Fundamental Research
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Fundamental Science in the Harry R. Allcock Research Group
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 high polymers with a degree of polymerization of 15,000 or above, and molecular weights of two million or higher. The linear polymers depicted in 1 represent only one of several different architectures. Star structures, block copolymers with organic polymers or polysiloxanes, cyclolinear species, and graft or comb macromolecules have also been produced in our program (see the following chart).
Altogether, more than 700 different polyphosphazenes have been synthesized, with properties that, in many cases, cannot be generated from classical macromolecules.
Synthesis Polyphosphazenes are synthesized in our program by two methods - (a) by ring-opening polymerization followed by macromolecular substitution, a route that was discovered by us in the mid-1960's, and (b) via a living cationic condensation polymerization developed in the late 1990's. These processes are summarized in the following Scheme. Each method has its advantages and disadvantages.
The ring-opening polymerization / macromolecular substitution route involves the thermal polymerization of a cyclic phosphazene with chloro- side units to give high molecular weight poly(dichlorophosphazene). The chlorine atoms in this reactive macromolecular intermediate are then replaced by organic or organometallic groups such as alkoxy, aryloxy, amino, or organosilicon groups. Cyclic phosphazenes with organic side groups can also be polymerized. This process has led to the synthesis of a broad range of polymers, with different side groups or combinations of two or more side groups, and often with unique combinations of properties. The advantages of this synthesis method are the commercial availability of the starting cyclic trimer, the fact that more than 250 different reagents have been shown to undergo the substitution reaction, and the wide range of structures and properties that are accessible (see chart on the following page). The main disadvantage is the lack of control over the polymer chain lengths and relatively high temperatures need for the polymerization process.
The second synthesis method involves the room temperature living cationic condensation polymerization of an N-silyl-chlorophosphoranimine at room temperature to give poly(dichlorophosphazene) which can then be subjected to substitution reactions. The advantages of this method are that it allows control of the chain lengths and is the main access route to block, graft, and comb copolymers of polyphosphazenes with organic polymers and polysiloxanes. Star polymers are also synthesized by this route. The disadvantages of this process are the non-availability of a commercial source of the monomer and the somewhat lower molecular weights of polymers produced by this method.
The ongoing research in our program seeks to understand the mechanisms and scope of the two polymerization processes, to define the limits to the macromolecular substitution reactions, to understand the molecular structure-property relationships in these polymers, to study secondary reactions that place functional units on the organic side groups, and to explore alternative ways to produce polyphosphazenes and other inorganic backbone polymers. The synthetic techniques used are typical "wet chemistry" procedures of the type found in most organic or inorganic programs, as well as vacuum line and dry box manipulation of air-sensitive compounds. We make extensive use of polymer characterization techniques such as gel-permeation chromatography, light scattering, NMR, DSC, TGA, X-ray diffraction, MALDI mass spectrometry, and methods for electro-optical evaluation. The normal scale of our polymer synthesis work is at the 100-200 gram level, although we have on occasions scaled reactions up to the 1 kilogram level using equipment available elsewhere.
2. Small Molecule Model Chemistry
Small molecules are easier to synthesize and study than are their counterparts at the high polymer level. Thus, a large component of our research has been to explore new synthesis reaction and their mechanisms and the molecular structures of the small molecule products as a prelude to transposing the same chemistry to the high polymer level. Some examples of small molecule systems first studied in our program are shown below.
Questions that we attempt to answer at the small molecule level are:
· Will the replacement of halogen atoms linked to phosphorus by specific nucleophiles be accompanied by cleavage of the phosphorus-nitrogen skeletal bonds? At the high polymer level this would lead to a dramatic shortening of the polymer chains and loss of valuable properties.
· Are there differences between the reactions of nucleophilic reagents with cyclic phosphazenes with different ring sizes or with short chain phosphazenes?
· What is the stability of the small molecule model compounds to heat, ultraviolet or gamma radiation?
· What is the stability to hydrolysis under different pH conditions? This is important for an assessment of potential biological uses.
· What are the bond angles, bond lengths, and molecular conformations associated with the presence of different side groups attached to phosphorus? This information is accessible through single crystal X-ray diffraction studies and it may provide clues to the details of the high polymer structures.
· Do special properties exist in the small molecules, such as optical, electronic, or biological characteristics, that might encourage a sustained effort to synthesize the corresponding high polymer?
These aspects of our work are crucial for the design of new high polymer systems. For example, phosphazene cyclic trimers or tetramers with polyaromatic side groups are excellent synthetic models for their high polymeric counterparts and for preliminary measurements to probe photonic possibilities such as refractive indices, liquid crystalline behavior, NLO characteristics, etc. Cyclic phosphazenes with branched alkyl ether side groups are models for polymers that may serve as solid lithium ion conductors. Small molecules such as the ferrocenyl derivative shown above are both models for electroactive polymers and monomers for ring-strain driven polymerizations to linear high polymers. Phosphazene cyclic trimers with amino acid ester or glyceryl side groups were found to hydrolyze slowly to phosphate, ammonia, and amino acid or glycerol, and this provided the impetus for us to develop the corresponding high polymers for several biomedical applications.
Our continuing work on the small molecule chemistry involves studies of the role of side group steric hindrance on the substitution chemistry of small molecule phosphazenes, on the sensitivity to hydrolysis of phosphazenes with biological important side groups, and on ring and chain conformations. Important practical properties such as chemical and thermal stability, radiation stability, and biomedical compatibility can often be assessed at this level. We attempt to address these issues through synthesis and structural studies using the full range of characterization methods such as NMR, UV/vis spectroscopy, electrochemistry, and mass spectrometry. The molecular structures of high polymers are difficult to determine by X-ray diffraction techniques, but single crystal X-ray structure determination of cyclic trimers, tetramers, and short chain phosphazenes provides valuable bond angle and bond length information that provides a starting point for structural studies on the corresponding high polymers.
Occasionally, the small molecules themselves may prove to have properties that indicate potential uses. For example, some of the small molecules synthesized in our program have proved to be excellent fire-retardant additives for organic polymers or have been used to enhance the ionic conductivity of lithium ion conductive membranes.
3. Phosphazene Clathrates
A major development that arose from our small molecule work was the discovery that certain cyclic phosphazenes with spiro organic side groups crystallize from organic solvents to give crystals with solvent molecules trapped in tunnels or cavities within the lattice. Examples of compounds that are of interest in understanding this phenomenon are shown in the following chart.
Compound 2, which is prepared by the reaction of (NPCl2)3 with catechol in the presence of a base, was the first member of the series. Its clathration behavior was discovered by accident when it was found that recrystallized material always gave the wrong microanalysis, but vacuum sublimed material analyzed correctly. It was later shown that solvent molecules are trapped in 5 A diameter tunnels that are aligned down the c-axis of the hexagonal crystal structure. Guest trapping by recrystallization can be used to selectively clathrate one component of a solvent mixture. Moreover, guest molecules are also absorbed from the vapor state, and this also leads to molecular separations based on molecular recognition and guest size. More recent work has demonstrated that organic vinyl compounds trapped in the tunnels may be polymerized by gamma irradiation to yield polymers that in some cases are not obtainable outside the clathrate. Classical polymer molecules such as polyethylene or poly(ethylene oxide) are spontaneously absorbed into the tunnel structure in ways that allow polymer separations on the basis of chain length, linearity or branched character, or end group structure.
The clathration behavior is a consequence of the trigonal paddle-wheel
architecture of the host molecules which pack together in two possible ways. In
the absence of a guest, the side groups bend and the molecules assemble into a
monoclinic lattice. In the presence of a guest they pack into a hexagonal
lattice which generates the tunnels, as shown in the picture below. The
hexagonal form is stabilized by the guest molecules. Removal of the guest
molecules causes reversion of the lattice to monoclinic. Re-exposure to guest
molecules brings about re-formation of the hexagonal modification.
Compound 4, with longer side arms, behaves in the same way, but the tunnels are wider (10 A), and larger molecules are accommodated. Deeper side groups, as found in 6 and 7, bend or twist in the solid state and generate cavities rather than continuous tunnels. Thus, guest molecules cannot be absorbed and lost easily. The side groups in compound 6 are twisted in the solid state, and this essentially fills space that would be allotted to tunnels or cavities. Hence this phosphazene does not form clathrates.
The ongoing work on this topic involves a search for other host molecules that have unusual solid state properties due to their shape-fitting characteristics. We are also interested in the use of these systems for the storage of unstable guest molecules and for carrying out chemical reactions in the confined space of the tunnels or cavities.