Fundamental Research


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Synthesis and Reactions  |  Structure and Properties  |  Small Molecule Chemistry  |


Fundamental Science in the Harry R. Allcock Research Group

POLYMER SYNTHESIS AND REACTIONS

1. Phosphazene Polymers

             Polyphosphazenes form a broad series of macromolecules all of which contain a backbone of alternating phosphorus and nitrogen atoms and with two organic, organometallic, or inorganic side groups attached to each phosphorus atom (structure 1).

1

These are typically high polymers with a degree of polymerization of 15,000 or higher, and molecular weights of two million or more.  The linear polymers depicted in 1 represent only one of several different architectures.  Star structures, dendrimers, block copolymers with organic polymers or polysiloxanes, cyclolinear species, graft or comb macromolecules, and polymers with phosphorus, nitrogen, and carbon or sulfur have also been produced in our program.  The diversity of structures is illustrated by the matrix shown in the following picture.  Altogether more than 700 different phosphazene polymers have been synthesized, most of them in our laboratory.  In many cases these polymers have combinations of properties that cannot be generated from classical macromolecules.

2. Development of Polymerization Reactions

             The development of new polymers and materials that combine organic and inorganic structures requires non-traditional methods of synthesis.  Whereas structural diversity in most classical organic polymers is achieved by finding ways to polymerize or copolymerize a wide range of different petrochemical monomers, polyphosphazene research uses a different approach.  In this method, techniques are developed to synthesize a relatively small number of inorganic backbone polymers that have halogen side units, and these halogen atoms are then replaced by organic or organometallic units using macromolecular substitution processes. Thus, there are two aspects to the fundamental polymer research in this program - development of polymerization reactions, and macromolecular substitution processes.

 Two alternative methods are used in our program to assemble the polyphosphazene backbone.  The first involves an elevated temperature ring-opening polymerization of a commercially available cyclic phosphazene, and the second makes use of the condensation reactions of organosilicon-phosphazene monomers.  Both are then followed by replacement of halogen side units by organic or organometallic groups

 

            (a) Ring-Opening Polymerization.  This is the process that allowed us to pioneer the synthesis of the first stable polyphosphazenes.  It is still the main method for polyphosphazene laboratory synthesis and manufacture.  A critical intermediate in the synthesis of poly(organophosphazenes) is poly(dichlorophosphazene).  This is usually obtained by the thermal ring-opening polymerization of hexachlorocyclotrphosphazene, although fluoro- and mixed organo-halogeno cyclic phosphazenes can also be used.

 

 

 

            (b) Living Cationic Condensation Polymerizations.  An alternative method for the synthesis of poly(dichlorophosphazene) is via the living condensation polymerization of an N-silylphosphoranimine such as Me3Si-N=PCl3 under the influence of a Lewis acid initiator.  This process was discovered through a collaboration between our group and the program of Prof. Manners (now at the University of Bristol in the UK).  This reaction takes place at room temperature, and it provides access to linear block copolymers, including copolymers with organic macromolecules.  It is also being used to synthesize star, dendrimer, and comb-type structures.  An example of the use of this method to synthesize block copolymers in shown in the following scheme.

 

 

 

3. Macromolecular Substitution Reactions

 

The conversion of reactive poly(halogenophosphazenes) to poly(organophosphazenes) is accomplished by the reactions of these polymers with one or more organic or organometallic nucleophiles.  This is an extraordinary reaction, since typically 30,000 or more halogen atoms per molecule must be replaced to ensure that no P-halogen bonds remain.  It is a process that is successful only because of the very high reactivity of phosphorus-halogen bonds, and it provides a distinct contrast to the behavior of classical all-organic polymers.  Note that the polymer properties also depend on the ratios of two or more different side groups linked to the same chain and to the disposition of those units along the chain.

Much of our current research involves examining the reactions of an ever-widening list of organic nucleophiles (alkoxides, aryloxides, amines, and organometallic reagents) in order to produce new macromolecules with hitherto unseen combinations of properties.  These macromolecular substitution reactions are normally preceded by small molecule model reactions (see separate section).

 

 

POLYMER STRUCTURE-PROPERTY RELATIONSHIPS

1. Analytical Methods Employed

 The determination of molecular structure is an essential component of all synthesis research.  In our program we routinely use the following analytical techniques to establish the structure and properties of each new polymer:  phosphorus, carbon, hydrogen, and often silicon or fluorine, solution state NMR spectroscopy; DSC techniques to measure glass and melting transitions, TGA to estimate thermal behavior, and GPC analysis to estimate chain lengths and molecular weight distributions.  In specialized cases we also measure refractive indices, optical dispersion, UV and IR absorption, water contact angles to determine hydrophobicity of hydrophilicity, SCM,  ESCA, and STM for surface analyses, TEM, electrical conductivity measurements, and for biomedical polymers we examine resistance or susceptibility to hydrolysis.  Our collaborators also carry out extensive biological testing on specific polymers.  We also use techniques such as electrostatic spinning to produce nanostructures and spin casting to produce materials for special tests.    

 2. Purpose of Structure-Property Studies

 New property combinations can be generated through control of the polymer structure in polyphosphazenes.  This is accomplished in three ways:  first though choice of the skeletal architecture (linear, branched, star, dendrimer, block copolymer, etc.); second, through understanding the role played by the inorganic elements in the skeleton; and, third, by studies of the influence of different organic or organometallic side groups on the polymer and materials properties.  A long-range objective in our program is to understand these factors to the extent that we can confidently anticipate the properties of any polyphosphazene before it is synthesized.

3. Influences of the Backbone

 The following properties are imparted to polyphosphazenes by the presence of the phosphorus-nitrogen backbone:  thermo-oxidative stability, fire resistance, electrochemical stability, very high torsional mobility (low barrier to skeletal bond twisting), transparency from the 230 nm region in the ultraviolet to the near infrared, high refractive index, hydrophilicity, and hydrolysis to phosphate and ammonia when specific side groups are present.  We are also exploring the effects of introducing other inorganic elements and carbon into the backbone structure.     

4. Influence of the Side Groups

 The side groups in polyphosphazenes control solubility, secondary reaction chemistry, thermal decomposition, resistance to hydrolysis, and they also have an influence on polymer chain flexibility, bioerosion, and on ion conductivity.  The side groups are responsible for overall hydrophilicity or hydrophobicity; for optical properties such as refractive index and dispersion, UV-visible absorption, NLO and liquid crystalline character; and they play a major role in the formation of gels, membranes, drug delivery systems, and in the interactions with other materials in polymer blends and composites.  Being able to control different combinations of properties by the introduction of two or more different types of side groups is a major objective in our program.

 

 

SMALL MOLECULE CHEMISTRY

1. General Purpose

 Long-range fundamental science in this program is directed toward the development of new chemistry that promises to be important in materials science and biomedical research.  Small-molecule chemistry, including synthesis, mechanisms, and molecular structure, plays a significant part in our research.  It becomes manifest in three main ways - first, a curiosity-driven investigation of new reactions of hybrid organic-inorganic molecules; second via the use of small molecules as models for the reactions, mechanisms, and structures of high polymers; and third, through the study of molecular inclusion adducts.  Techniques used include synthesis studies, X-ray crystallography, reaction kinetics, NMR, and mass spectrometry.  It should also be noted that many of the reactions studied in our program can be understood in terms of physical-organic principles worked out by chemists in an earlier era, with the difference that they are here being applied to main group inorganic moleules rather than to simple organic species.

2. New Chemistry of Hybrid Organic-Inorganic Small Molecules

 Although the physical-organic chemistry of classical organic small molecules is well explored, the same cannot be said of many molecules that are hybrids of organic and inorganic structures.  We are interested primarily in small molecule organophosphazenes but we also have a strong interest in organosilicon chemistry and in heterocyclic hybrids that contain main group elements and transition metals.  Examples include ring systems and short chain species that contain phosphorus and nitrogen plus carbon, sulfur, boron, and metals in the skeleton together with organic groups linked to the skeleton.  This work leads directly into the model compound aspects described below.

  3. Small-molecule Models for High Polymers

 Small molecules are easier to synthesize and study than are their counterparts at the high polymer level. For this reason, the use of small molecule studies to explore reactions and properties that could be useful fot high polymers and materials is a prominent feature of our work.  Nearly all of the new reactions that we apply to phosphazene high polymers are examined first using small molecules such as cyclic trimers or tetramers or short chain linear phosphazenes as models.  Subsequent studies then examine the degree to which the small-molecule chemistry can be used to design and synthesize macromolecules and materials.

 An example is the use of azide chemistry to provide access to new molecular structures.  The reaction scheme below illustrates the replacement of chlorine atoms by azido groups, en route to the formation of phosphinimines.  This provides access to the products of insertion reactions such as those that involve nitrenes.

 

 

Other examples include the use of amino acid esters, oligopeptides, or etheric alkoxides as nucleophiles for the replacement of chlorine atoms in cyclophosphazenes.  The pattern of halogen replacement (gem or non-gem, cis or trans) has wide implications for the high polymer chemistry, and is studied at the small-molecule level by NMR spectroscopy and other techniques.  Amino acid ester derivatives hydrolyze to amino acids, phosphate, ammonia, and the ester function, processes that are investigated via reaction kinetics to establish hydrolysis reaction mechanisms.  Other mechanistic studies are directed toward the protection and deprotection of pendent amino or carboxylic acid groups as steps in the preparation of functional molecules.  The reactions of electrophilic and other reagents with aryl groups attached to a phosphazene skeleton are being examined as a means to introduce sulfonic acid, phosphonic acid, or sulfonimide substituents to give species that are models for fuel cell membranes.  Many of these interactions are accompanied by side reactions, and finding conditions that favor the preferred products is a continuing objective.

3. Molecular Inclusion Adducts (Clathrates)

 A number of spirocyclic trimeric phosphazenes that were first synthesized in our program for another purpose have the unusual property of crystallizing in a form that traps other molecules in tunnels or cavities in the crystal lattice.  These hosts can be used to separate small molecules from the liquid or vapor states because the entry or departure of the small molecules into or from the lattice depends on the diameter of the tunnels in the host structure.  The 5-10 A tunnels have also been used to store reactive guest molecules and to serve as Angstom-level reaction chambers for the polymerization of unsaturated organic monomers.  Different sized side groups linked to the phosphazene ring, such as those shown below, generate different diameters of tunnels or shapes of cavities.  Hence, the behavior of the guest molecules may be fine-tuned via the structure of the host spiro side groups.  Some polymers synthesized in the tunnels cannot be prepared by normal solution polymerization techniques.  This is possible because the small diameter of the tunnels prevents crosslinking reactions and favors the formation of linear macromolecules.  The tunnel systems are also permit the unlikely process of allowing linear macromolecules to enter and occupy the free volume, a process that facilitates the separation of linear from branched polymers of the same species.

 

 

 


 

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Copyright 2006 H. R. Allcock Research Group
Last modified: 10/15/2017