Tuning a robust system: N,O zinc guanidine catalysts for the ROP of lactideElectronic supplementary information (ESI) available. CCDC 1880806-1880809. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt04938f

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 [ZnCl 2 (TMG4NMe 2 asme)], [ZnCl 2 (TMG5Clasme)], [ZnCl 2 (TMG5Measme)] and [ZnCl 2 (TMG5NMe 2 asme)] 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 [ZnCl 2 (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. Four non-toxic, highly active and robust complexes have been used in the ring opening polymerisation of non-purifed lactide to substitute Sn(Oct) 2 .