Journal of Inorganic and Organometallic Polymers and Materials

, Volume 16, Issue 4, pp 273–276

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

Authors

    • Department of ChemistryUniversity of Vermont
Article

DOI: 10.1007/s10904-006-9053-8

Cite this article as:
Allen, C.W. J Inorg Organomet Polym (2006) 16: 273. doi:10.1007/s10904-006-9053-8

Abstract

Selected highlights of the career of Professor Harry R. Allcock are presented. The theme of the interplay of fundamental and applied chemistry, a hallmark of Dr. Allcock’s research program, is exemplified by discussions of the mechanistic and synthetic studies of phosphazene polymerization followed by applications in the areas of biomedical materials and solid state conductivity.

Keywords

Phosphazenespolyphosphazenesbiomedical materialsionic conductivity

It is most fitting that the Journal of Inorganic and Organometallic Polymers and Materials should devote this issue to honor the long, productive and high impact career of Professor Harry Allcock. If Inorganic Polymers were first established as an important area of basic and applied research by Rochow, then Harry Allcock has been the prime mover in establishing the breadth and depth the area over the past 40 years of his still highly productive career at the Pennsylvania State University. This productivity has been manifested in three single authored monographs [13], two coauthored text books, (one in three [4] editions, the other in two editions [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 nature of Allcock’s scientific and technical contributions, I will start by showing how Allcock’s early ground breaking investigations took a chemical curiosity and transformed it into a major area of current research. Following this I will focus on three areas of Allcock’s research program which demonstrate the range of fundamental and applied chemistry. These studies while based on the poly(phosphazene) platform have much broader implications for Polymer and Materials Science.

Prior to Allcock’s investigations in the early 60s, poly(dichlorophosphazene) referred to at the time as poly(phosphonitrilic chloride), usually merited no more than a few lines in text books and monographs under the title of inorganic rubber. The thermal synthesis from hexachlorocyclotriphosphazene (N3P3Cl6) was often irreproducible and the material while elasomeric after preparation underwent degradative hydrolysis in the open atmosphere. Clearly, the elastomeric character and the fact that the phosphazene and siloxane repeart units are isoelectronic demonstrated the potential of the
https://static-content.springer.com/image/art%3A10.1007%2Fs10904-006-9053-8/MediaObjects/10904_2006_9053_Sch1_HTML.gif
Scheme 1.

 Phosphazene and siloxane repeat units.

poly(phosphazenes) as a target of research and applications (Scheme 1). This potential was realized in a series of publications in the years of 1964–1966. The application of rigorous physiochemical methods allowed for an in-depth understanding of the mechanism of the thermal ring opening polymerization (Scheme 2) of N3P3Cl6 and provided a reliable synthesis route to (NPCl2)n [3, 9]. The broad diversity of substitution reactions which had been investigated in depth for the cyclophosphaznes [2, 10, 11] could now be applied to the poly(phosphazenes) [1214]. This process, dubed by Allcock the “macromolecular substitution route” (Scheme 2), has lead to several hundred new polymers all derived from poly(dichlorophosphazene). The scope of this process is unique in polymer chemistry. Of equal importance is the fact that replacement of the highly reactive phosphorus–chlorine bond with stable entities, particularily those with phosphorus–oxygen bonds, provides polymers which are stable under demanding conditions including high temperature or acid and alkaline solutions.
https://static-content.springer.com/image/art%3A10.1007%2Fs10904-006-9053-8/MediaObjects/10904_2006_9053_Sch2_HTML.gif
Scheme 2.

Synthesis and reactions of poly (dichlorophosphazene).

Clearly with over 500 papers from the Allcock group as well as 3,000 directly related and another 3,000 indirectly related publications from other laboratories, an in-depth review of Allcock’s contributions [3] is beyond the scope of this introduction. Thus, I will focus on three diverse areas where he has made significant contributions. The first of these involves the basic science involved in the synthesis of poly(phosphazenes) from small molecule monomers. Building on his initial mechanistic work [10], he went on to examine, using for example force field calculations, the conformational properties of the poly(phosphazenes) [15, 16] and the fundamentals of ring-ring and ring-chain equilibria [17] in phosphazene polymerization [18]. Using this knowledge he explored the ring opening polymerization of organo− [19] and metalloorganophosphazenes [20, 21]. In this work he systematized and elucidate the role of the cosubstituents and more importantly substituent induced ring strain in the ring opening polymerization process [22]. In recent years, the Allcock group has developed a cationic living polymerization of phosphoranimine monomers, such as Cl3PNSilMe3, to provide linear, [2325] block [26, 27], star [28] telechelic [29, 30] poly(phosphazenes) (Scheme 3).
https://static-content.springer.com/image/art%3A10.1007%2Fs10904-006-9053-8/MediaObjects/10904_2006_9053_Sch3_HTML.gif
Scheme 3.

 

Allcock has always maintained a dual approach of fundamental and applications driven research to his studies. An area that maintains an on going appeal to him in this regard is that of biomedical applications. This work involves the synthesis and property evaluation of poly(phosphazenes with bioactive side groups such as amino acid esters [31, 32], steroids [33], anesthetics [34], heparin [35], glyceryl [36], oligopeptides [37], glycolic and lactide esters [38, 39] (biodegradible materials) and hydroxyapatite composites [40]. Certain of these materials as well as the paracarboxyphenoxy phosphazene serve as effective microencapsulations hydrogels [4143]. In a process in which the elegance is matched by conceptual simplicity, the carboxylate salts of +1 cations are soluble in aqueous solutions but may be cross-linked to form hydrogels simply by addition of a divalent cation [41]. The resulting microspheres can encapsulate reagents and even whole cells (and allow for cellular integrity and function to continue), which are present in the solution where then ionic cross-linking process occurs. Enzyamatic immobilization within poly(phosphazenes) hydrogels with polyether side chains has also been explored [44]. The design of poly(phosphazenes) with certain of the side groups described above for control release of bioactive materials has also been accomplished [4547]. Other biomedical targets which have been explored include skeletal tissue regeneration [48, 49], antibacterial activity and mutagenicity [50], tissue engineering [51] and bone repair studies [52, 53]. In addition to the broad and exciting range of biomedical applications noted above, this work shows a signature aspect of the Allcock research program i.e. the use of the poly(phosphazene) platform as a spring board into seemingly distant but crucially important areas of science and technology.

As a last, of many possible, example of the transformation of fundamental work involving the poly(phosphazene) platform to the production and characterization of new materials with significant applications potential, I will move to the area of Materials Science. The specific focus will be in the area of solid polymer electrolytes. The core concept was to exploit the stability and low glass transition temperature of the poly(phosphazene) backbone along with metal ion, specifically lithium, bind capacity of polyether side chains such as the ethoxyethoxymethoxy (MEEP) entity [5460]. The synthetic work focused on stubstituent and additive control of the conductivity and increased dimensional stability needed for long term integrity of devices obtained from these electrolytes. Further work in the synthetic domain explored additional poly(phosphazenes) such as gels [61], organic polymers with pendant cyclophosphazenes containing polyether substituents [6265], and phosphazene-ethylene oxide block coplymers [66] as lithium carrier solid electrolytes. Typical of the Allcock approach synthesis is only one part of a multifaceted approach to polymer science. In the polymer electrolyte work, numerous publications explore studies of conductivity, transport properties and mechanisms of conductivity in these materials [6772].

The choice of topics for this snapshot of the diversity and creativity exhibited by the output from Allcock’s laboratory is by necessity brief and idiosyncratic with the author. One could have treated each of these in more depth and looked at other topics such as organometallic phosphazenes, phosphazene clathrates, radiation chemistry, phosphazene membranes, electrical/optical materials, and phosphazene surface modification. He has been recognized by important institutional appointments, the Evan Pugh Professorship is the Pennsylvania State University’s highest academic honor, a Guggenheim Fellowship, visiting scientist at Stanford, Imperial College of Science and Technology, London and the IBM Almaden Laboratories as well as numerous endowed lectureships. His work has lead to major honors by professional organizations including the Chemical Pioneer Award of the American Institute of Chemists and three of the American Chemical Society major awards: the National Award in Polymer Chemistry (1984), National Award in Materials Chemistry (1992), the Herman Mark Award in Polymer Chemistry (1994) and most recently, the Award in Applied Polymer Science (2007). So inclusion I will return to where I started by saying that it is most fitting and proper that the Journal exclusively focused on Inorganic and Organometallic Polymers and Materials should dedicate this issue to a true pioneer and continuing significant contributor to this fascinating and important area of science.

Copyright information

© Springer Science+Business Media, LLC 2006