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Imine- and phosphinimine-supported indium complexes were used as catalysts in the polymerization of racemic lactide and ε-caprolactone as well as their copolymerization by the sequential and simultaneous addition of monomers. Tuning the electronics and sterics of the indium centers by either (i) changing the nature of the nitrogen donors and (ii) coordinating a hemilabile side group had a significant effect on the reactivity of the complexes, their stability, and their control in the synthesis of block copolymers. Specifically, the imine-supported complex (

We report the alternating copolymerization of caprolactone and epoxide through the in situ formation of pre-sequenced spiroorthoester monomer.

Neutral zinc complexes supported by H[PNNO], a diaminophenolate ligand bearing a pendant phosphine group, were synthesized and characterized.

A systematic rheological study on the influence of valency of different counterions on the properties of CNC hydrogels was carried out. Rheo-polarized microscopy was used to prove that preshear of 500 s

The controlled synthesis of block copolymers offers great potential for the valorization of biodegradable polyesters, many of which can be bioderived. Combining polyester blocks with other oxygenated polymers through ring-opening polymerization (ROP) and copolymerization (ROCOP) reactions allows for the synthesis of new materials with tunable properties. In this review article, we describe recent advances in the synthesis of diblock and multiblock polyesters, as well as polyether and polycarbonate block copolymers bearing biodegradable polyester block segments. Due to the great diversity of oxygenated monomers available from petrochemical and biomass sources, a great number of polymerization strategies have been developed involving metal- and organo-catalysts with different degrees of control. This review aims to provide an overview of the strategies available for the synthesis of different block copolymers: from the more widespread sequential addition methods to the more rare systems displaying high degrees of kinetic control in a mixture of monomers or those with controlled switchable behavior.

Neutral heavier group 13 metals aluminum, gallium, and indium have been utilized as Lewis acid catalysts in various organic transformations ranging from classical organic reactions to polymerization reactions. The introduction of cationic charge can enhance the Lewis acidity of metal centers and allow cationic group 13 complexes to be excellent catalysts in Lewis acid catalysis, including most of the transformations achieved with neutral group 13 complexes. While cationic aluminum complexes have been investigated extensively in catalysis, there is a more recent push to explore the catalytic reactivities of cationic gallium and indium complexes. The field of cationic group 13 complexes has been expanding with discrete cationic complexes supported by purposely designed ligands. This review aims to provide an overview of what has been done to date and ideas of what can be possibly done from now in the growing field of cationic group 13 complexes as catalysts.

The transformation of CO2 to value added commodity chemicals presents an impactful strategy to obtain products that are less dependent on fossil fuels. In this study, indium tribromide (InBr3) mixed with tetrabutylammonium bromide (NBu4Br) co-catalyst has been identified as a simple, highly efficient catalyst for the synthesis of cyclic carbonates from epoxides and CO2 at sub-atmospheric pressures, room temperature, and under solvent-free conditions. The InBr3/NBu4Br catalytic system is tolerant toward different functional groups with high conversions and >99% selectivity for cyclic carbonate without resorting to high pressures and temperatures. Moreover, a combination of in situ IR, NMR spectroscopy, and substrate labelling experiments enabled the proof of key catalytic steps and detection of reaction intermediates to elucidate the reaction mechanism. This technology represents a potential scalable system for the utilization of waste CO2.

Bioderived cellulose nanocrystals (CNCs) are used to create light, flexible, biocompatible, and biodegradable electronic devices. Herein, surface modification of cellulose nanocrystals was employed to fabricate cationic and anionic CNCs. Subsequently, we demonstrated rectification behavior from a fixed junction between two agarose hydrogels doped with cationic and anionic cellulose nanocrystals. The current rectification ratio reaches 70 reproducibly, which is significantly higher than that for analogous diodes generated with microfibrillated cellulose (∼15) and the first polyelectrolyte gel diode (∼40). The current-voltage characteristics of the CNC-hydrogel diode are influenced by concentration, gel thickness, scanning frequency, and applied voltage. The high surface area of CNC resulted in high charge density after surface modification, which in turn resulted in good rectification behavior from only small amounts of dopant material.

Cationic indium complexes were used as catalysts for the copolymerization of epoxides and racemic lactide (rac-LA) via sequential addition to form high-molecular weight block copolymers. Mechanisti...

We report a metal-mediated sequential addition synthetic route for copolymerization of lactide (or e-caprolactone) and methyl methacrylate. We synthesized a series of neutral and cationic indium co...

We report imine- and amine-based dinucleating ligands bearing a bisphenol backbone and explore their coordination chemistry with zinc to form zinc alkyl, alkoxide, acetate, and amide complexes. Full characterization of the complexes shows that this ligand framework can support dinuclear and trinuclear complexes. We explore the reactivity of the zinc alkyl and alkoxide complexes as catalysts for the ring opening polymerization of lactide and compared this reactivity to analogous mononuclear complexes. We show that 1) The amine-based complexes are more reactive than the imine-based analogues; 2) The trinuclear zinc alkyl species show unusual control and reproducibility for lactide polymerization; and 3) The extent of bimetallic cooperation is hampered by the ability of the ligand framework to form trinuclear clusters.

This is a comprehensive study of the effects of rationally designed hemilabile ligands on the stability, reactivity, and change in catalytic behavior of indium complexes. We report cationic alkyl indium complexes supported by a family of hemi-salen type ligands bearing hemilabile thiophenyl (2a), furfuryl (2b) and pyridyl (2c) pendant donor arms. Shelf-life and stability of these complexes followed the trend 2a < 2b < 2c, showing direct correlation to the affinity of the pendant donor group to the indium center. Reactivity towards polymerization of epichlorohydrin and cyclohexene oxide followed the trend 2a > 2b > 2c with control of polymerization following an inverse relationship to reactivity. Surprisingly, 2c polymerized racemic lactide without an external initiator, likely through an alkyl-initiated coordination-insertion mechanism., This is an in-depth study into the role hemilabile ligands play in tuning stability and reactivity in cationic indium complexes.

Group 13 metal complexes have emerged as powerful catalysts for transforming CO

We report the alternating copolymerization of caprolactone and epoxide through the

Poly(lactic acid) (PLA) is by far the most commonly produced biodegradable polyester. Metal-mediated ring opening polymerization of lactide is the most studied route for the synthesis of PLA, with hundreds of catalysts reported to date. Discrete metal initiators are comprised of an electropositive metal center, an ancillary ligand, and an initiating group such as an amide or an alkoxide. This difference in polarity between the electrophilic metal centers and the nucleophlic initiators often leads to catalyst aggregation. Partially due to this phenomenon, bimetallic catalysts, either dimeric, tethered, or dinucleating, are gaining attention as catalysts. The involvement of two metals in each of these motifs can affect the mechanism, and thus the activity, of the catalytic systems differently. This review is divided into three main segments: A) Dinuclear or dimeric species arising from aggregation of two discrete metal centers through bridging ligands; B) Tethered species involving two non-bridged metal centers on the same ligand architecture that can react independently and C) Dinucleating catalysts involving a multidentate ligand platform bound to two different metals, which are also bridged by a secondary ligand and can react in tandem. This review explores the structure-function relationship for each of these motifs by examining the effect of ligand design on various modes of bimetallic cooperativity in the ring opening polymerization mechanism of lactide.

We report the first example of discrete cationic indium complexes for the copolymerization of epoxides, cyclic ethers, and lactide. [SalenIn][SbF6] in particular proved to be a highly active catalyst for the homo-polymerization of functionalized epoxides and their copolymerization with other cyclic ethers THF, oxetane and oxepane. This catalyst also proved competent in the polymerization of epichlorohydrin and lactide, forming copolymers with good activity and control. Investigation of the role of counteranions and solvent donors on the kinetics of polymerization of epoxides revealed a subtle effect of solvents on initiation rates.

A series of aryl-capped poly(lactide)s (PLAs) were synthesized by living ring-opening polymerization with a chain transfer agent (also referred to as immortal polymerization, iROP) using a previously reported dinuclear indium catalyst, [(NNO)InCl]2(μ-Cl)(μ-OEt) (A). For understanding the extent and strength of association caused by the arylated chain ends, end-functionalized PLAs were analysed using thermal, rheological, and mechanical techniques. We show that the end-group has a greater effect on the properties of low molecular weight PLAs due to the larger number density of the aryl end groups; significant interactions can be induced under oscillatory shear conditions in the low frequency flow regime (terminal zone). The effects of associations on the extensional properties are also discussed.

We introduce an air- and moisture-stable hydroxy-bridged indium salan complex as a highly active and controlled catalyst for the ring-opening polymerization of cyclic esters in air. The reversible activation of this complex with linear and branched alcohols leads to immortal polymerization, allowing the controlled formation of block copolymers in air. It is the only reported example of a living catalyst that remains controlled after multiple exposures to ambient air at high temperatures. Although the prevalent catalyst for ring-opening polymerization, tin octanoate, is robust, it does not promote controlled polymerization. Our indium catalyst is exceptional in being both robust and controlled.

In order to further understand the relaxation behavior of binary blends of star and linear chains, new polymer blends consisting of linear poly(hydroxybutyrate) (PHB) matrix and PHB star molecules are designed, and their dynamics is investigated by varying the star concentrations and the molar mass of the linear matrix, while keeping few or no star–star entanglements in the blends. By studying the constraint release Rouse (CRR) relaxation of the star polymer diluted in the linear matrix at concentrations low enough to neglect star–star entanglements, we first point out the importance of the number of short linear chain entanglements on the CRR time of the long chains. For the blends composed of a larger proportion of star molecules, we then use this new definition of CRR time to determine the necessary time of a star–star entanglement segment to relax by CRR and explore its dilated tube, at the rhythm of the disentanglement/re-entanglements of the short chains. By considering this as the new reference tim...

Although the rheology of cellulose nanocrystalline (CNC) suspensions has been widely studied, less attention has been paid to the modified cellulose nanocrystals such as cationic and anionic cellulose hydrogels. In this work, the rheological behavior of cellulose nanocrystals (CNCs), anionic CNCs (pCNCs), and cationic CNCs (nCNCs), was comparatively studied. The rheological behavior demonstrated that the nCNC and pCNC form hydrogen bonding, which significantly contributes to the increase in the gel strengths in the sonicated state. The formation of such structures between individual fibers prevents flocculation due to the increased suspension stability. In addition, the extensive formation of hydrogen bonding in the case of nCNC compared to that of pCNC explains its enhanced rheological properties. The effect of pre-shear has been studied in detail for these systems by considering a combination of pre-shear and startup of steady shear in different shearing directions with certain rest/recovery time in between to eliminate strain history and thus eliminate the possible bias of pre-shearing on structure formation.

Ionic driven devices have been increasingly investigated in the drive to develop flexible and biointegrable electronics. One such device is a polyelectrolyte gel diode capable of rectifying ionic current. However, the underlying mechanism behind the rectification of current in polyelectrolyte gel diodes is not fully understood. Based on experimental data, it has been proposed that the rectification is due to the asymmetric distribution of ions at the interface between two gels doped with a cationic polyelectrolyte on one side and an anionic polyelectrolyte on the other. Additionally, an electrochemical model has been proposed to explain the mechanism quantitatively. Here, we explore the mechanism proposed by the Yamamoto–Doi model and validate it by using experimental data. We show that the diode operates via a physical mechanism that involves the electrochemical generation of proton and hydroxyl ions at the electrodes to generate current. Exponential currents (J) in the forward bias were observed and J = A − V (with A inversely proportional to the gel ionization and V the potential) in the backward bias, which coincides with predictions of the electrochemical Yamamoto–Doi model. Additionally, we also confirm the dependence of the electrochemical model on the dopant concentration in the backward bias regime.

Highly crystalline poly(hydroxybutyrate) suffers from high melting point and entanglement molecular weight. This leads to low melt strength, limits processing through regular techniques, and precludes many applications. In this work we report a series of racemic and enantiopure zinc catalysts supported by variously substituted diaminophenolate ancillary ligands which form high melt strength PHBs with different molecular weights and microstructure. These complexes are active for the highly controlled polymerization of β-butyrolactone (BBL); some can polymerize 2000 equiv of BBL in less than 30 min. Changing the steric bulk of the ligand forms PHBs of varied syndiotacticity (Pr = 0.75 to 0.55). These are highly robust systems capable of polymerizing an unprecedented 20000 equiv of BBL in the presence of 5000 equiv of benzyl alcohol. Thermorheological investigations reveal that the synthesized PHBs have surprisingly high melt strength at above the melting point. For processable PHBs, high density of entangle...

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.

A series of poly(lactide) (PLA) samples, exhibiting various levels of syndiotactic enrichment, were formed via the ring-opening polymerization of meso-lactide using two families of dinuclear indium catalysts: (RR/RR)-[(NNO)InCl]2(μ-Cl)(μ-OEt) (1) and (RR/RR)-[(ONNO)In(μ-OEt)]2 (2). Isotactic and heterotactic PLAs were also synthesized using known methodologies, and the thermal and rheological behaviors of these PLAs with different microstructures were compared. Solution rheological studies showed that the values of intrinsic viscosities and hydrodynamic radii as functions of molecular weight (Mw) were highest for iso-PLAs, followed by hetero and then syndio-PLAs. The viscosities of the heterotactically enriched PLAs were in agreement with literature values reported for atactic PLAs. The molecular weight between entanglements (Me) was greatest for the syndiotactically enriched PLAs, giving rise to the lowest zero-shear viscosity. In addition, hetero- and isotactically enriched PLA had higher flow activatio...

Indium and zinc complexes, [(NNOtBu)InCl]2(μ-Cl)(μ-OTHMB) (2) and (NNiOtBu)Zn(CH2CH3) (3), were used to produce monodispersed three- and six-armed star-shaped PHBs using tris(hydroxymethyl)benzene (THMB) and dipentaerythritol as the chain transfer agents. Reactions catalyzed by complex 2 were highly controlled, with THMB:catalyst ratios of up to 590:1, resulting in star-shaped PHBs with predictable molecular weights (Mn = 1.25–219 kDa) and narrow dispersities (Đ = 1.02–1.08). The zinc-based catalyst, 3, was less controlled than the indium analogue but nevertheless generated moderately syndiotactic PHBs with maximum Mn values of ∼100 kDa. Importantly, 3 allowed the formation of previously unknown 6-armed star PHBs, allowing us to compare the effects of the different PHB architectures on the rheological behavior of the materials. High molecular weight linear and star polymers were characterized using solution and melt viscoelastic studies. Zero-shear viscosity of linear PHBs exhibited a power law relationsh...

The first example of a one-component precursor to star-shaped polyesters, and its utilization in the synthesis of previously unknown star-shaped poly(hydroxybutyrate)-poly(lactic acid) block copolymers, is reported. A series of such mono- and bis-benzyl alkoxy-bridged complexes were synthesized, fully characterized, and their solvent dependent solution structures and reactivity were examined. These complexes were highly active catalysts for the controlled polymerization of β-butyrolactone to form poly(hydroxybutyrate) at room temperature. Solution studies indicate that a mononuclear propagating species formed in THF and that the dimer-monomer equilibrium affects the rates of BBL polymerization. In the presence of linear and branched alcohols, these complexes catalyze well-controlled immortal polymerization and copolymerization of β-butyrolactone and lactide.

Inexorably, the environmental persistence and damage caused by polyolefins have become major drawbacks to their continued long-term use. Global shifts in thinking from fossil-fuel to renewable biobased resources have urged researchers to focus their attention on substituting fossil-fuel based polymers with renewable and biodegradable alternatives on an industrial scale. The recent development of biodegradable polyesters from ring opening polymerization (ROP) of bioderived cyclic ester monomers has emerged as a promising new avenue toward this goal. Ever increasing numbers of metal-based initiators have been reported in the literature for the controlled ROP of cyclic esters, in particular for the polymerization of lactide to produce poly(lactic acid) (PLA). PLA has several material weaknesses, which hinder its use as a replacement for commodity plastics. Despite many advances in developing highly active and controlled catalysts for lactide polymerization, no single catalyst system has emerged to replace industrially used catalysts and provide access to PLA materials with improved properties. We reported the first example of indium(III) for the ring opening polymerization of lactide. Since then, indium(III) has emerged as a useful Lewis acid in initiators for the controlled polymerization of lactide and other cyclic esters. In particular, we have developed a large family of chiral dinuclear indium complexes bearing tridentate diaminophenolate ligands and tetradentate salen and salan ligands. Complexes within our tridentate ligand family are highly active initiators for the moderately isoselective living and immortal polymerization of rac-lactide, as well as other cyclic esters. We have shown that subtle steric effects influence aggregation in these systems, with polymerization typically proceeding through a dinuclear propagating species. In addition, profound effects on polymerization activities have been observed for central tertiary versus secondary amine donors in these and other related systems. In contrast, our well-controlled and highly active chiral indium salen systems are more isoselective than the tridentate analogues and polymerize lactide via a mononuclear propagating species. Again, we have noticed that subtle steric and electronic changes to the ligand can influence both polymerization activity and stereoselectivity via aggregation phenomena. Recently, we have reported a promising new chiral indium catalyst supported by a tetradentate salan ligand. This catalyst is remarkably water and air stable and can be activated by linear and branched alcohols to provide controlled access to multiblock copolymers in air. This catalyst represents an important step forward toward generating new, commercially relevant catalysts for ROP of cyclic esters to produce novel biodegradable polymers, and highlights the unique value of indium-based catalysts in the field.

This report describes two different methodologies for the synthesis of aryl end-functionalized poly(lactide)s (PLAs) catalyzed by indium complexes. In the first method, a series of para-functionalized phenoxy-bridged dinuclear indium complexes [(NNO)InCl]2(μ-Cl)(μ-OPhR) (R = OMe (1), Me (2), H (3), Br (4), NO2 (5)) were synthesized and fully characterized. The solution and solid state structures of these complexes reflect the electronic differences between these initiators. The polymerization rates correlate with the electron donating ability of the phenoxy initiators: the para-nitro substituted complex 5 is essentially inactive. However, the para-methoxy variant, while less active than the ethoxy-bridged complex [(NNO)InCl]2(μ-Cl)(μ-OEt) (A), shows sufficient activity. Alternatively, aryl-capped PLAs were synthesized via immortal polymerization of PLA with A in the presence of a range of arylated chain transfer agents. Certain aromatic diols shut down polymerization by chelating one indium centre to form a stable metal complex. Immortal ROP was successful when using phenol, and 1,5-naphthalenediol. These polymers were analysed and chain end fidelity was confirmed using 1H NMR spectroscopy, MALDI-TOF mass spectrometry, and UV-Vis spectroscopy. This study shed light on possible speciation when attempting to generate PLA-lignin copolymers.

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.

A DFT study of the ring-opening polymerization of lactide (LA) induced by a dinuclear indium catalyst supported by a chiral diamino phenoxy ligand, [(NNHO)InCl]2(μ-Cl)(μ-OEt) (1), is reported. The nature of the active catalyst, mononuclear vs dinuclear, was investigated and was shown to be dinuclear because of the high energetic cost of its dissociation. The selectivity of the system was investigated for the polymerization of LA with the dinuclear (R,R/R,R)-1 catalyst. In complete agreement with experimental results we observed that (1) selectivity is controlled by the nucleophilic addition of LA to the alcoholate, resulting in the chain-end control of polymerization, (2) a slight kinetic preference for the polymerization of l-LA over d-LA is found that translates to a krel value of ∼14, which is identical with the experimental value, and (3) when rac-LA is used, no clear preference for d- vs l-LA insertion is found, leading to isotactic PLA.

The dinuclear indium catalyst [(NNO)InCl]2(μ-OEt)(μ-Cl), previously reported to be highly active for the living ring-opening polymerization of cyclic esters lactide (LA) and β-butyrolactone (BBL), was used to generate a series of triblock copolymers of poly(lactic acid) (PLA) and poly(hydroxybutyrate) (PHB). Copolymers PLLA–PDLLA–PLLA and PLLA–PDLLA–PDLA, synthesized via sequential monomer addition, showed low molecular weight distributions and excellent correlation between the calculated and experiment molecular weights. Significantly, triblock copolymers of the type PLA–PHB–PLA were also synthesized for the first time through a sequential addition technique. Analysis of polymers after each addition of monomer showed that although only 85% conversion was achieved after addition of BBL, the remaining chain ends were active and addition of more lactide yielded a triblock. Rheological studies of PLLA–PHB–PDLA indicated solid like behavior even well above the temperature at which stereocomplex formation was ...

In this study, different compositions of nearly monodispersed diblock copolymers of dl -lactide or d -lactide and l -lactide were synthesized by living ring-opening polymerization with a dinuclear indium catalyst. The effects of molecular weight and block length ratio on the rheological behavior of dl and l -lactide diblock copolymers in the disordered state were investigated. For comparison, blends of PDLLA and PLLA homopolymers of equivalent molecular weights to the diblock copolymers were prepared. We found that the time–temperature (t–T) superposition principle is applicable to the diblock copolymers PLLA- b -PDLLA and blends in the disordered state. However, the t–T superposition failed at low temperatures close to the temperature of crystallization. In contrast, diblock copolymers PLLA- b -PDLA formed stereocomplex crystallites of high melting point (slightly above 200 °C) that causes a viscosity enhancement. The failure of t–T superposition was found due to existing of micro homo or stereocomplex crystallites. The non-isothermal crystallization behavior was investigated using differential scanning calorimetry (DSC). The DSC thermograms of blends exhibited a single glass transition at 50–60 °C followed by melting point of PLLA at 177 °C. With decreasing of the PLLA content in the blends, the intensity of the melting peak decreased. In addition, different crystallization behavior was observed for diblock copolymers compared to their equivalent blends. Specifically, low temperatures and enthalpies of melting peaks were observed for diblock copolymers. These also show improvement in elongation at break and tensile strength as compared to their counterpart homopolymer blends.

The wall slip and melt fracture behaviour of several commercial polylactides (PLAs) as well as their rheological properties under shear and extensional have been investigated. The PLAs have had weight-average molecular weights in the range of 104–105 g/mol and studied in the temperature range of 160–200°C. The solution properties and linear viscoelastic behaviour of melts indicate linear microstructure behaviour. PLAs with molecular weights greater than a certain value were found to slip, with the slip velocity to increase with decrease of molecular weight. The capillary data were found to agree well with linear viscoelastic envelope once correction for slip effects was applied. The onset of melt fracture for the high molecular weight PLAs was found to occur at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature. Addition of 0.5 wt.% of a polycaprolactone (PCL) into the PLA that exhibits melt fracture was found to be effective in eliminating and delaying the onset of melt fracture to higher shear rates. This is due to significant interfacial slip that occurs in the presence of PCL.

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.

Reaction of the phosphine-tethered isocyanide iron(II) complex 1, [CpFe(CO)(PCN)]I, with primary and secondary amines forms the corresponding acyclic (diamino)carbene complexes [CpFe(CO)(PCXNH)]I; ...

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.

Poly(lactic acid) (PLA) particles were generated by rapid expansion of supercritical PLA/CO2 solutions (RESS). Two different PLA samples, one with high (PDI = 2.4) and the other one with low (PDI = 1.4) polydispersity but similar number average molecular weight, were compared. After micronization, the polymers were analysed by rapid-scan infrared spectroscopy, scanning electron microscopy, size-exclusion chromatography, differential scanning calorimetry, and NMR spectroscopy. Our investigation reveals that the polydispersity of the polymers strongly affects the size but not the shape of the particles. We found larger particles (∼730 nm) for the PLA with high polydispersity than for the PLA with low polydispersity (∼270 nm). In both cases, spherical particles were formed. Moreover, our results clearly show that PLA with high polydispersity is less suitable for RESS processing because the low-molecular weight chains are depleted over time and process conditions are thus not constant.

Electron rich phosphinimine-imine proligands Ph(3)PN(C(6)H(4))C(Ph)(NAr) (L(Ar)) (Ar = 4-(OEt)C(6)H(4) (OEt), 3,5-Me(2)C(6)H(3)(Xyl)) were synthesized in three steps from 2-aminobenzophenone. These compounds, along with previously reported L(Mes) and L(Tol) (Mes = 2,4,6-Me(3)C(6)H(2), Tol = 4-MeC(6)H(4)) were used to synthesize a series of tetracarbonyltungsten(0) complexes: L(Mes)W(CO)(4) (1), L(Tol)W(CO)(4) (2), L(OEt)W(CO)(4) (3), and L(Xyl)W(CO)(4) (4). The ligands were evaluated by analysis of the carbonyl stretching frequencies of the tungsten complexes and were shown to be better sigma-donors and poorer pi-acceptors compared to similar ligands in the literature. The coordination chemistry of the proligands was expanded to other late transition metals and L(Mes)CoCl(2) (5), L(Tol)CoCl(2) (6), L(Mes)NiBr(2) (7), L(Tol)NiBr(2) (8), L(Mes)ZnCl(2) (9), and L(Tol)ZnCl(2) (10) were synthesized by the direct reaction of L(Mes) and L(Tol) with the respective metal dihalide precursors. The complexes were fully characterized and the molecular structures of complexes 3, 6, 7, and 10 were reported. The synthesis of zinc complexes 9 and 10 was dependent on the steric bulk of the ligand. Complex 10 proved to be resistant to derivatization via a number of routes.

Donor-functionalized (phosphino)(amino)- and (silyl)(amino)carbenes are generated via nucleophilic attack at the carbon atom of a coordinated isocyande on a piano-stool iron(II) complex. The template synthesis methodology involves the formation of ylidene complexes, which are reduced to yield the desired carbene complexes. The electronic properties of the resulting carbene complexes are similar and indicate that in the (phosphino)(amino)carbene the phosphine lone pair is not interacting with the carbene carbon and is available for further reactivity.

The synthesis and characterization of triphenylphosphinimine−arylimine bidentate ligands (aryl = 2,4,6-trimethylphenyl or 4-methylphenyl) along with their dichloropalladium(II) complexes are reported. These complexes form tridentate orthopalladated species upon heating in the presence of sodium acetate. The reverse reaction occurs upon addition of HCl·Et2O. The rate of orthopalladation and reprotonation is highly dependent on the steric bulk of the ligand. Related cationic species were also synthesized and characterized and found to be inert in the presence of 1-hexene and styrene.

Chiral diaminophenoxy proligands, H(NNR′OR), where R = t-Bu, H and R′ = Me, H, have been developed and their respective zinc ethyl complexes, (NNR′OR)ZnEt, 3a (R = t-Bu, R′ = Me), 3b (R = H, R′ = Me), 3c (R = t-Bu, R′ = H), have been synthesized. The reactivity of 3a with alcohols was explored in detail and compared to a compound reported by Hillmyer, Tolman et al., LZnEt (L = 2,4-di-tert-butyl-6-{[(2′-dimethylaminoethyl)methylamino]-methyl}phenolate). Unlike LZnEt, 3a was inert toward ethanol (as well as methanol, isopropanol, and water). It reacted with phenol and with hydrochloric acid to form (NNMeOtBu)ZnOPh, 4a, and (NNMeOtBu)ZnCl, 5a, respectively. Racemic and enantiopure forms of 4a, (±)-4a and (R,R)-4a, were synthesized. The phenoxide complex catalyzed the ring opening polymerization of lactide to atactic poly(lactic acid).

InI_3 is able to catalyze the conversion of methanol to a mixture of hydrocarbons at 200 °C with one highly branched alkane, 2,2,3-trimethylbutane (triptane), being obtained in high selectivity. The mechanism for InI_3-catalyzed reactions appears to be basically the same as that proposed for the previously studied ZnI_2-catalyzed system in which sequential methylation of olefins is followed by competing reactions of the resulting carbocation: proton loss to give the next olefin vs hydride transfer to give the corresponding alkane. Although the reaction conditions and typical triptane yields achievable with ZnI_2 and InI_3 are quite similar, the two systems behave rather differently in a number of important particulars, including significant differences between the detailed product distributions. Most of the differences in behavior can be ascribed to the stronger Lewis acidity of InI_3, including the ability to activate some alkanes, the higher activity for methylation of arenes, and the fact that methanol conversion can be observed at somewhat lower temperatures with InI_3 than with ZnI_2.

A methodology for controlling aggregation in highly active and isoselective indium catalysts for the ring opening polymerization of racemic lactide is reported., A methodology for controlling aggregation in highly active and isoselective indium catalysts for the ring opening polymerization of racemic lactide is reported. A series of racemic and enantiopure dinuclear indium ethoxide complexes bearing salen ligands [(ONNOR)InOEt]2 (R = Br, Me, admantyl, cumyl, t-Bu) were synthesized and fully characterized. Mononuclear analogues (ONNOR)InOCH2Pyr (R = Br, t-Bu, SiPh3) were synthesized by controlling aggregation with the use of chelating 2-pyridinemethoxide functionality. The nuclearity of metal complexes was confirmed using PGSE NMR spectroscopy. Detailed kinetic studies show a clear initiation period for these dinuclear catalysts, which is lacking in their mononuclear analogues. The polymerization behavior of analogous dinuclear and mononuclear compounds is identical and consistent with a mononuclear propagating species. The isotacticity of the resulting polymers was investigated using direct integration and peak deconvolution methodologies and the two were compared.

A series of (±)- and (R,R)-tridentate diamino, ortho/para disubstituted phenolate proligands H(NNO(R)) with various phenolate substituents was synthesized and used to prepare indium dichloride complexes (NNO(R))InCl2via salt metathesis of the deprotonated ligands with indium trichloride. These complexes are dinuclear in the solid state, in contrast to previously reported complexes with t-butyl or methyl phenolate substituents. Solution state (1)H and PGSE NMR spectroscopy suggests that a fast exchange between the monomeric and dimeric forms of these complexes may exist in solution and is likely influenced by the chirality of the complexes undergoing aggregation. The indium dichloride complexes were utilized to synthesize dinuclear indium ethoxide complexes via salt metathesis with sodium ethoxide. These complexes were active for the polymerization of lactide. In situ and bulk polymerization data confirmed differences in the activity and selectivity of these systems based on the phenolate substituents as well as the ligand chirality.