Harry R. Allcock – A True Pioneer in the Field of Inorganicand Organometallic Polymers

Christopher W. Allen

Accepted August 5, 2006

Selected highlights of the career of Professor Harry R. Allcock are presented. The theme of theinterplay of fundamental and applied chemistry, a hallmark of Dr. Allcock�s research pro-

gram, is exemplified by discussions of the mechanistic and synthetic studies of phosphazenepolymerization followed by applications in the areas of biomedical materials and solid stateconductivity.

KEY WORDS: Phosphazenes; polyphosphazenes; biomedical materials; ionic conductivity.

It is most fitting that the Journal of Inorganicand Organometallic Polymers and Materials shoulddevote this issue to honor the long, productive andhigh impact career of Professor Harry Allcock. IfInorganic Polymers were first established as animportant area of basic and applied research byRochow, then Harry Allcock has been the primemover in establishing the breadth and depth the areaover the past 40 years of his still highly productivecareer at the Pennsylvania State University. Thisproductivity has been manifested in three singleauthored monographs [1–3], two coauthored textbooks, (one in three [4] editions, the other in twoeditions [5]) as well as edited [6] and two co-edited [7,8] volumes, well over 500 peer reviewed articles,reviews and chapters and 57 patents.

In order to provide some insight into the natureof Allcock�s scientific and technical contributions, Iwill start by showing how Allcock�s early groundbreaking investigations took a chemical curiosity andtransformed it into a major area of current research.Following this I will focus on three areas of Allcock�sresearch program which demonstrate the range offundamental and applied chemistry. These studies

while based on the poly(phosphazene) platform havemuch broader implications for Polymer and Materi-als Science.

Prior to Allcock�s investigations in the early60s, poly(dichlorophosphazene) referred to at thetime as poly(phosphonitrilic chloride), usually mer-ited no more than a few lines in text books andmonographs under the title of inorganic rubber. Thethermal synthesis from hexachlorocyclotriphospha-zene (N3P3Cl6) was often irreproducible and thematerial while elasomeric after preparation under-went degradative hydrolysis in the open atmosphere.Clearly, the elastomeric character and the factthat the phosphazene and siloxane repeart unitsare isoelectronic demonstrated the potential of thepoly(phosphazenes) as a target of research andapplications (Scheme 1). This potential was realizedin a series of publications in the years of 1964–1966.The application of rigorous physiochemical methodsallowed for an in-depth understanding of the mech-anism of the thermal ring opening polymerization(Scheme 2) of N3P3Cl6 and provided a reliablesynthesis route to (NPCl2)n [3, 9]. The broaddiversity of substitution reactions which had been

P

R

RN Si

Me

MeOvs

Scheme 1. Phosphazene and siloxane repeat units.

Department of Chemistry, University of Vermont, Cook Science

Building, 82 University Place, Burlington, VT 05405-0125, USA.

E-mail: ChristopherW.Allen@uvm.edu

Journal of Inorganic and Organometallic Polymers and Materials, Vol. 16, No. 4, December 2006 (� 2006)DOI: 10.1007/s10904-006-9053-8

2731574-1443/06/1200-0273/0 � 2006 Springer Science+Business Media, Inc.

investigated in depth for the cyclophosphaznes [2,10, 11] could now be applied to the poly(phosphaz-enes) [12–14]. This process, dubed by Allcock the‘‘macromolecular substitution route’’ (Scheme 2),has lead to several hundred new polymers allderived from poly(dichlorophosphazene). The scopeof this process is unique in polymer chemistry. Ofequal importance is the fact that replacement of thehighly reactive phosphorus–chlorine bond with sta-ble entities, particularily those with phosphorus–oxygen bonds, provides polymers which are stableunder demanding conditions including high temper-ature or acid and alkaline solutions.

Clearly with over 500 papers from the Allcockgroup as well as 3,000 directly related and another3,000 indirectly related publications from otherlaboratories, an in-depth review of Allcock�s contri-butions [3] is beyond the scope of this introduction.Thus, I will focus on three diverse areas where he hasmade significant contributions. The first of theseinvolves the basic science involved in the synthesis ofpoly(phosphazenes) from small molecule monomers.Building on his initial mechanistic work [10], hewent on to examine, using for example force fieldcalculations, the conformational properties of thepoly(phosphazenes) [15, 16] and the fundamentals of

ring-ring and ring-chain equilibria [17] in phospha-zene polymerization [18]. Using this knowledge heexplored the ring opening polymerization of organo)[19] and metalloorganophosphazenes [20, 21]. In thiswork he systematized and elucidate the role of thecosubstituents and more importantly substituentinduced ring strain in the ring opening polymeriza-tion process [22]. In recent years, the Allcock grouphas developed a cationic living polymerization ofphosphoranimine monomers, such as Cl3PNSilMe3,to provide linear, [23–25] block [26, 27], star [28]telechelic [29, 30] poly(phosphazenes) (Scheme 3).

Allcock has always maintained a dual approach offundamental and applications driven research to hisstudies. An area that maintains an on going appeal tohim in this regard is that of biomedical applications.This work involves the synthesis and property evalu-ation of poly(phosphazenes with bioactive side groupssuch as amino acid esters [31, 32], steroids [33],anesthetics [34], heparin [35], glyceryl [36], oligopep-tides [37], glycolic and lactide esters [38, 39] (biodegra-dible materials) and hydroxyapatite composites [40].Certain of these materials as well as the paracarboxy-phenoxy phosphazene serve as effective microencapsu-lations hydrogels [41–43]. In a process in which theelegance is matched by conceptual simplicity, thecarboxylate salts of +1 cations are soluble in aqueoussolutions but may be cross-linked to form hydrogelssimply by addition of a divalent cation [41]. Theresulting microspheres can encapsulate reagents andeven whole cells (and allow for cellular integrity andfunction to continue), which are present in the solutionwhere then ionic cross-linking process occurs. Enzya-matic immobilization within poly(phosphazenes) hy-drogels with polyether side chains has also beenexplored [44]. The design of poly(phosphazenes) withcertain of the side groups described above for controlrelease of bioactive materials has also been accom-plished [45–47]. Other biomedical targets which havebeen explored include skeletal tissue regeneration [48,49], antibacterial activity and mutagenicity [50], tissueengineering [51] and bone repair studies [52, 53]. Inaddition to the broad and exciting range of biomedicalapplications noted above, this work shows a signatureaspect of the Allcock research program i.e. the use ofthe poly(phosphazene) platform as a spring board intoseemingly distant but crucially important areas ofscience and technology.

As a last, of many possible, example of thetransformation of fundamental work involving thepoly(phosphazene) platform to the production andcharacterization of new materials with significantScheme 3.

Scheme 2. Synthesis and reactions of poly (dichlorophosphazene).

274 Allen

applications potential, I will move to the area ofMaterials Science. The specific focus will be in the areaof solid polymer electrolytes. The core concept was toexploit the stability and low glass transition temper-ature of the poly(phosphazene) backbone along withmetal ion, specifically lithium, bind capacity of polye-ther side chains such as the ethoxyethoxymethoxy(MEEP) entity [54–60]. The synthetic work focused onstubstituent and additive control of the conductivityand increased dimensional stability needed for longterm integrity of devices obtained from these electro-lytes. Further work in the synthetic domain exploredadditional poly(phosphazenes) such as gels [61],organic polymers with pendant cyclophosphazenescontaining polyether substituents [62–65], and phos-phazene-ethylene oxide block coplymers [66] as lith-ium carrier solid electrolytes. Typical of the Allcockapproach synthesis is only one part of a multifacetedapproach to polymer science. In the polymer electro-lyte work, numerous publications explore studies ofconductivity, transport properties and mechanisms ofconductivity in these materials [67–72].

The choice of topics for this snapshot of thediversity and creativity exhibited by the output fromAllcock�s laboratory is by necessity brief and idio-syncratic with the author. One could have treatedeach of these in more depth and looked at othertopics such as organometallic phosphazenes, phos-phazene clathrates, radiation chemistry, phosphazenemembranes, electrical/optical materials, and phos-phazene surface modification. He has been recog-nized by important institutional appointments, theEvan Pugh Professorship is the Pennsylvania StateUniversity�s highest academic honor, a GuggenheimFellowship, visiting scientist at Stanford, ImperialCollege of Science and Technology, London and theIBM Almaden Laboratories as well as numerousendowed lectureships. His work has lead to majorhonors by professional organizations including theChemical Pioneer Award of the American Institute ofChemists and three of the American ChemicalSociety major awards: the National Award in Poly-mer Chemistry (1984), National Award in MaterialsChemistry (1992), the Herman Mark Award inPolymer Chemistry (1994) and most recently, theAward in Applied Polymer Science (2007). So inclu-sion I will return to where I started by saying that it ismost fitting and proper that the Journal exclusivelyfocused on Inorganic and Organometallic Polymersand Materials should dedicate this issue to a truepioneer and continuing significant contributor to thisfascinating and important area of science.

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275Harry R. Allcock – A True Pioneer

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72. R. Frech, S. York, H. R. Allcock, and E. C. Kellam, Macro-molecules 23, 8699 (2004).

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