Michael P. Shaver

Controlled Radical Polymerization Mediated by Amine–Bis(phenolate) Iron(III) Complexes

Tetradentate amine-bis(phenolate) iron(III) halide complexes containing chloro substituents on the aromatic ring are extremely efficient catalysts for controlled radical polymerization. Molecular weights are in good agreement with theoretical values and polydispersity indexes (PDIs) are as low as 1.11 for styrene and methyl methacrylate polymerizations. Complexes containing alkyl substituents on the aromatic ring are less efficient. Kinetic data reveal activity for styrene polymerization among the fastest reported to date and initial studies implicate a multimechanism system. Despite the highly colored polymerization media, simple work-up procedures yield pure white polymers.

Tacticity Control in the Synthesis of Poly(lactic acid) Polymer Stars with Dipentaerythritol Cores

The synthesis of a family of polymer stars with arms of varied tacticities is discussed. The effect of polymer tacticity on the physical properties of these polymer stars is presented. Dipentaerythritol cores support six poly(lactic acid) (PLA) arms. Lewis acidic tin and aluminum catalysts control the polymerization to afford polymer stars of variable tacticity. The analysis of these polymers by NMR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and differential scanning calorimetry reveals the effects of tacticity control on the physical properties of the polymer stars. Preliminary decomposition studies suggest that the biodegradation profile of a polymer star may also be tuned by stereochemical control. This is the first systematic altering of tacticity in PLA polymer stars, showing that polymer tacticity can have a great impact on star properties.

Tacticity-induced changes in the micellization and degradation properties of poly(lactic acid)-block-poly(ethylene glycol) copolymers.

Poly(lactic acid)-block-poly(ethylene glycol) copolymers (PLA-b-PEG) featuring varying tacticities (atactic, heterotactic, isotactic) in the PLA block were synthesized and investigated for their micellar stability, degradation, and thermal properties. Utilizing tin(II) bis(2-ethylhexanoate), aluminum salan, and aluminum salen catalysts, the copolymers were synthesized through the ring-opening polymerization of d-, l-, rac-, or a blend of l- and rac-lactide using monomethoxy-poly(ethylene glycol) as a macroinitiator. The critical micelle concentration, which reflects the micellar stability, was probed using a fluorescence spectroscopic method with pyrene as the probe. The copolymers were degraded in a methanolic solution of 1,5,7-triaza-bicyclo[4.4.0]dec-5-ene and the degradation was measured by (1)H NMR spectroscopic and gel permeation chromatographic analyses. Differential scanning calorimetry and thermogravimetric analysis provided information on the thermal properties of the copolymers. Atactic and heterotactic microstructures in the PLA block resulted in lower micellar stability, as well as faster degradation and shorter erosion time compared to polymers with high isotactic enchainment (Pm). By modification of the Pm, micellar stability, degradation, and erosion rates of the copolymers can be tuned to specific biomedical applications. Interestingly, while tin(II) bis(2-ethylhexanoate) and aluminum salan-catalyzed PLA-b-PEG copolymers exhibited similar micellization behavior, the aluminum salen-catalyzed PLA-b-PEG exhibited unique behavior at high micelle concentration in the presence of the pyrene probe. This unique behavior can be attributed to the disintegration of the micelles through the interactions of long isotactic stereoblock segments.

Atom Transfer Radical Polymerization (ATRP) and Organometallic Mediated Radical Polymerization (OMRP) of Styrene Mediated by Diaminobis(phenolato)iron(II) Complexes: A DFT Study

This study has addressed the radical polymerization of styrene mediated by the diaminobis(phenolate) complexes [Fe(O-2,4-Y2C6H2-5-CH2)2NCH2CH2NMe2], abbreviated as [Fe(II)]. The system is known to be well controlled when Y = Cl but not when Y = alkyl. The control was proposed to occur by a dual ATRP+OMRP mechanism. We have used DFT calculations to address the Y = Cl and Y = CH3 systems. The growing radical chain, ATRP dormant chain, and OMRP dormant chain were simplified to PhCH(CH3)(•), PhCH(CH3)-Cl, and [PhCH(CH3)-Fe(III)]. The idealized ATRP activation/deactivation equilibrium involves [Fe(III)-Cl] (I(Y)) and PhCH(CH3)(•) on the active side and [Fe(II)] (II(Y)) and PhCH(CH3)-Cl on the dormant side, whereas the OMRP activation/deactivation relates [Fe(II)] and PhCH(CH3)(•) with [PhCH(CH3)-Fe(III)] (III(Y)). A benchmarking of various functionals against the known spin properties of alkylporphyriniron(III) shows B3PW91* to be a suitable functional. For the purpose of bond dissociation energy calculations, a dispersion correction was made (B3PW91*-D3). For both Y systems, the ground state is a spin sextet for I, a spin quintet for II, and a spin quartet for III. The calculations show a greater energy cost for the ATRP activation process involving Cl atom addition to II(Cl) to yield I(Cl) (7.2 kcal/mol) relative to the process transforming II(Me) to I(Me) (2.1 kcal/mol). On the other hand, the alkyl addition transforming II to III provides slightly greater stabilization for II(Cl) (27.1 kcal/mol) than for II(Me) (26.1 kcal/mol). As a result, both ATRP and OMRP trapping processes provide greater stabilization for the Y = Cl system, in agreement with the observed better control. The charge analysis attributes these minor but determining energy differences to the inductive electron withdrawing effect of the phenolato Cl substituents. The ATRP and OMRP activation/deactivation pathways have been analyzed in relation to the spin state change; they show in each case the absence of an activation barrier, and both processes corresponding to spin allowed single-state pathways occurring on the quartet (OMRP) and quintet (ATRP) potential energy surfaces. Molecular volume calculations suggest that the balance between ATRP and OMRP may shift in favor of the latter at higher pressures.

Tuning thermal properties and microphase separation in aliphatic polyester ABA copolymers

Four alkyl substituted β-lactones were investigated as monomers in ring opening polymerisation to produce a family of poly(3-hydroxyalkanoate)s. Homopolymers were synthesised using a robust aluminium salen catalyst, resulting in polymers with low dispersity (Đ < 1.1) and predictable molecular weights. ABA triblock copolymers were prepared using poly(L-lactic acid) as the A block and the aforementioned poly(3-hydroxyalkanoate) as the B block via a sequential addition method. Characterisation of these copolymers determined they were well controlled with low dispersities and predictable molecular weight. DSC analysis determined copolymers prepared from β-butyrolactone or β-valerolactone yielded polymers with tunable and predictable thermal properties. Copolymers prepared from β-heptanolactone yielded a microphase separated material as indicated by SAXS, with two distinct Tgs. The polymers could be readily cast into flexible films and their improved tensile properties were explored.

Organometallic mediated radical polymerization of vinyl acetate using bis(imino)pyridine vanadium trichloride complexes

The synthesis and characterization of one novel proligand and six novel vanadium(III) trichloride complexes is described. The controlled radical polymerization activity towards vinyl acetate of these, and eight other bis(imino)pyridine vanadium trichloride complexes previously reported, is investigated. Those complexes possessing variation at the N-aryl para-position with no steric protection offered by ortho-substituents (4 examples) result in poor control over poly(vinyl acetate) polymerization. Control is improved with increasing steric bulk at the ortho-position of the N-aryl substituent (4 examples) although attempts to increase steric bulk past isopropyl were unsuccessful. Synthesizing bis(imino)pyridine vanadium trichloride complexes with substituted imine backbones restores polymerization control when aliphatic substituents are used (4 examples) but ceases to make any drastic improvements on catalyst lifetime. Modification of the polymerization conditions is also investigated, in an attempt to improve the catalyst lifetime. Expansion of the monomer scope to include other vinyl esters, particularly those derived from renewable resources, shows promising results.

An aromatic/aliphatic polyester prepared via ring-opening polymerisation and its remarkably selective and cyclable depolymerisation to monomer

The ring-opening polymerisation of 2,3-dihydro-5H-1,4-benzodioxepin-5-one (2,3-DHB) with aluminium salen or organocatalysts gives polyester homopolymers and copolymers with L-lactide or rac-β-butyrolactone that contain both aromatic and aliphatic linkages, the first polymers with an aromatic ring in the backbone prepared by this key method. The same Al salen catalyst catalyses a remarkably selective depolymerisation to monomer under modified reaction conditions. The process may be cycled to repeatedly recycle polymer to monomer and maintain the polymers low dispersity.

Amine-bis(phenolate) Iron(III)-Catalyzed Formal Hydroamination of Olefins

A practical synthesis of highly functionalized amines by the formal hydroamination reaction of alkenes with nitroarenes catalyzed by an air stable amine-bis(phenolate) iron(III) complex is reported. The reaction uses an easily handled silane, low catalyst loadings, and mild reaction conditions. A wide range of substrates are transformed with synthetically useful yields (21 examples).

ATRP/OMRP/CCT Interplay in Styrene Polymerization Mediated by Iron(II) Complexes: A DFT Study of the α‐Diimine System

A DFT study of various model systems has addressed the interference of catalytic chain transfer (CCT) as a function of the R(2) substituent in the atom-transfer radical polymerization (ATRP) of styrene catalyzed by [FeCl2 (R(1) N=C(R(2))C(R(2))=NR(1))] complexes. All model systems used R(1) =CH3 in place of the experimental Cy and tBu substituents and 1-phenylethyl in place of the polystyrene (PS) chain. A mechanistic investigation of 1) ATRP activation, 2) radical trapping in organometallic-mediated radical polymerization (OMRP), and 3) pathways to the hydride CCT intermediate was conducted with a simplified system with R(2)=H. This study suggests that CCT could occur by direct hydrogen-atom transfer without any activation barrier. Further analysis of more realistic models with R(2)=p-C6 H4 F or p-C6 H4 NMe2 suggests that the electronic effect of the aryl para substituents significantly alters the ATRP activation barrier. Conversely, the hydrogen-atom-transfer barrier is essentially unaffected. Thus, the greater ATRP catalytic activity of the p-NMe2 system makes the background CCT process less significant. The DFT study also compares the [FeCl2 (R(1) N=C(R(2))=C(R(2))=NR(1))] systems with a diaminobis(phenolato) derivative for which the CCT process shows even greater accessibility but has less incidence because of faster ATRP chain growth and interplay with a more efficient OMRP trapping. The difference between the two systems is attributed to destabilization of the Fe(II) catalyst by the geometric constraints of the tetradentate diaminobis(phenolato) ligand.

Ring-opening polymerization of rac-lactide and ε-caprolactone using zinc and calcium salicylaldiminato complexes

Tridentate Schiff base complexes of zinc and calcium were prepared and tested in the ring-opening polymeriza- tion of 3-caprolactone and rac-lactide to generate biodegradable polymeric materials from biocompatible metals. Alteration of the pendant donor arm attached to the imine backbone provides some control over catalyst composition and polymeriza- tion activity. Complexes of the formula (ONN)ZnN(SiMe3)2, where (ONN) = 2-(N-donor arm-imine)(4,6-di(tert-butyl)phen- oxide), were isolated with ethyldimethylamine, ethylpiperidine, and ethylmorpholine substituents, while disproportionation led to the isolation of (ONN)2Zn complexes with methylpyridine, quinoline, and ethyldiisopropylamine derivatives, two of which were crystallographically characterized. Calcium complexes were more stable and novel (ONN)CaN(SiMe3)2 com- plexes with ethylpiperidine and ethyldiisopropylamine substituents were reported. Zinc and calcium catalysts coordinated to a single tridentate ligand were effective at initiating the polymerization of 3-caprolactone, but did not control the polymeriza- tions, whereas the bis(ligand) complexes produced no polymer. These catalysts were effective at controlling the polymeriza- tion of rac-lactide. Coordinatively saturated complexes inhibit the polymerization, while initiation from either the amido or ligand alkoxide functionalities produces poly(lactic acid) with low polydispersities.