The ring-opening polymerization of l-lactide initiated by rare earth metal silylamido complexes [Ln(OSSO){N(SiHMe2)2}(THF)] (1−3:  Ln = Y, Lu; OSSO = 1, ω-dithiaalkanediyl-bridged bisphenolato) was studied. MALDI-TOF mass spectrometry and 1H NMR spectroscopy suggested that the polymerization proceeded via a conventional coordination−insertion mechanism involving silylamide ligand as the initiating group and the cleavage of acyl−oxygen bond of the monomer. A two-stage linear relationship between ln([LA]0/[LA] t ) and the polymerization time was observed for the yttrium complex [Y(pdtbp){N(SiHMe2)2}(THF)] (pdtbp = 1,5-dithiapentanediyl-bis{4,6-di-tert-butylphenolato}, 3). In both stages, the polymerization showed first-order kinetics for the monomer concentration. The first-order dependency of the initiator concentration was only observed when the monomer conversion to PLA was less than 50−60%. The aggregation of the active growing polymer chain into dimeric structure occurred in the second stage. In contrast, the in situ generated alkoxide initiator [Ln(OSSO){N(SiHMe2)2}(THF)]/ i PrOH showed a different behavior. When 3 was reacted with 2-propanol in 1:2 ratio, the in situ generated alkoxides initiated the living polymerization of l-lactide. Neither aggregation nor intramolecular transesterification was observed over the entire conversion range. Polylactides with controlled molecular parameters (M n, end groups) and low polydispersities were formed as a result of fast alkoxide/alcohol exchange.

Currently there is considerable interest in the controlled ringopening polymerization (ROP) of lactides (LAs) by welldefined metal initiators because of the biodegradable and biocompatible nature of polylactides (PLAs) and their potentially wide-ranging commercial applications. [1, 2] Particularly interesting is that some discrete metal complexes initiate the ROP of rac-LA or meso-LA in a stereoselective manner, [3][4][5][6][7][8][9] and thus PLAs with a variety of architectures ranging from isotactic, [3d-i] syndiotactic, [5] to heterotactic [6][7][8] can be obtained. Enantiomerically pure or racemic aluminum complexes with chiral salen-type ligands (salen = N,N'-bis-(salicylidene)ethylenediamine) were reported to polymerize rac-LA to form isotactic or stereoblock/stereogradient PLA, [3a-g] and to polymerize meso-LA to form syndiotactic PLA through enantiomorphic site control.[5a] Achiral aluminum-salen or salan complexes (salan = N,N'-bis(orthohydroxybenzyl)ethylenediamine) produce isotactic or heterotactic PLA from rac-LA through a chain-end control mechanism.[3h,i, 6] Magnesium, [7b,d] zinc, [7c,d] calcium, [7d,e] and yttrium [8] complexes are highly active for ROP of rac-LA, in some cases showing significant preference for heterotactic dyad enchainment. Despite considerable efforts devoted to initiator design, [3][4][5][6][7][8][9][10] factors governing stereocontrol during the ROP of lactides are still not well understood. Herein we report that a series of scandium complexes with 1,w-dithiaalkanediyl-bridged bisphenolato (OSSO)-type ligands (Table 1) show high heterotactic selectivity during the ROP of rac-LA. This selectivity involves a new type of dynamic monomer recognition based on the fluxionality of the ancillary ligand. Group 3 complexes with such ligands have previously been found to be active initiators for ROP of l-LA, [11a,b] and a slight heterotactic preference during the ROP of rac-LA has been noted for the corresponding aluminum complexes.

Monomeric rare earth metal bis(phenolato) complexes [(L s,s )Ln{N(SiHMe 2 ) 2 }(THF)] (1a-4c) were isolated from the reaction of silylamido complexes [Ln{N(SiHMe 2 ) 2 } 3 (THF) x ] (Ln = Sc, x = 1; Ln = Y, Lu, x = 2) and one equivalent of tetradentate 1,ω-dithiaalkanediyl-bridged bis(phenol)s etbmpH 2 , ptbmpH 2 , edtbpH 2 and pdtbpH 2 in moderate to high yields. In contrast to the unsymmetrical scandium complexes 1a and 3a, the scandium complex 2a, the yttrium complexes 1b and 4b as well as the lutetium complexes 1c-4c show C s or C 2 symmetry due to the relatively fast dissociation of THF on the NMR time scale at room temperature. The monomeric structures of the complexes 2a and 4b were confirmed by X-ray diffraction studies. The six-coordinate central metal with the tetradentate ligand, the silylamido group, and one THF, adopts a C 1 symmetrical configuration with trans(O,O) or cis(O,O) orientation of the two oxygen donors of the ligand. Distorted octahedral and trigonal prismatic coordination geometries are found for 2a and 4b. Substitution reaction with 2,2,6,6-tetramethyl-3,5-heptanedione afforded the corresponding complexes 6b, 6c, 7c and 8b with dimeric structures and with trityl alcohol the alkoxide complex [(etbmp)Y(OCPh 3 )(THF)] (9). All new complexes efficiently initiated the ring-opening polymerization of -lactide in THF. High molecular weight poly(-lactide)s with narrow molecular weight distributions (M w /M n 1.15-1.41) were obtained using complexes 1a-4c. Dimeric β-diketonato complexes were only active in the presence of THF or excess isopropanol.

A series of aluminium alkyl complexes (BDI)AlEt(2) (3a-m) bearing symmetrical or unsymmetrical beta-diketiminate ligand (BDI) frameworks were obtained from the reaction of triethyl aluminium and the corresponding beta-diketimine. The monomeric structure of the aluminium complex 3k was confirmed by an X-ray diffraction study, which shows that the aluminium center is coordinated by both of the nitrogen donors of the chelating diketiminate ligand and the two ethyl groups in a distorted tetrahedral geometry. Attempt to synthesize beta-diketiminate aluminium alkoxide complexes by the reactions of monochloride complex "(BDI-2a)AlMeCl" (4) with alkali salts of 2-propanol gave unexpectedly an aluminoxane [(BDI-2a)AlMe](2)(micro-O) (7) as characterized by X-ray diffraction methods. Complexes 3a-m and [(2,6-(i)Pr(2)C(6)H(3)NCMe)(2)HC]AlEt(2) (8) were found to catalyze the ring-opening polymerization (ROP) of epsilon-caprolactone with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-donating substituents at the para-positions of the aryl rings in the ligand resulted in an apparent decrease in catalytic activity. Complex 3h showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the meta-trifluoromethyl substituents on the aryl rings. The increase of steric hindrance of the ligand by introducing ortho-substituents onto the phenyl moieties also resulted in a decrease in the catalytic activity. Although the viscosity average molecular weights (M(eta)) of the obtained poly(caprolactone)s increased with the enhancement of monomer conversion, the ROPs of epsilon-caprolactone initiated by complexes 3a-m and 8 were not well-controlled, as judged from the broad molecular weight distributions (PDI = 1.66-3.74, M(w)/M(n)) of the obtained polymers and the nonlinear relationship of molecular weight versus monomer conversion.