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Switchable catalysis is a powerful tool in the polymer chemist's toolbox as it allows on demand access to a variety of polymer architectures. Switchable catalysts operate by the generation of a species which is chemically distinct in behaviour and structure to the precursor. This difference in catalytic activity has been exploited to allow spatiotemporal control over polymerisations in the synthesis of (co)polymers. Although switchable methodologies have been applied to other polymerisation mechanisms for quite some time, for ring opening polymerisation (ROP) reactions it is a relatively young area of research. Despite its infancy, the field is accelerating rapidly. Here, we review recent developments for selected external stimuli for ROP, including redox chemistry, light, allosteric and mechanical control. Furthermore, a brief review on switch catalysis involving exogeneous gases will also be provided, although this area differs from traditional switchable catalysis techniques. An outlook on the future of switchable catalysis is also provided. [ABSTRACT FROM AUTHOR]

Herein, we report the preparation, characterisation and catalytic applications of air-stable Fe(III)–acetate complexes consisting of salan, salen and salalen ligand frameworks. Owing to the simple synthetic protocol employed, a wide range of complexes have been prepared and structure–activity-relationships investigated. X-ray diffraction confirmed the solid-state structures for eight of the complexes. These represent the first Fe(III)–acetate complexes applied for the selective coupling of CO2/epoxide and lactide polymerisation. The coupling of CO2 and challenging cyclohexene oxide substrate was performed under mild, solvent-free conditions (80 °C, 10 bar CO2) to selectively form the cis-cyclohexene carbonate as the exclusive product (selectivity >99%) with a metal loading of 0.08 mol%. A reduced aminopiperidine ligand backbone was found as the most active catalyst, and after investigating four co-catalysts, showed high functional group tolerance and robustness when applied to a broad, commercially available, terminal epoxide substrate scope with high conversions observed. The ring-opening polymerisation of rac-lactide was achieved using the Fe(III)OAc complexes using triethylamine and benzyl alcohol for initiation, interestingly isoselectivity was observed in some cases. [ABSTRACT FROM AUTHOR]

Non-toxic, highly active and robust complexes are the holy grail as ideal green catalysts for the polymerisation of biorenewable and biodegradable polylactide. Four new zinc guanidine complexes [ZnCl2(TMG4NMe2asme)], [ZnCl2(TMG5Clasme)], [ZnCl2(TMG5Measme)] and [ZnCl2(TMG5NMe2asme)] with different electron-donating and electron-withdrawing groups on the ligand's aromatic backbone have been synthesised. Ligands are derived from low-cost commercially available compounds and have been converted by a three- or four-step synthesis process into the desired ligand in good yields. The compounds have been fully characterised and tested in the ROP of rac-LA under industrially relevant conditions. The complexes are based on the recently published structure [ZnCl2(TMGasme)] which has shown high activity in the polymerisation of lactide at 150 °C. Different substituents in the para-position of the guanidine moiety significantly increase the polymerisation rate whereas positioning substituents in the meta-position causes no change in the reaction rate. With molecular weights over 71 000 g mol−1 being achievable, the best system produces polymers for multiple industrial applications and its polymerisation rate approaches that of Sn(Oct)2. The robust systems are able to polymerise non-purified lactide. The initiation of the polymerisation is suggested to occur due to impurities in the monomer. [ABSTRACT FROM AUTHOR]

Four dimeric lanthanide alkoxide complexes bearing ONNO bipyrrolidine salan ligands (LMeH2/LtBuH2) have been prepared with Nd, Sm and Yb. Depending on the metal and substituents, these complexes adopt varying coordination geometries. While investigating the hydrolytic degradation of these complexes, three dimeric mixed alkoxide/hydroxide and bis-hydroxide products were also prepared, isolated and characterised. Despite paramagnetism, 1H NMR and diffusion ordered spectroscopy (DOSY) allowed additional characterisation alongside elemental and single-crystal X-ray diffraction analyses. These systems were very active for the controlled ring-opening polymerisation (ROP) of rac-lactide (LA), under industrially relevant melt conditions and in solution, yielding complete conversion within 5 minutes at [Ln] : [LA] ratios of up to 3000 : 1 in toluene, and at 80 °C, whilst retaining low dispersities (Đ = 1.1). 1H DOSY NMR spectroscopy was employed to monitor polymer growth from the metal centres in situ, and revealed a dinuclear catalytically active species. [ABSTRACT FROM AUTHOR]

A range of ligands based upon 2-(aminomethyl)piperidine have been successfully complexed to Mg(ii), Zn(ii) and group IV metal centres. These complexes have been characterised both in solution and solid state with different coordination geometries realised dependant on the nature of the ligand. For the Mg(ii) and Zn(ii) complexes, M(1–2)2, were isolated and analysed by DOSY NMR spectroscopy. These ligands also furnished diastereomeric group IV complexes, M(1–2)2(OiPr)2. Group IV salalen and salan complexes, M(4–5)(OR)2 were also found to be diastereomeric in nature, with either β-cis or α-cis geometriesrespectively. The tridentate ligand, 6H2, yielded five coordinate complexes with both Ti(iv) and Zr(iv). All complexes were screened for the ring opening polymerisation of rac-lactide under both solvent and melt conditions. For the Mg(ii) and Zn(ii) complexes, good activity was observed with Zn(1–2)2 demonstrating immortal polymerisation characteristics. [ABSTRACT FROM AUTHOR]

A series of monometallic and bimetallic Al(iii) complexes with substituted naphthyl based Schiff base ligands have been prepared and characterised. When 1-aminonaphthalene based ligands were reacted with AlMe3 monometallic complexes were isolated, however, with 1,5 and 1,8-diaminonaphthalene based ligands bimetallic complexes were formed. In all cases 4-coordinate tetrahedral Al(iii) centres were observed in the solid state and in solution. There was little difference in rate of polymerisation of rac-lactide between the monometallic and bimetallic complexes based on 1,5-diaminonaphthalene. However, for the 1,8-diaminonaphthalene the complex was an order of magnitude faster than the monometallic and the analogous 1,5-system. Moreover, this complex was active at room temperature, which is rare for aluminium initiators, and PLA with a high degree (Pm = 0.82) of isotacticity was observed. [ABSTRACT FROM AUTHOR]

Unsymmetrical amine tris(phenolate) ligands have been prepared and complexed to Zr(IV). In the solidstate dimeric species are observed, which persist in solution. The complexes show interesting coordination chemistry and the nature of which is discussed. These complexes are analogues of a highly stereoselective C3-symmetric Zr(IV) amine tris(phenolate) alkoxide and have been prepared, characterised and trialled for the ring-opening of rac-lactide providing insight into the effect of symmetry and ligand steric bulk on selectivity. The polymerisations have been investigated in solution and under the industrially preferred melt conditions (130 °C). The rate law for the ROP has been determined - with Zr2(1)2(OiPr)2, a pseudo first order dependency was seen, however for Zr2(2)2(OiPr)2 this changed to a half order dependency in initiator. [ABSTRACT FROM AUTHOR]

In this paper we report the synthesis and full characterisation of a range of Ti(IV)-catecholato systems complexed to piperazine or homopiperazine salan ligands. The steric/electronic environment of the catecholate moiety has been varied and the effect this has on cytotoxicity discussed. It was observed that the 7-membered homopiperazine complexes are more stable to hydrolysis than their piperazine cousins in biological media. In general the homopiperazine complexes show higher cytotoxicity than the piperazine complexes, with the most cytotoxic complex exhibiting IC50 (μM) values of 3 ± 0.5 μM (HT-29) and 4 ± 1 μM (OVCAR). [ABSTRACT FROM AUTHOR]

In this paper nine various salalen ligands have been prepared and characterised. The steric and electronic effects of both the salen and salan fragments have been varied in a systematic fashion to ascertain how this affects the selectivity for the ROP of rac-lactide. These were complexed to AlMe3 to generate pseudo trigonal bypyramidal metal centred complexes. Upon addition of benzyl alcohol the active initiator can be easily prepared. The complexes were screened for the ROP of rac-lactide in solution and melt conditions. PLA with narrow molecular weight distributions (PDIs range from 1.07–1.67) could be isolated with moderate degrees of tacticity. Significantly it was found that chloro groups on the imine fragment increased the degree of heterotactic enchainment in the polymer. The kinetics for one series of salalen–Al complexes was also investigated. [ABSTRACT FROM AUTHOR]

In this paper we report the first use of Al(iii) salalen complexes for the ring opening polymerisation of rac-lactide. Polylactides with narrow polydispersities (PDIs range from 1.04–1.65) and moderate degrees of stereoselectivity were formed. Eight salalen Al(iii) complexes have been prepared and fully characterised by solution-state NMR spectroscopy and, where appropriate, single crystal X-ray diffraction. With ligand 3H2either a monomeric or dimeric Al(iii) species was formed, the dimeric species was favoured at low concentrations. The complexes were tested for the ring opening polymerisation of rac-lactide in toluene at 80 or 100 °C. Interestingly, various tacticities of polymer were formed, which were dependent upon the nature of the group bound to the amine nitrogen centre. [ABSTRACT FROM AUTHOR]

In this paper we report the synthesis and solid-state structures for a series of pyridine based Cu(ii) complexes and preliminary data for the asymmetric Henry reaction. Interestingly, the solid-state structures indicate the incorporation of an alcohol into one of the imine groups of the ligand, forming a rare α-amino ether group. The complexes have been studied viasingle crystal X-ray diffraction, EPR spectroscopy and mass spectrometry. Intriguingly, it has been observed that the alcohol only adds to one of the imine moieties. Density functional theory (DFT) calculations have also been employed to rationalise the observed structures. The Cu(ii) complexes have been tested in the asymmetric Henry reaction (benzaldehyde + nitromethane or nitroethane) with ee's up to 84% being achieved as well as high conversions and modest diastereoselectivities. [ABSTRACT FROM AUTHOR]