Approximately 90% of all manufactured chemicals rely on catalysis sometime during their production.  By both necessity and design, we have tried to develop broad expertise across a number of chemical disciplines related to catalysis:  synthetic organic chemistry, physical organic chemistry, organometallic chemistry, inorganic chemistry, industrial chemistry, polymerization chemistry, polymer science, environmental chemistry, and theoretical chemistry.  Since 2001 our research group has identified several new, selective, and efficient catalysts for both small molecule transformations and polymerizations.  Our targeted catalysts are often relevant to industrial applications, offering mechanistic insight, improved catalytic behavior, or altogether new pathways for catalytic bond formation.

Syndiotactic polypropylene (s-PP)—the stereochemical cousin of the commercially dominant isotactic form—was first made in significant amounts by discrete organometallic catalysts in 1988 [1].  Such single-site catalysts have revolutionized the polyolefin industry in the last ten years, allowing for the synthesis of many polyolefins with novel architectures and important commercial applications.  One of our primary goals has been to devise syndioselective copolymerization catalysts for producing new materials that are functional substitutes for polyvinyl chloride (PVC).  Copolymers of s-PP are targeted because s-PP itself has an anomalously high impact strength—even larger than that of PVC.  Worldwide efforts toward PVC abatement are growing because of the recognized environmental incompatibility of the PVC chemical lifecycle [2].  Our research efforts have resulted in the most syndioselective propylene polymerization catalyst and the highest melting s-PP [3].  Features included in the original design [4] also afforded minimal discrimination between small and large monomers, an ideal trait for a copolymerization catalyst [5].  More recently, we have demonstrated exquisite control in the preparation of syndiotactic-copoly[propylene/α-olefin], wherein α-olefin comonomer (C4 and greater) incorporation introduces branches into an otherwise linear and crystalline s-PP polymer. 

 

A promising area of research toward which we are directing increasing efforts is aldimine coupling with cyanide-based catalysts. While simple aldimine dimerization has been known since 1928 [6], cyclization and polymerization via aldimine coupling are novel transformations that we have recently reported [7]. This simple carbon-carbon bond forming reaction is leading us to a variety of new structures [8], including: heterocycles, carbocycles, alpha-diimines, alpha-diketones, vicinal diamines, alpha-amino ketones, novel dyes, conjugated polymers, non-conjugated polymers, macrocycles, carbenes, organocatalysts, and ligands for transition metal mediated homogeneous catalysis, including chiral variants. The examples below illustrate the generality and utility of cyanide catalyze aldimine coupling.

Our research group has been exploring the use of biorenewable oxygen-rich monomers—more directly obtained from Nature—for replacing petroleum-based feedstocks that are required for the vast majority of current polymer syntheses.  Methanol (wood alcohol) can be obtained via wood distillation and it is the C1 feedstock employed for making trioxane.  We have demonstrated the cationic copolymerization of trioxane and long-chain epoxides to produce polyoxymethylene (POM) copolymers [9].  Importantly, the long-chain epoxide comonomers can be obtained from the α-olefins generated via ethenolysis of triglycerides, which are found abundantly in vegetable oils.  In a strategy akin to that for linear low density polyethylene (LLDPE), the controlled incorporation of long chain epoxides into linear POM affords a new family of branched POM copolymers with finely-tuned mechanical properties.


Lactic acid is readily obtained by the controlled fermentation of starches, usually from corn.  The ring-opening polymerization (ROP) of the corresponding cyclic dimer, lactide, provides polylactic acid (PLA), an increasingly important biodegradable thermoplastic polyester.  Because a toxic tin-based catalyst is normally used for this polymerization, a more benign catalyst is desirable for medical grade PLA.  With attention to this issue, we developed a sodium-based catalyst for the highly controlled ROP of lactide [10].  Moreover, its activity is second only to a certain zinc-based catalyst under comparable conditions.


While PLA is an increasingly popular plastic, it suffers from at least one major drawback.  Its low glass transition temperature near 60°C precludes its use in many applications—notably hot beverage containers.  Our goal is to devise a PLA copolymer that is a suitable polystyrene substitute.  These PLA copolymers should readily biodegrade and not persist in the environment like petroleum-based polystyrene plastics. Our most recent results in this endeavor have identified suitable catalysts and polymerization conditions for the copolymerization of a biorenewable rigid monomer with lactide [11].  Our focus now turns to measuring the thermomechanical properties of these novel copolymers as a function of the fractional incorporation of comonomer.

 

Our efforts in experimental polymerization chemistry are accompanied by a sizeable thrust in theoretical polymer chemistry.  We have applied derived analytical equations and Monte Carlo simulations to assist in the understanding of polyolefin tacticity (stereochemistry), specifically in a class of isotactic-hemiisotactic elastomeric polypropylenes [12].   These methods and techniques are being extended to copolymers based on syndiotactic PP, POM, and PLA.  A more detailed understanding of the copolymer linear sequence will clarify issues related to polymer crystallinity and should be critical in optimizing, for example, the opposing properties of tensile strength and impact resistance for a given targeted application.

 

Density functional theory calculations have settled some controversies regarding the thermodynamics of carbon dioxide/olefin copolymerizations [13].  These calculations and our inability to reproduce certain reported experiments [14] temper the enthusiasm for this potential route to polyesters and provide important information to the community interested in carbon dioxide utilization in organic synthesis. Other theoretical efforts demonstrate the first chemical applications of the unusual S_2∞ and C_∞ point groups in devising an algorithm for polymer chirality determination not reliant upon translational symmetry operations [15].

We have developed a catalytic (2.5 mol%), nickel-based alternative to stoichiometric chromium reagents for common laboratory oxidations. The combination of nickel (II) salts and aqueous sodium hypochlorite (bleach) generates 3-5 nm nanoparticles that function as high surface area heterogeneous catalysts for the oxidation of primary alcohols, secondary alcohols, and aldehydes to give carboxylic acids, ketones, and carboxylic acids, respectively  [16].  This inexpensive, practical, and efficient system also converts α,β-unsaturated carboxylic acids to epoxy-acids, which is essentially impossible with any other standard epoxidation catalyst because of the substrate’s electron deficiency.  In most cases these oxidations can be performed without an organic solvent, making this approach a “greener” alternative to conventional methods.

[1] Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem. Soc.  1988, 110, 6255-6256.
[2] Thornton, J. Pandora’s Poison: Chlorine, Health, and a new Environmental Strategy; MIT Press:  Cambridge, Massachusetts, 2000.
[3] Irwin, L. J.; Miller, S. A.  “Unprecedented Syndioselectivity and Syndiotactic Polyolefin Melting Temperature:  Polypropylene and Poly(4-methyl-1-pentene) from a Highly Active, Sterically Expanded eta(1)-Fluorenyl-eta(1)-Amido Zirconium Complex” J. Am. Chem. Soc. 2005, 127, 9972-9973.
[4] Irwin, L. J.; Reibenspies, J. H.; Miller, S. A. “Synthesis and characterization of sterically expanded ansa-eta(1)-fluorenyl-amido complexes” Polyhedron, 2005, 24, 1314-1324.
[5] Irwin, L. J.; Reibenspies, J. H.; Miller, S. A. “A Sterically Expanded “Constrained Geometry Catalyst” for Highly Active Olefin Polymerization and Copolymerization: An Unyielding Comonomer Effect” J. Am. Chem. Soc. 2004, 126, 16716-16717.
[6] Strain, H. H.  J. Am. Chem. Soc. 1928, 50, 2218-2223.
[7] Reich, B. J. E.; Justice, A. K.; Beckstead, B. T.; Reibenspies, J. H.; Miller, S. A. “Cyanide-Catalyzed Cyclizations via Aldimine Coupling” J. Org. Chem. 2004, 69, 1357-1359.
[8] Reich, B. J. E.; Greenwald, E. E.; Justice, A. K.; Beckstead, B. T.; Reibenspies, J. H.; North, S. W.; Miller, S. A. “Ene-diamine versus Imine-amine Isomeric Preferences” J. Org. Chem. 2005, 70, 8409-8416.
[9] Ilg, A. D.; Price, C. J.; Miller, S. A., submitted.
[10] Chen, H.-Y.; Zhang, J.; Lin, C.-C.; Reibenspies, J. H.; Miller, S. A., "Efficient and controlled polymerization of lactide under mild conditions with a sodium-based catalyst" accepted to Green Chemistry.
[11] Ilg, A. D.; Miller, S. A., manuscript in preparation.
[12] Miller, S. A. “Isotactic Block Length Distribution in Polypropylene:  Bernoullian vs. Hemiisotactic” Macromolecules 2004, 37, 3983-3995.
[13] Price, C. J.; Reich, B. J. E.; Miller, S. A. “Thermodynamic and Kinetic Considerations in the Copolymerization of Ethylene and Carbon Dioxide” Macromolecules 2006, 39, 2751-2756.
[14] Zou, F.; Li, Y.; Zou, X.; Qian, C.; Chen, R.; Song, Y. Chinese Patent 1334279, 2002.
[15] Miller, S. A. “Application of the S_2∞ and C_∞ point groups for the prediction of polymer chirality” Chem. Commun. 2006, 70, 862-864
[16] Grill, J. M.; Ogle, J. W.; Miller, S. A. “An Efficient and Practical System for the Catalytic Oxidation of Alcohols, Aldehydes, and α,β-Unsaturated Carboxylic Acids” J. Org. Chem. 2006, 71, 9291-9296.