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Synthesis and Reactions
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).
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
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.
alternative methods are used in our program to assemble the polyphosphazene
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
Both are then followed by replacement of halogen side units by organic or
(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).
1. Analytical Methods Employed
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
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.
Purpose of Structure-Property Studies
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
3. Influences of the Backbone
following properties are imparted to polyphosphazenes by the presence of the
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
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.
1. General Purpose
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
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.
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.