Parisa Mehrkhodavandi

Redox control of a ring-opening polymerization catalyst.

The activity of an yttrium alkoxide complex supported by a ferrocene-based ligand was controlled using redox reagents during the ring-opening polymerization of L-lactide. The oxidized complex was characterized by X-ray crystallography and (1)H NMR, XANES, and Mössbauer spectroscopy. Switching in situ between the oxidized and reduced yttrium complexes resulted in a change in the rate of polymerization of L-lactide. Synthesized polymers were analyzed by gel permeation chromatography. Polymerization of trimethylene carbonate was also performed with the reduced and oxidized forms of an indium alkoxide complex. The indium system showed the opposite behavior to that of yttrium, revealing a metal-based dependency on the rate of polymerization.

Mechanism of living lactide polymerization by dinuclear indium catalysts and its impact on isoselectivity.

A family of racemic and enantiopure indium complexes 1-11 bearing bulky chiral diaminoaryloxy ligands, H(NNO(R)), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNO(R))InX](2)[μ-Y][μ-OEt] (5, R = (t)Bu, X = Y = Cl; 8, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly(lactic acid) (PLA) at room temperature. In particular, complex 5 showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (R,R/R,R)-5 is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex 8. Importantly, the selectivities obtained for the ROP of racemic LA with (R,R/R,R)-5 and (R,R/R,R)-8 are different and, along with kinetics investigations, suggest a dinuclear propagating species for these complexes.

Redox control of group 4 metal ring-opening polymerization activity toward l -lactide and ε-caprolactone

The activity of several group 4 metal alkoxide complexes supported by ferrocene-based ligands was controlled using redox reagents during the ring-opening polymerization of l-lactide and ε-caprolactone. Switching in situ between the oxidized and reduced forms of a metal complex resulted in a change in the corresponding rate of polymerization. Opposite behavior was observed for each monomer used. One-pot copolymerization of the two monomers to give block copolymers was also achieved.

Highly controlled immortal polymerization of β-butyrolactone by a dinuclear indium catalyst

An ethoxy-bridged dinuclear indium catalyst was used for the ring opening polymerization of the cyclic ester β-butyrolactone to form the biodegradable polyester poly(hydroxybutyrate) (PHB). The catalyst shows remarkable activity and control during polymerization, allowing for formation of diblock polymers. Addition of high ratios of alcohols to the catalyst leads to fast chain transfer and immortal polymerization.

Synthesis and structural studies of chiral indium(III) complexes supported by tridentate diaminophenol ligands.

Indium(III) dimethyl, dihalide, and alkoxy-bridged complexes bearing a chiral diaminophenoxy tridentate ligand [NN(H)O](-) were synthesized. The dimethyl complex (NN(H)O)InMe(2) (1) was unreactive toward ethanol and 2-propanol and only partially reactive toward the more acidic phenol. The dihalide complexes (NN(H)O)InX(2) (X = Cl (3), Br (4), I (5)) reacted with NaOEt to form robust alkoxy-bridged complexes with the formula {[(NN(H)O)InX](2)(mu-X)(mu-OEt)} (X = Cl (6), Br (7), I (8)). The reaction of the alkoxy-bridged complexes with water produced hydroxy-bridged dinuclear indium compounds. The hydroxy-bridged complex bearing a chloride ligand [(NN(H)O)InCl(mu-OH)](2) (9) was significantly more reactive toward dissociation and formation of a pyridine adduct than the iodo analogue [(NN(H)O)InI(mu-OH)](2) (10). All compounds were fully characterized in solution by NMR spectroscopy and in the solid state by single-crystal X-ray diffraction. In addition, DFT calculations were used to help explain the reactivity trends observed.

Solution and melt viscoelastic properties of controlled microstructure poly(lactide)

A series of controlled microstructure poly(lactide) (PLA) samples were synthesized using a novel chiral dinuclear indium catalyst capable of living polymerization of lactide. PLAs with different ratios of L- to D- monomer ratios of 100:0, 90:10, 75:25, 50:50, and 0:100 were investigated. The relationship between intrinsic viscosity and the absolute molar mass distribution of the samples obtained by light scattering gel permeation chromatography in tetrahydrofuran gives [η] = 0.014 + 0.75 Mw, a scaling law of typical coil dimensions of linear macromolecules in good solvent. The melt rheological study includes determination of zero-shear viscosity and its relationship with the molecular weight, the relaxation spectrum, and its relation with molecular weight characteristics, as well as plateau modulus and other important rheological parameters that are helpful in predicting the linear viscoelasticity of PLA. Emphasis is placed on the uniaxial melt behavior of these polymers. At low temperatures, significant strain hardening is observed, which gradually disappears with an increase in temperature and decrease of Hencky strain rate. The K-BKZ constitutive equation is used to model the experimental data. It is concluded that in spite of their linear structure, PLAs exhibit strain hardening which is not due to strain-induced crystallization, and it is solely due to the dynamics of molecular relaxation.

Role of aggregation in the synthesis and polymerization activity of SalBinap indium alkoxide complexes.

The reaction of racemic SalBinap ligand, (±)-H2(ONN*OMe), with InCl3 and excess NaOEt generated a mixture of two dinuclear compounds [(μ-κ(2)-ONN*OMe)In(μ-OEt)]2 (1a) and [κ(4)-ONN*OMe)In(μ-OEt)]2 (1b), which were isolated and fully characterized. Polymerization of racemic lactide with 1a and 1b was slow in refluxing THF and showed only modest stereoselectivity. Catalyst 1b displayed better control than 1a, with the experimental molecular weights of the resulting poly(lactic acid) in agreement with the expected values. The higher-than-expected molecular weights observed in polymers formed by 1a were due to partial initiation of the catalyst. The reaction of (±)-H2(ONN*OtBu) with InCl3 yielded (κ(4)-ONN*OtBu)InCl (2); however, further reactivity of the compound formed a mixture of products. An attempt to prevent aggregation by reacting (±)-H2(ONN*OMe) with InCl3 and excess NaO(i)Pr yielded an intractable mixture, including [(μ-κ(2)-ONN*OMe)In]2(μ-Cl)(μ-OH) (3). The thermal stabilities of compounds 1a and 1b under polymerization conditions were investigated. Examination of the polymerization behavior of complexes 1a and 1b and the reaction equilibrium between the two illustrates the importance of aggregation in indium salen complexes compared to their aluminum counterparts.

Probing the Role of Secondary versus Tertiary Amine Donor Ligands for Indium Catalysts in Lactide Polymerization

The role of the central amine donor in a previously reported dinuclear indium catalyst, [N(Me2)N(H)O)InCl]2(μ-Cl)(μ-OEt) (1), for the polymerization of lactide was investigated through experimental methods. The solid state structural data of a series of dimeric complexes related to 1, including the previously reported bromide derivative [(N(Me2)N(H)O)InBr](μ-Br)(μ-OEt) (2) and the newly synthesized methylated derivative [(N(Me2)N(Me)O)InCl]2(μ-Cl)(μ-OEt) (6), showed that weak hydrogen bonding may be present within some of these complexes in the solid state. The polymerization of rac-lactide with 2, 6, and a related achiral complex [(L(H))InCl]2(μ-Cl)(μ-OEt) (8) synthesized for this study indicates that hydrogen bonding may not influence the reactivity of these compounds. The nature of the central amine donor may play a role in tuning the reactivity of these types of catalysts. Catalysts with central secondary amine donors, such as complexes 1, 2, and 8, are 2 orders of magnitude more reactive than those with central tertiary amine donors, such as complex 6.

Dinucleating Ligand Platforms Supporting Indium and Zinc Catalysts for Cyclic Ester Polymerization

The synthesis of the first alkoxide-bridged indium complex supported by a chiral dinucleating ligand platform (1), along with its zinc analogue (2), is reported. Both complexes are synthesized in a one-pot reaction starting from a chiral dinucleating bis(diamino)phenolate ligand platform, sodium ethoxide, and respective metal salts. The dinucleating indium analogue (7) based on an achiral ligand backbone is also reported. Indium complexes bearing either the chiral or achiral ligand catalyze the ring-opening polymerization of racemic lactide (rac-LA) to afford highly heterotactic poly(lactic acid) (PLA; Pr > 0.85). The indium complex bearing an achiral ligand affords essentially atactic PLA from meso-LA. The role of the dinucleating ligand structure in catalyst synthesis and polymerization activity is discussed.

The Role of Nitrogen Donors in Zinc Catalysts for Lactide Ring-Opening Polymerization

The electronic effects of nitrogen donors in zinc catalysts for ring-opening polymerization of cyclic esters were investigated. Alkyl and benzyloxy zinc complexes supported by tridentate diamino- and aminoimino phenolate ligands were synthesized, and their solid-state and solution structures characterized. The solution-state structures showed that the alkyl complexes are mononuclear, while the alkoxy complexes are dimeric with the ligands coordinated with different denticities depending on the nature of the ligand donors. The catalytic activities of these compounds toward the ring-opening polymerization of racemic lactide were studied and showed that catalysts with secondary and imine nitrogen donors are more active than analogues with tertiary amines.