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Bob Howell

​​​ Photo of Bob Howell Professor
Dow Science Complex 263
  • B.A., Chemistry, Berea College, 1964
  • Ph.D., Physical Organic Chemistry, Ohio University, 1971
  • Postdoctoral Associate, Organometallic Chemistry, Iowa State University, 1971-74
  • Industrial Internship, Polymer Science, Dow Chemical Company, 1985
  • Academic-Industrial Exchange, Polymer Science, Dow Chemical Company, 1990
Research Fields
  • Flame retardants for polymeric materials
  • New polymeric fuel-cell membranes
  • Polymerization techniques
  • Thermal methods of analysis
  • Polymer-supported organoplatinum antitumor agents
  • Barrier plastic packaging
  • Bioplastics
  • Polymers from renewable sources
​​Current Research Projects
Release Platforms for Organoplatinum Antitumor Agents

Organoplatinum compounds are broad spectrum, very effective antitumor agents. Cisplatin [cis-dichlorodiammineplatinum(II)] is currently the most widely used cancer drug. It is often used in combination with an organic antitumor compound or Carboplatin [1,1-cyclobutanedicarboxyalato(diammine)platinum(II); the second platinum drug to gain widespread commercial use]. The potential of these drugs has been limited because of the severe side effects which accompany their administration. Among the most debilitating side effects induced by organoplatinum drugs are 1.) severe kidney damage and 2.) extreme nausea (as a class the platinum compounds are among the most effective nausea producing agents known – to the point that some patients refuse to complete the treatment regimen). In an attempt to mitigate the toxicity of these drugs we have, for some time, been using water soluble polymers as platforms on which a platinum drug or prodrug might be supported and from which it might be slowly released into the extracellular fluid. This approach has several major potential advantages over the traditional forced hydration therapy currently practiced. First, the solubility of the drug formulation may be dramatically enhanced such that the volume of fluid required to introduce a satisfactory dose of drug is strongly diminished [Cisplatin has a solubility of about 10 mg/L in aqueous saline]. More importantly, if the release rate is optimal, the drug is released into the blood stream at a level that is beneath the toxicity threshold such that side effects may be mitigated. Most recently we have been able to place approximately forty (1,2-diaminocyclohexane)platinum(II) moieties on the surface of a generation 4.5 PAMAM dendrimer. This polymer-drug conjugate displays a very good release profile for the platinum species [cis-(diaquo)(1,2-diaminocyclohexane)platinum(II)].

Vinylidene Chloride Polymers for Barrier Plastic Packaging

Vinylidene chloride copolymers ontaining >85% vinylidene chloride display outstanding barrier to the transport of oxygen (to prevent food spoilage) and other small molecules as well as flavor/aroma molecules (to prevent flavor scalping on the supermarket shelf). For this reason these materials occupy a place of prominence in the food packaging industry. Unfortunately, these resins tend to undergo degradative dehydrochlorination under process conditions. To permit commercial exploitation of these materials the degredation must be controlled. We are involved in an on-going program to develop vinylidene chloride polymers with enhanced thermal stability. A promising approach is the synthesis of polymers containing a comonomer capable of reacting with (and consequently removing) hydrogen chloride as it is formed to expose functionality capable of scavaging radical species.

Poly(styrene) Containing No Head-to-Head Units

Poly(styrene) is a large volume commodity polymer. It is widely used in inexpensive packaging for baked goods (cookies, muffins, rolls, pastries, etc.) and similar items. Conventional poly(styrene) is produced by radical techniques. Polymerization terminination occurs by radical coupling to introduce a head-to-head unit into the mainchain. At elevated temperature during processing this unit may undergo homoloysis to generate macroradicals which can extrude styrene monomer. The presence of styrene monomer in the finished item, even at very low level, may impart objectionable flavor/aroma to the food item contained. This problem could be avoided by using poly(styrene) containing head-to-head units. We have been able to develop methods for the synthesis of such a material and to demonstrate its superior thermal stability.

Dual Functional Flame Retardants for Polymeric Materials

For most applications polymeric materials must be flame retarded. Traditionally, flame retardants have been organohalogen compounds, particularly brominated aromatics. These compounds are readily available, inexpensive, and very effective as flame retardants. They function by releasing hydrogen halide into the gas phase which traps flame propagating radicals. Because of the release of halogen into the atmosphere, the use of these compounds is increasingly being opposed in many parts of the world. Compounds which might promote solid phase (charring) flame retandance as well as gas phase activity should permit the achievement of an adequate level of flame retardance with much reduced levels of halogen. This has been demonstrated for several series of compounds containing both bromine and phosphorus or bromine, nitrogen, and phosphorus. Several potentially very effective but environmentally friendly flame retardants are under development.

Phospholane Initiators for Radical Polymerization

Phosphplane containing a strained carbon-carbon bond may be prepared from a hindered 1,2-diol and dichloro(phyenyl)phosphine or related compound. The compounds undergo homolysis of the carbon-carbon bond at modest temperature to afford a diradical capable of initiating vinyl polymerization. The polymers produced contain a phosphorus moiety in the mainchain and as a consequence exhibit flammability much lower than the corresponding polymer containing phosphorus. Since the phosphorus unit is part of the polymer structure it is not lost during processing or product manufacture as a simple additive might be.

Alkoxyamine Initiator/Mediators for Radical Polymerization via Diels-Alder Reaction

Nitroxyl-mediated radical polymerization offers a method for the generation of low polydispersity styrene polymers. Hindered dienes, e.g., 1,4-diphenyl-1,3-butadine, react with nitrosobenzene to generate substituted 1,2-oxazines. The carbon-oxygen bond in these compounds undergo thermolysis at modest temperatures (70 – 100° C) to generate both a carbon radical capable of initiating polymerization and a stable nitroxyl radical suitable for mediation of the propagation reaction.

Green Polymeric Materials from Renewable Sources

Renewable raw material bases for polymers are of increasing interest both because of environmental concerns (biodegradability) and the ever-increasing cost of petroleum feed stocks. Succinic acid is available from a number of plant sources. It can be reduced to 1,4-butanediol. Thus, both monomers (the diol and succinic acid) required for the generation of poly(butylene succinate) are available from a single source. The properties of this and related polymers, as both the homopolymers and as blends with more traditional materials, are being examined.

Lignin as a Source of Green Flame Retardants

Lignin is a natural polymer readily available as a by-product of the wood pulping industry. It is a highly aromatic material containing a wealth of functionality (largely phenolic groups) for reaction with phosphorus reagents. The phenolic groups are also activated for electrophilic halogenation. Thus lignin may be converted via several conventional reactions to a material with flame retardant properties. Such materials are being generated and evaluated as flame-retardant additives.

Selected Publications
  • Bob A. Howell and Wenxiao Sun. Thermal degradation of esters/ethers derived from tartaric acid, Journal of Thermal Analysis and CalorimetryASAP.
  • ​B. A. Howell and Y.-J. Cho. “Thermal Decomposition of 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane”, J. Therm. Anal. Cal., 101, 2 (2010), 517-521.
  • B.A. Howell, P Chhetri, A. Dumitrascu, and K.N. Stanton. “Thermal Degradation of Platinum(IV) Precursors to Antitumor Drugs”, J. Therm. Ana. Cal. 102, 2 (2010),499-503.
  • B.A. Howell and K.E. Carter. “Thermal Stability of Phosphinated Diethyl Tartrate”, J. Therm. Anal. Cal. 102, 2 (2010), 493-498.
  • B.A. Howell and A. Dumitrascu. “Thermal Stability of Bidendate Nitrogen Ligands Tethered to Multiwall Carbon Nanotubes”, J. Therm. Anal. Cal. 102, 2 (2010), 505-512.
  • B.A. Howell, "Strained Organophosphorus Compounds as Reactive Flame Retardants for Polymeric Materials," in T.R. Hull and B.K. Kandola, Ed., Fire Retardancy of Polymers: New Strategies and Mechanisms, Royal Society of Chemistry, 2009, Chapt 2, 28-34.
  • B.A. Howell, "Development of additives possessing both solid-phase and gas-phase flame retardant activities", Polymer Degradation and Stability, 93 (2008) 2052-2057 .
  • B.A. Howell, "Utilization of Thermogravimetry in the Study of Reaction Mechanism", J. Therm. Anal. Cal., 93, 27 (2008).
  • B.A. Howell, “Thermal Properties of Compounds Possessing Both Solid-Phase and Gas-Phase Flame Retardant Potential”, J. Therm. Anal. Cal., 89, 373 (2007).
  • B.A. Howell, “The Utilization of TG/GC/MS in the Establishment of the Mechanism of Poly(styrene) Degradation”, J. Therm. Anal. Cal., 89, 393 (2007).
  • B.A. Howell and A.O. Odelana, “Stability of Vinylidene Chloride Copolymers Containing 4-Vinylpyridine Units”, J. Therm. Anal. Cal., 89, 449 (2007).
  • B.A. Howell and J. Uzibor, “Pentaphenyl-1,3,2-dioxaphospholane as a Reactive Flame Retardant”, J. Vinyl Addit. Technol., 12, 192 (2006).
  • B.A. Howell, Y. Cui, K. Chaiwong and H. Zhao, “Polymerization as a Means of Stabilization for Poly(styrene)”, J. Vinyl Addit. Technol., 12, 198 (2006).
  • B.A. Howell and J. Zhang, “Stabilization of Vinyl Chloride/Vinylidene Chloride Copolymers Using N-Substitutedmaleimides,” J. Vinyl Addit. Technol., 12, 88 (2006).

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