0049] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X² is selected from chloro, bromo, (1-4C)alkoxy, benzyloxy and phenoxy, wherein the (1- 4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0050] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from methyl, isopropyl and tertbutyl, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0051] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

M¹ and M² are each independently selected from scandium, yttrium, lanthanum, cerium and lutetium; X¹ is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy; and

X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with two substituents selected from methyl, isopropyl and tertbutyl, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0052] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X² is selected from chloro, benzyloxy, -0-2,6-dimethyl-phenoxy, -0-2,6-diisopropyl-phenoxy and -0-2,4-ditertbutyl-phenoxy, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0053] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X¹ is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy; and

[0054] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from (1-4C)alkyl and (1- 4C)alkoxy; and X² is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0055] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from methyl, isopropyl and tertbutyl; and

[0056] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with two substituents selected from methyl, isopropyl and tertbutyl; and

[0057] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X¹ is selected from chloro, benzyloxy, -0-2,6-dimethyl-phenoxy, -0-2,6-diisopropyl-phenoxy and -0-2,4-ditertbutyl-phenoxy; and X² is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0058] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

[0059] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

[0060] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X¹ is selected from chloro, bromo, (1-4C)alkoxy, benzyloxy and phenoxy, wherein the (1- 4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy; and X² is selected from chloro, bromo, (1-4C)alkoxy, benzyloxy and phenoxy, wherein the (1- 4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0061] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X¹ is selected from chloro, bromo, (1-4C)alkoxy, benzyloxy and phenoxy, wherein the (1- 4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy; and

X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from (1-4C)alkyl and (1- 4C)alkoxy, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0062] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

[0063] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from methyl, isopropyl and tertbutyl; and X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with two substituents selected from methyl, isopropyl and tertbutyl, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran).

[0064] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

[0065] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

[0066] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from (1-4C)alkyl and (1- 4C)alkoxy, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran). [0067] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

[0068] In an embodiment, the compound of formula (I) has a structure according to formula (la), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

[0069] In an embodiment, the compound of formula (I) has a structure according to formula (lb) shown below:

wherein M¹ , M², X¹ and X² have any of those definitions recited hereinbefore. [0070] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

[0071] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

[0072] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

X² is selected from chloro, bromo, (1-4C)alkoxy, benzyloxy and phenoxy, wherein the (1- 4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy, or X² is a complexed solvent (e.g. pyridine or tetrahydrofuran). [0073] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

[0074] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

[0075] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

[0076] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein M¹ and M² are each independently selected from scandium, yttrium, lanthanum, cerium and lutetium;

[0077] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from (1-4C)alkyl and (1- 4C)alkoxy; and

[0078] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

[0079] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein M¹ and M² are each independently selected from scandium, yttrium, lanthanum, cerium and lutetium;

[0080] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

X¹ is selected from chloro, benzyloxy, -0-2,6-dimethyl-phenoxy, -0-2,6-diisopropyl-phenoxy and -0-2,4-ditertbutyl-phenoxy; and

[0081] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

[0082] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein M¹ and M² are each independently selected from scandium, yttrium, lanthanum, cerium and lutetium;

[0083] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

[0084] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

[0085] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein M¹ and M² are each independently selected from scandium, yttrium, lanthanum, cerium and lutetium;

[0086] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

[0087] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

[0088] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

M¹ and M² are each independently selected from yttrium and lutetium; X¹ is selected from chloro, bromo, (1-4C)alkoxy, benzyloxy and phenoxy, wherein the (1- 4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy; and

[0089] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

[0090] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

[0091] In an embodiment, the compound of formula (I) has a structure according to formula (lb), wherein

M¹ and M² are each independently selected from yttrium and lutetium;

[0092] In a particularly suitable embodiment, the compound of formula (I) has any one of the following structures:

Ar = 2.6-dirr ethyl-phenyl _{Ar =} 2,4-ditertbutylphenyl

or benzyloxy

[0093] In an embodiment, the one or more cyclic esters or cyclic amides has a structure according to formula (II) shown below

wherein

Q is selected from O or NR_y, wherein R_y is selected from hydrogen, (1-6C)alkyl, (2-6C)alkenyl and (2-6C)alkynyl; and

ring A is a 4-23 membered heterocycle containing 1 to 4 O or N ring heteroatoms in total, wherein the heterocycle is optionally substituted with one or more substituents selected from oxo, (1-6C)alkyl, (2-6C)alkenyl, (2-6C)alkynyl, (1-6C)alkoxy, aryl and heteroaryl. [0094] In an embodiment, Q is selected from O or NR_y, wherein R_y is selected from hydrogen, (1-3C)alkyl, (2-3C)alkenyl or (2-3C)alkynyl.

[0095] In an embodiment, Q is O.

[0096] In an embodiment, ring A is a 4-18 membered heterocycle containing 1 to 3 O or N ring heteroatoms in total, wherein the heterocycle is optionally substituted with one or more substituents selected from oxo, (1-6C)alkyl, (1-6C)alkoxy and aryl.

[0097] In an embodiment, ring A is a 4-, 6-, 7- or 16-membered heterocycle containing 1 to 3 O or N ring heteroatoms in total, wherein the heterocycle is optionally substituted with one or more substituents selected from oxo, (1-6C)alkyl, (1-6C)alkoxy and aryl.

[0098] In an embodiment, ring A does not contain any N ring heteroatoms.

[0099] In an embodiment, the cyclic ester or cyclic amide is a lactone.

[00100] In an embodiment, the cyclic ester or cyclic amide is a lactam.

[00101] In a particular embodiment, the cyclic ester or cyclic amide is a lactide (e.g. the cyclic diester of 2-hydroxypropionoic acid).

[00102] In an embodiment, the mole ratio of the compound of formula (I) to the cyclic ester or cyclic amide is 1 :5 to 1 :10,000. Suitably, the mole ratio of the compound of formula (I) to the cyclic ester or cyclic amide is 1 :10 to 1 : 1000. More suitably, the mole ratio of the compound of formula (I) to the cyclic ester or cyclic amide is 1 :25 to 1 :250.

[00103] In an embodiment, the process is not conducted in a solvent.

[00104] In an embodiment, the process is conducted in a solvent selected from toluene, tetrahydrofuran and methylene chloride.

[00105] In an embodiment, the process is conducted for a period of 1 minute to 96 hours. Suitably, the process is conducted for a period of 5 minute to 72 hours.

[00106] In an embodiment, the process is conducted at a pressure of 0.9 to 5 bar. Suitably, the process is conducted at a pressure of 0.9 to 2 bar.

[00107] In an embodiment, the process is conducted at a temperature of 10 - 150°C. Suitably, the process is conducted at a temperature ³50°C (e.g. 50 - 150°C. More suitably, the process is conducted at a temperature greater than ³60°C (e.g. 60 - 150°C). Most suitably, the process is conducted at a temperature greater than ³70°C (e.g. 70 - 150°C or 70 - 120°C). [00108] In an embodiment, the process is conducted in the presence of a chain transfer agent suitable for use in the ring opening polymerisation of a cyclic ester or cyclic amide. Suitably, the chain transfer agent is a hydroxy-functional compound (e.g. an alcohol, diol or polyol). More suitably, the chain transfer agent is selected from the group consisting of tert-butanol, benzyl alcohol and iso-propanol.

Preparation of compounds of formula (I)

[00109] The compounds of formula (I) may be formed by any suitable process known in the art. Particular examples of processes for the preparing compounds for formula (I) are set out in the accompanying examples.

[00110] Generally, when U and L² are permethylpentalene (Pn*), the process of preparing a compound of formula (I) comprises:

i. reacting [U₂(tmeda)][Pn*] (tmeda = tetramethylethylenediamine), shown below

©

Li ₂ (tmeda)

with a compound of formula (A) shown below:

M^a(X)₃

(A) wherein

M^a has any of the identities discussed herein in relation to M¹ or M², and

X is a halide (e.g. chloro); and

ii. optionally reacting the product of step i with a compound of formula (B) shown below:

M^bR

(B) wherein M^b is an alkali metal (e.g. potassium), and

R is (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1- 3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-5C)alkyl, (2-5C)alkenyl, (2-5C)alkynyl and (1- 5C)alkoxy.

[00111] Generally, when L¹ and L² are (hydro)permethylpentalene (Pn*(H)), the process of preparing a compound of formula (I) comprises:

i. reacting LiPn*(H), shown below

with a compound of formula (A) shown below:

M^a(X)₃

(A) wherein

M^a has any of the identities discussed herein in relation to M¹ or M², and

X is a halide (e.g. chloro); and

M^bR

(B) wherein

M^b is an alkali metal (e.g. potassium), and

R is (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1- 3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-5C)alkyl, (2-5C)alkenyl, (2-5C)alkynyl and (1- 5C)alkoxy. [00112] It will be appreciated that when X¹ (and, optionally X²) is halo, step ii need not be carried out.

[00113] It will be appreciated that any suitable solvent may be used in steps i and ii. Depending on the nature of X¹, L¹ and L², the solvent used in step i and/or step ii may complex to M¹/M² as neutral ligand, X². Suitably, the solvent used in steps i and ii is tetrahydrofuran.

[00114] A person of skill in the art will be able to select suitable reaction conditions (e.g. temperature, pressures, reaction times, agitation etc.) for syntheses.

EXAMPLES

[00115] One or more examples of the invention will now be described, for the purpose of illustration only, with reference to the accompanying figures, in which:

Fig. 1 shows ¹ H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn^*Y(p-CI)(thf)]₂. ^* denotes residual protio solvent and bound thf, and + denotes residual tmeda.

Fig. 2 shows ¹H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn*Lu(p-CI)(thf)]₂. * denotes residual protio solvent and bound thf, and + denotes residual tmeda.

Fig. 3 shows ¹H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn*(H)Y(p-CI)CI]₂. * denotes residual protio solvent.

Fig. 4 shows ¹H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn^*(H)Lu(p-CI)CI]_2. ^* denotes residual protio solvent.

Fig. 5 shows ¹ H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn*Y(p-OAr^Me)(thf)]₂. * denotes residual protio solvent.

Fig. 6 shows ¹H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn*(H)Y(p-OAr^tBu)(OAr^tBu)]_{2 *} denotes residual protio solvent.

Fig. 7 shows the polymerisation of of -lactide using [Pn*Y(p-CI)(thf)]₂ at 80 °C with [LA]₀ = 0.5 M, [LA]o/[Y]o = 50 in thf-cfe (black square), in benzene-Gf₆/thf-cf8 (black circle) and with two equivalents of benzyl alcohol in benzene-cf₆/thf-cf₈ (black triangle).

Fig. 8 shows the polymerisation of -lactide at 23 °C with [LA]₀ = 0.5 M, [LA]o/[Y]o = 50 in benzene-Gf₆/thf-cf₈ using [Pn*Y(p-CI)(thf)]₂ (black diamond) and [Pn*Y(p-OAr^Me)(thf)]₂ (black circle).

Fig. 9 shows the polymerisation of L-lactide using [Pn*Y(p-CI)(thf)]₂with [LA]o = 0.5 M, [LA]o/[Y]o = 50 in benzene- cfe/thf- da at 80 °C (black diamond) and 23 °C (black circle). Fig. 10 shows the polymerisation of L-lactide at 80 °C with [LA]₀ = 0.5 M, [LA]o/[M]₀ = 50 in benzene-cfe/thf-cfe using [Pn^*Y(p-OAr^Me)(thf)]₂ (black circle), Pn^*Zr(0-2,6-Me-C6H3)₂ (black square), Pn*ZrCp(0-2,6-Me-CsH₃) (black triangle) and Pn*ZrCp(0‘Bu) (down triangle).

Fig. 11 shows temperature variation in the polymerisation of L-lactide using [Pn*Y(p-CI)(thf)]₂. Polymerisation condition: CeD₆, 40 mg of lactide, [LA]₀ = 0.5 M and [LA]₀/[M]₀ = 50.

Fig. 12 shows the polymerisation of L- and rac-lactide using [Pn*Y(p-OAr^Me)(thf)]₂ in thf-cfe or benzene-cfe. Polymerisation condition: 25 °C, 40 mg of lactide, [LA]₀ = 0.5 M and [LA]₀/[M]₀ = 50. 90% conversion correspond to 2.2.

Fig. 13 shows homodecoupled ¹H{¹ H} studies of L- and rac- lactide polymerised using [Rh*U(m- OAr^Me)(thf)]₂ and [Pn*Y(p-CI)(thf)]₂. Polymerisation condition: 40 mg of lactide, [LA]o = 0.5 M and [LA]o/[M]o = 50.

Fig. 14 shows the ¹³C{¹ H} NMR spectra of poly(e-caprolactone) synthesised using [Rh^*U(m- OAr^Me)(thf)]₂. Polymerisation condition: 48.5 mg of e-caprolactone, [e-CLJo = 0.5 M and [e- CL]o/[Y]o = 25.

General details

[00116] General Considerations. All manipulations were carried out using standard Schlenk line or drybox techniques under an atmosphere of dinitrogen. Protio solvents were i) (pentane, hexane, toluene, benzene) degassed by sparging with dinitrogen, dried by passing through a column of activated sieves and stored over potassium mirrors ii) (thf, diethyl ether) distilled from sodium metal and stored activated 4 A molecular sieves (thf). Deuterated solvents were dried over potassium (C₆D₆) or CaH₂ (CDCh), distilled under reduced pressure, freeze-pump-thaw degassed three times prior to use. ¹H NMR spectra were recorded at 298 K, unless otherwise stated, on Varian Mercury VX-Works 300 or Bruker AVA 500 spectrometers and ¹³C{¹H} or ¹³C spectra on the same spectrometers at operating frequencies of 75 and 125 MHz respectively. Two dimensional ¹ H-¹H and ¹³C-¹ H correlation experiments were used, when necessary, to confirm ¹ H and ¹³C assignments. All NMR spectra were referenced internally to residual protio solvent (¹H) or solvent (¹³C) resonances and are reported relative to tetramethylsilane (d = 0 ppm). Chemical shifts are quoted in d (ppm) and coupling constants in Hertz. Elemental analyses were carried out at London Metropolitan University. MALDI-ToF-MS were collected using a Voyager DE-STR from Applied Biosystems equipped with a 337 nm nitrogen laser. All other reagents were purchased and used without further purification. Polymer molecular weights were determined by GPC using a Polymer Laboratories Plgel Mixed-D column (300 mm length, 7.5 mm diameter) and a Polymer Laboratories PL-GPC50 Plus instrument equipped with a refractive index detector. L- and rac-lactide was recrystallised twice from toluene and sublimed (70 °C, 103 mbar).

[00117] X-ray crystallography. Crystals were mounted on glass fibres using perfluoropolyether oil, transferred to a goniometer head on the diffractometer and cooled rapidly to 150 K. Data collections were performed using an Enraf-Nonius FR590 KappaCCD diffractometer, utilising graphite-monochromated Mo K_a X-ray radiation (l = 0.71073 A). Intensity data were processed using the DENZO-SMN package and corrected for absorption using SORTAV. The structures were solved using direct methods (SIR-92) or a charge flipping algorithm (SUPERFLIP) and refined by full-matrix least-squares procedures.

[00118] Polymerisation procedure. The lactide monomer (40 mg) and the complex were introduced in an NMR tube following the desired monomerinitiator ratio. Then 0.57 mL of chloroform-cfi was added to the compounds, leading to an initial monomer concentration of [LA]₀ = 0.5 M. The solution was monitored by ¹FI NMR spectroscopy. The conversion was determined by comparing the integration of the methine resonance of the polymer to the monomer.

Example 1 - Synthesis of catalytic complexes

1.1 - Synthesis of permethylpentalene halide complexes

[00119] Flaving regard to Scheme 1 below, reaction of MCh (M = Y or Lu) with [Li₂(tmeda)][Pn*] in thf for 30 minutes afforded a yellow-orange solution containing [Rh*M(m- Cl)(thf)]₂ in quantitative yield as judged by ¹FI NMR spectroscopy.

Scheme 1 : Synthesis of [Pn*M(p-CI)(thf)]₂ (M = Y or Lu)

[00120] Fig. 1 shows the ¹ H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn*Y(p-CI)(thf)]₂. Fig. 2 shows the ¹H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn*Lu(p-CI)(thf)]₂. The ¹H NMR spectra of both complexes show two singlets in a ratio of 12H:6H corresponding to the non- wing-tip and wing-tip methyl group protons respectively.

1.2 - Synthesis of (hvdro)permethylpentalene halide complexes

[00121] Having regard to Scheme 2 below, reaction of MCI3 (M = Y or Lu) with LiPn^*(H) in thf for 30 minutes afforded yellow-orange [Pn*(H)M(p-CI)CI]2 in quantitative yields. Removal of the volatiles in vacuo and extraction in pentane afforded crystals suitable for a single crystal X-ray diffraction study. Connectivity obtained from the data set showed a [YCl2]_n cluster that had both bound THF and Pn*(H)^_ ligands. This suggests that the dimeric [Pn*(H)M(p-CI)CI]₂ is likely stabilised by coordinating THF when in solution.

27 C 0.5 h

Scheme 2: Synthesis of [Pn^*(H)M(p-CI)CI]₂ (M = Y or Lu)

[00122] Fig. 3 shows the ¹ H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn*(H)Y(p-CI)CI]₂. Fig.

4 shows the ¹ H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn*(H)Lu(p-CI)CI]₂. The ¹ H NMR spectra of [Pn^*(H)M(p-CI)CI]2 show a quartet at ~3.2 ppm and doublet at ~1.1 ppm which are characteristic of the methine proton and methyl group in the C1 position in Pn*(H)^_ complexes.

5 singlets define the remaining methyl groups. Small amounts of other isomers (by virtue of planar chirality) are clearly visible in the NMR spectra but it is clear that a single major isomer is formed under ambient conditions.

1.3 - Synthesis of permethyloentalene aryloxide complexes

[00123] In order to avoid any aggregation during desolvation of the permethylpentalene rare earth halide complexes, these complexes were generated in situ and then reacted with potassium aryloxides in an effort to afford the corresponding aryloxide-substituted complexes. It was expected that the bulky aryloxide ligands would be both solubilising and stabilising. [00124] Having regard to Scheme 3 below, reaction of [Pn*M(p-CI)(thf)]₂ (M = Y or Lu) with 2 equivalents of KOAr^R derivatives (OAr^R = 0-2,6-Me-C₆H₃ or Ar^Me, 0-2,6-'Pr-C₆H₃ or OAr'^Pr) led to instant consumption of starting materials as judged by ¹ H NMR spectroscopy and products consistent with the desired aryloxide complexes [Pn*M(p-OAr)(thf)]₂.

Scheme 3: Synthesis of [Pn*M(p-OAr^R)(thf)]₂ (M = Y or Lu, OAr = 0-2,6-Me-C₆H₃ (0Ar^Me), O-

2,6-'Pr-CsH₃ (OAr^iPr))

[00125] Fig. 5 shows the ¹H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Pn*Y(p-OAr^Me)(thf)]₂. The ¹H NMR spectrum of [Pn*Y(p-OAr^Me)(thf)]₂ contains two singlets at 1.78 and 1.86 ppm for the permethylpentalene resonances, a singlet at 2.13 ppm for the aryloxide methyl groups. Two multiplets close to the thf-cfe solvent resonances indicate bound thf.

1.4 - Synthesis of (hvdro)permethylpentalene aryloxide complexes

[00126] In order to avoid any aggregation during desolvation of the (hydro)permethylpentalene rare earth halide complexes, these complexes were generated in situ and then reacted with potassium aryloxides in an effort to afford the corresponding aryloxide-substituted complexes. It was expected that the bulky aryloxide ligands would be both solubilising and stabilising.

[00127] Having regard to Scheme 4 below, reaction of [Pn*(H)Y(p-CI)CI]₂ with 4 equivalents of KO-2,4-‘Bu-C₆H₃ afforded a product consistent with [Pn^*(H)Y(p-OAr^tBu)(OAr^tBu)]₂ as judged by ¹H NMR spectroscopy.

Scheme 4: Synthesis of [Pn^*(H)Y(p-OAr^tBu)(OAr^tBu)]₂ (OAr^tBu = 0-2,4-tBu-C₆H₃).

[00128] Fig. 6 shows the ¹ H NMR spectrum (400 MHz, 298 K, thf-cfe) of [Rh*(H)U(m- OAr^tBu)(OAr^tBu)]₂ (OAr^tBu = 0-2,4-ⁱBu-0₆H₃).

Example 2 - Polymerisation studies

[00129] The ability of the complexes of Example 1 to catalyse the polymerisation of L-lactide was assessed according to the protocol discussed in the general details.

[00130] Fig. 7 shows the polymerisation of lactide with [Pn^*Y(p-CI)(thf)]₂ in two solvent mixtures, as well as with the addition of benzyl alcohol. It is noticeable that when the alcohol is added, the polymerisation is much faster due to the presence of the alkoxide active centre. The change of rate when benzene-cfe is added show that the thf-cfe has a retarding effect on the polymerisation; perhaps due to competition with the active centre.

[00131] Fig. 8 follows on from the findings outlined in Fig. 7 and shows that the alkoxide complex ([Pn*Y(p-OAr^Me)(thf)]₂) demonstrated a much higher rate of lactide polymerisation than the halide complex ([Pn*Y(p-CI)(thf)]₂). In particular, Fig. 8 shows that the alkoxide complex had consumed all of the available lactide within 1 hour. Fig. 8 also shows that the alkoxide complex is extremely active even at room temperature.

[00132] Fig. 9 shows that [Pn*Y(p-CI)(thf)]₂ is more catalytically active in lactide polymerisation at elevated temperatures.

[00133] Fig. 10 compares the catalytic activity of [Pn*Y(p-OAr^Me)(thf)]₂in the polymerisation of lactide with various other permethylpentalene-based complexes known to polymerise lactide. Fig. 10 shows that the rare earth-based [Pn*Y(p-OAr^Me)(thf)]₂is significantly more active than the zirconium-based comparators. [00134] Fig. 11 shows that the rate of polymerisation of L-lactide at room temperature is very slow when [Pn*Y(p-CI)(thf)]₂ was used. However, there is a huge rate increase between 65 and 80°C, certainly due to a high barrier of activation.

[00135] Fig. 12 shows that [Pn^*Y(p-OAr^Me)thf]₂ is extremely fast at room temperature; too fast to properly monitor. There is little variation in rates between rac- and L-lactide and thf does not significantly slow down the polymerisation.

[00136] Fig. 13 shows NMR spectra highlighting heterotactic biased (>85%) PLA obtained using [Pn*Y(p-CI)(thf)]₂. Atactic biased PLA were obtained for polymerisation of rac-lactide using [Pn*Y(p-OAr^Me)thf]₂. Isotactic PLA was obtained from L-lactide using [Pn*Y(p-OAr^Me)thf]₂, demonstrating no epimerisation.

[00137] Fig. 14 shows the ¹³C{¹ H} NMR spectra highlighting instant polymerisation of e- caprolactone at room temperature using [Pn*Y(p-OAr^Me)thf]₂. Gel formation after less than 2 minutes demonstrates full conversion and extremely active initiators.

[00138] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.

Claims

1. A process for the ring opening polymerisation (ROP) of a cyclic ester or a cyclic amide, the process comprising the step of contacting a compound having a structure according to formula (I) shown below with one or more cyclic esters or cyclic amides:

wherein

M¹ and M² are each independently a group 3 metal or a lanthanide,

or X² is a neutral ligand (e.g. a complexed solvent), and

Li and L_å are each independently selected from permethylpentalene and (hydro)permethylpentalene.

2. The process of claim 1 , wherein M¹ and M² are each independently selected from

scandium, yttrium, lanthanum, cerium and lutetium.

3. The process of claim 1 or 2, wherein M¹ and M² are each independently selected from yttrium and lutetium

4. The process of claim 1 , 2 or 3, wherein X¹ is selected from halo, (1-6C)alkoxy, aryl(1- 3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1- 4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy.

5. The process of any preceding claim, wherein X¹ is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy.

6. The process of any preceding claim, wherein X¹ is selected from chloro, bromo, (1- 4C)alkoxy, benzyloxy and phenoxy, wherein the (1-4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy.

7. The process of any preceding claim, wherein X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from (1-4C)alkyl and (1-4C)alkoxy.

8. The process of any preceding claim, wherein X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from methyl, isopropyl and tertbutyl.

9. The process of any preceding claim, wherein X¹ is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with two substituents selected from methyl, isopropyl and tertbutyl.

10. The process of any preceding claim, wherein X¹ is selected from chloro, benzyloxy, -O- 2,6-dimethyl-phenoxy, -0-2,6-diisopropyl-phenoxy and -0-2,4-ditertbutyl-phenoxy.

11. The process of any preceding claim, wherein X² is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl, (2-4C)alkenyl, (2-4C)alkynyl and (1-4C)alkoxy, or X² is a complexed solvent.

12. The process of any preceding claim, wherein X² is selected from halo, (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy, wherein the (1-6C)alkoxy, aryl(1-3C)alkoxy and aryloxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy, or X² is a complexed solvent.

13. The process of any preceding claim, wherein X² is selected from chloro, bromo, (1- 4C)alkoxy, benzyloxy and phenoxy, wherein the (1-4C)alkoxy, benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from halo, (1-4C)alkyl and (1-4C)alkoxy, or X² is a complexed solvent.

14. The process of any preceding claim, wherein X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from (1-4C)alkyl and (1-4C)alkoxy, or X² is a complexed solvent.

15. The process of any preceding claim, wherein X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with one or more substituents selected from methyl, isopropyl and tertbutyl, or X² is a complexed solvent.

16. The process of any preceding claim, wherein X² is selected from chloro, benzyloxy and phenoxy, wherein the benzyloxy and phenoxy groups may be optionally substituted with two substituents selected from methyl, isopropyl and tertbutyl, or X² is a complexed solvent.

17. The process of any preceding claim, wherein X² is selected from chloro, benzyloxy, -O- 2,6-dimethyl-phenoxy, -0-2,6-diisopropyl-phenoxy and -0-2,4-ditertbutyl-phenoxy, or X² is a complexed solvent.

18. The process of any preceding claim, wherein when X² is a complexed solvent, it is selected from ether and pyridine.

19. The process of claim 18, wherein the ether is tetrahydrofuran.

20. The process of any preceding claim, wherein X¹ and X² have the same identity.

21. The process of any preceding claim, wherein and l_2 are both permethylpentalene, such that the compound of formula (I) has a structure according to formula (la) shown below:

22. The process of any one of claims 1 to 20, wherein and l_2 are both

(hydro)permethylpentalene, such that the compound of formula (I) has a structure according to formula (lb) shown below:

23. The process of claim 1 , wherein the compound has any one of the following structures:

24. The process of any preceding claim, wherein the one or more cyclic esters or cyclic amides has a structure according to formula (II) shown below:

wherein

ring A is a 4-23 membered heterocycle containing 1 to 4 O or N ring heteroatoms in total, wherein the heterocycle is optionally substituted with one or more substituents selected from oxo, (1-6C)alkyl, (2-6C)alkenyl, (2-6C)alkynyl, (1-6C)alkoxy, aryl and heteroaryl.

25. The process of claim 24, wherein Q is selected from O or NR_y, wherein R_y is selected from hydrogen, (1-3C)alkyl, (2-3C)alkenyl or (2-3C)alkynyl.

26. The process of claim 24 or 25, wherein Q is O.

27. The process of claim 24, 25 or 26, wherein ring A is a 4-18 membered heterocycle containing 1 to 3 O or N ring heteroatoms in total, wherein the heterocycle is optionally substituted with one or more substituents selected from oxo, (1-6C)alkyl, (1-6C)alkoxy and aryl.

28. The process of any one of claims 24 to 27, wherein ring A is a 4-, 6-, 7- or 16- membered heterocycle containing 1 to 3 O or N ring heteroatoms in total, wherein the heterocycle is optionally substituted with one or more substituents selected from oxo,

(1 -6C)alkyl, (1-6C)alkoxy and aryl.

29. The process of any one of claims 24 to 28, wherein ring A does not contain any N ring heteroatoms.

30. The process of any one of claims 24 to 29, wherein the cyclic ester or cyclic amide is a lactone.

31. The process of any one of claims 24 to 29, wherein the cyclic ester or cyclic amide is a lactide.

32. The process of any one of claims 24 to 28, wherein the cyclic ester or cyclic amide is a lactam.

33. The process of any one of claims 24 to 32, wherein the mole ratio of the compound of formula (I) to the cyclic ester or cyclic amide is 1 :5 to 1 : 10,000.

34. The process of any one of claims 24 to 33, wherein the mole ratio of the compound of formula (I) to the cyclic ester or cyclic amide is 1 : 10 to 1 : 1000.

35. The process of any one of claims 24 to 34, wherein the mole ratio of the compound of formula (I) to the cyclic ester or cyclic amide is 1 :25 to 1 :250

36. The process of any one of claims 24 to 35, wherein the process is not conducted in a solvent.

37. The process of any one of claims 24 to 35, wherein the process is conducted in a solvent selected from toluene, tetrahydrofuran and methylene chloride.

38. The process of any one of claims 24 to 37, wherein the process is conducted for a period of 1 minute to 96 hours.

39. The process of any one of claims 24 to 38, wherein the process is conducted for a period of 5 minute to 72 hours.

40. The process of any one of claims 24 to 39, wherein the process is conducted at a

pressure of 0.9 to 5 bar.

41. The process of any one of claims 24 to 40, wherein the process is conducted at a

pressure of 0.9 to 2 bar.

42. The process of any one of claims 24 to 41 , wherein the process is conducted at a

temperature of 10 - 150°C.

43. The process of any one of claims 24 to 42, wherein the process is conducted in the presence of a chain transfer agent suitable for use in the ring opening polymerisation of a cyclic ester or cyclic amide.

44. The process of claim 43, wherein the chain transfer agent is a hydroxy-functional

compound (e.g. an alcohol, diol or polyol).