Marina Lamberti

Gradient Isotactic Multiblock Polylactides from Aluminum Complexes of Chiral Salalen Ligands

Aluminum complexes of enantiomerically pure aminomethylpyrrolidine-based salalen ligands and their application in the stereoselective polymerization of lactide are described. Poly(lactic acid) featuring the new gradient isotactic multiblock microstructure was synthesized by isoselective catalysts, which operate by a combination of enantiomorphic-site and chain-end control mechanisms.

Coordination Chemistry and Reactivity of Zinc Complexes Supported by a Phosphido Pincer Ligand

The preparation and characterization of new Zn(II) complexes of the type [(PPP)ZnR] in which R = Et (1) or N(SiMe(3))(2) (2) and PPP is a tridentate monoanionic phosphido ligand (PPP-H = bis(2-diphenylphosphinophenyl)phosphine) are reported. Reaction of ZnEt(2) and Zn[N(SiMe(3))(2)](2) with one equivalent of proligand PPP-H produced the corresponding tetrahedral zinc ethyl (1) and zinc amido (2) complexes in high yield. Homoleptic (PPP)(2) Zn complex 3 was obtained by reaction of the precursors with two equivalents of the proligand. Structural characterization of 1-3 was achieved by multinuclear NMR spectroscopy ((1)H, (13)C, and (31)P) and X-ray crystallography (3). Variable-temperature (1)H and (31)P NMR studies highlighted marked flexibility of the phosphido pincer ligand in coordination at the metal center. A DFT calculation on the compounds provided theoretical support for this behavior. The activities of 1 and 2 toward the ring-opening polymerization of ε-caprolactone and of L- and rac-lactide were investigated, also in combination with an alcohol as external chain-transfer agent. Polyesters with controlled molecular parameters (M(n), end groups) and low polydispersities were obtained. A DFT study on ring-opening polymerization promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive zinc center. Preliminary investigations showed the ability of these complexes to promote copolymerization of L-lactide and ε-caprolactone to achieve random copolymers whose microstructure reproduces the composition of the monomer feed.

Bimetallic salen aluminum complexes: cooperation between reactive centers in the ring-opening polymerization of lactides and epoxides

Three dinuclear aluminum alkyl complexes of the general formula LAl2Me4, where L are salen ligands with an alkyl backbone of different lengths between the nitrogen atoms (1,3-propylene (1), 1,5-pentylene (2) and 1,12-dodecaylene (3)), have been prepared through alkane elimination reactions between each ligand and two equivalents of AlMe3. The related hemi-salen aluminum complex 4 was prepared by an analogous reaction between a phenoxy-imine ligand and a single equivalent of AlMe3. The activities of these aluminum complexes in the ring-opening polymerization (ROP) of rac-lactide and of several epoxides have been investigated and compared. The dinuclear complex 1, bearing the salen ligand with the shortest alkyl bridge, was the most active in the ROP of LA producing isotactic enriched PLA. Otherwise, the other complexes (2 and 3), in which the metal centers are remote, produced atactic PLA with inferior activity. Analogous differences in terms of activity emerged in the ROP of epoxides. The comparison of the catalytic behavior of the dinuclear complexes as well as their mononuclear counterparts suggests the cooperation between the two aluminum metal centers of the dinuclear species in which these are close enough.

Carbon Dioxide/Epoxide Reactions Catalyzed by Bimetallic Salalen Aluminum Complexes

Four new bimetallic aluminum complexes that bear salalen ligands were synthesized and characterized. They were employed as catalysts in the reaction of CO2 with both cyclohexene oxide and propylene oxide with tetrabutyl ammonium bromide as the cocatalyst. The effect of the reaction conditions on the productivity and selectivity of the catalytic system was evaluated. Under the optimized reaction conditions, cis‐cyclohexene carbonate and propylene carbonate were obtained as exclusive products, respectively. Interestingly, in the production of cis‐cyclohexene carbonate, turnover frequencies comparable to those of the most active systems described in the literature were obtained. Moreover the catalytic system was active even if it was operated at a low CO2 pressure (0.2 MPa) and with a low catalyst loading (0.04 mol %).

Ring-opening polymerization of ω-6-hexadecenlactone by a salicylaldiminato aluminum complex: a route to semicrystalline and functional poly(ester)s

The controlled and pseudo-living ring-opening polymerization of a large ring size lactone, ω-6-hexadecenlactone (6HDL), was achieved for the first time by a dimethyl(salicylaldiminato) aluminum initiator. The obtained poly(ω-6-hexadecenlactone) was functionalized by thiol–ene coupling reaction. By epoxidation of the double bond a poly(6,7-epoxy-ω-hexadecalactone) was formed. The subsequent opening of the epoxide by NaCNBH3 produced a poly(6,7-epoxy-ω-hexadecalactone) with occasional inter and intra ether-type crosslinks. Modifications occurred without any change in the degree of polymerization. The obtained polymeric samples were characterized by NMR, GPC and DSC and X-ray diffraction analysis. The “polyethylene-like” orthorhombic crystal lattice of poly(ω-6-hexadecenlactone) is deformed in the poly(6,7-epoxy-ω-hexadecalactone) and is replaced by a hexagonal packing when the poly(ester) chains are hydroxylated. Furthermore, copolymerization of 6HDL with smaller e-caprolactone produced random copolymers, with average sequence block lengths of 2, while the sequential addition of 6HDL and e-caprolactone or rac-lactide allowed the preparation of block-copolymers. The block and random copolymers were also semicrystalline.

Ring-opening polymerization of cyclic esters by phenoxy-thioether complexes derived from biocompatible metals

A series of novel Mg(II) and Zn(II) complexes of the form LMX or L2M, supported by phenoxy-thioether ligands bearing different substituents at the ortho position of the thiophenol rings [L(-) = 4,6-tBu2-OC6H2-2-CH2S(2-R-C6H4)(-)] [M = Zn, R = H, X = N(SiMe3)2 (1) and X = Et (2); M = Mg, X = n-Bu, R = H (3), R = CH3 (4), R = Br (5); M = Mg, R = H (6)], were synthesized and characterized. Reaction of the proper zinc precursor (Zn[N(SiHMe2)2]2 or ZnEt2) with one equivalent of the phenoxy-thioether proligand produced the corresponding amido (1) and ethyl (2) zinc complexes in high yields. The monoalkylmagnesium complexes (3-5) were all obtained by butane elimination reaction of the dialkylmagnesium reagent, Mg(n-Bu)2, with one equivalent of the proligand in good yields. The homoleptic complex 6 was obtained by reaction of 0.5 equivalent of Mg(n-Bu)2 and the proper proligand. Variable temperature (1)H NMR studies performed on 2 and 3 demonstrated that the named complexes are involved in fluxional processes concerning a fast conformational change of the six-membered metallacycle. DOSY (Diffusion Ordered SpectroscopY) (1)H experiments and ligand scrambling strongly suggested that complexes 1-6 exist as dimeric species in solution. All complexes were active as catalysts in the ring-opening polymerization of cyclic esters. In particular, magnesium complexes showed superior ε-caprolactone and lactide ROP behavior in terms of activity, control of molecular weights and molecular weight distributions.

Selective Synthesis of Cyclic Carbonate by Salalen-Aluminum Complexes and Mechanistic Studies

Salalen-aluminum complexes were synthesized and used as catalysts in the reactions of CO2 with different epoxides. The reaction of cyclohexene oxide and CO2 was thoroughly investigated. In particular, the effect of the reaction conditions (nature and equivalents of the co-catalyst, CO2 pressure, and temperature) and of the ligands (substituents on the ancillary ligand, nature of the labile ligand, and nature of the nitrogen-donor atoms) on the results of this reaction was studied. The cycloaddition reaction of CO2 with terminal epoxides bearing different functional groups was realized. Moreover, NMR mechanistic studies provided information on the catalytic cycle. Interestingly, the characterization of an intermediate species in the mechanism of the reaction of cyclohexene oxide with CO2 , catalyzed by one of the salalen-aluminum complexes, was accomplished.

Group 4 bis(chelate) metal complexes of monoanionic bidentate [E,O−] ligands (E = O, S): synthesis and application as α-olefin polymerization catalysts

Monoanionic bidentate phenoxy-ether [O(-),O] and phenoxy-thioether [O(-),S] ligands were synthesized and used to prepare octahedral group 4 metal complexes with general formula [O(-),E](2)ML(2) (complex 1: E = S, M = Hf, L = Bn; complex 2: E = O, M = Ti, L = NMe(2); complex 3: E = S, M = Ti, L = NMe(2); complex 4: E = S, M = Ti, L = Cl). Variable-temperature 1H NMR studies were performed on all the complexes 1-4 in the range of +20 to -70 degrees C. For the hafnium complex 1 a dynamic interchange of the Delta and Lambda enantiomers for a C2-symmetric isomer was revealed. Eyring analysis indicated that rearrangement of the ligands occurs via a non-dissociative mechanism. In the range of the explored temperatures, a broadening of the signals was observed for complexes 2-3. Finally, a reversible fluxional process between several isomers was revealed for complex 4. Complexes 1-4 were tested as pre-catalysts for ethylene and propene polymerization in combination with different activators and under variable conditions. Complex 1 was found to be inactive whereas complexes 2-4 showed moderate activities in the polymerization of ethylene and propene. A mixture of polymers with different microstructure was obtained in all cases, coherently with the fluxional behaviour of the pre-catalysts observed in solution.

Random l-lactide/ε-caprolactone copolymers as drug delivery materials

In this work, the degradation phenomena and the release kinetics of an active molecule from matrices systems made of random copolymers of ε-caprolactone (CL) and l-lactide (LA) were investigated by exposing the matrices, shaped as thin films, to simulated physiological environments. α-tocopherol was incorporated into the films as hydrophobic model molecule with the aim to investigate both its release pattern and its effect on erosion phenomena. In particular, the films have been kept at controlled conditions (temperature, stirring, pH) and they were characterized in terms of weight loss, water uptake, thermal properties, and change of number average molecular weight, in order to explain the molecule release kinetics and the degradation pathways of the copolymers. The main findings of this study are that the erosion phenomena take place significantly only when a critical value of the molecular mass was obtained in the sample; that the presence of the drug stabilizes the matrix and it decreases the rate of molecular mass decrease; and that crystallinity, reducing the chain mobility, causes lower erosion rates.

Ring-Opening Copolymerization of Epoxides with Cyclic Anhydrides Promoted by Bimetallic and Monometallic Phenoxy–Imine Aluminum complexes

Two dinuclear aluminum alkyl complexes bearing a salen ligand with alkyl backbone of different length between the nitrogen atoms (1,3‐propylene and 1,5‐pentylene backbone) and the related hemisalen aluminum complex were tested as catalysts in the ring‐opening copolymerization of cyclohexene oxide and limonene oxide with succinic anhydride and phthalic anhydride. The effects of different reaction conditions (co‐catalyst and solvent) on the productivity and selectivity of the reactions were evaluated. The comparison of the catalytic behavior of the dinuclear complexes bearing the aluminum reactive centers at different distances as well as of their mononuclear counterpart suggested that the copolymerization follows a monometallic pathway differently from what was observed in the homopolymerization of CHO.