A chemical method for the convenient surface functionalisation of polymers

Article abstract of DOI:10.1039/b414856h

A method for the application of carbenes as reactive intermediates for surface modification of polymeric substrates has been developed; the efficacy of the process has been demonstrated by irreversible dyeing of polystyrene, polythene, nylon, silica, and controlled pore glass, and to a limited extent, polytetrafluoroethylene. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b414856h

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A chemical method for the convenient surface functionalisation of
polymers{
Karim M. Awenat,a Philip J. Davis,a Mark G. Moloney*a and Warren Ebenezerb
Received (in Cambridge, UK) 27th September 2004, Accepted 19th November 2004
First published as an Advance Article on the web 12th January 2005
DOI: 10.1039/b414856h
volatile and highly reactive carbenes, high temperature, and metal
catalysts. Perhaps as a result of this, carbene insertion appears to
have been largely overlooked as a method applicable to polymer
modification.15 Nonetheless, changes in polymer surface tension,
permeability or photophysical characteristics were shown to be
possible. We considered that modification of a polymer surface by
carbene insertion was an excellent candidate worthy of further
investigation, particularly if the issues of generation of the carbene
could be addressed, since the well known reactivity of carbenes
with C–H and X–H (X 5 O, N, S) by bond insertion, and with
C–C double bonds by addition reactions, would be expected to
afford a very general process; notably, these reactions are
irreversible. We report here the results of some investigations for
the development of diarylcarbenes as reactive intermediates
suitable for polymer surface modification.
Diarylcarbenes have been widely studied16–18 and are easily
obtained by thermolysis or photolysis of the corresponding diazo
compound 2a, which in turn are readily prepared from the
benzophenone 1a in a two step sequence via an intermediate
benzophenone hydrazone (Scheme 1). We found that the required
diazo compounds were most readily prepared by treating the
benzophenone 1a with hydrazine hydrate in refluxing ethanol
overnight, followed by removal of the solvent and extraction into
dichloromethane;{ the hydrazone product needed to be used
immediately, since facile interconversion to the azine was observed
in all cases on storage even overnight. Oxidation of this
intermediate to the diazo compound 2a was most conveniently
performed with mercuric oxide in ether, and provided the
hydrazone starting material had been obtained in sufficiently
pure form, the diazo product needed no further purification.
A
method for the application of carbenes as reactive
intermediates for surface modification of polymeric substrates
has been developed; the efficacy of the process has been
demonstrated by irreversible dyeing of polystyrene, polythene,
nylon, silica, and controlled pore glass, and to a limited extent,
polytetrafluoroethylene.
The chemical modification of the surfaces of polymers is of
increasing importance in many diverse aspects of modern
technology, with applications which include solid phase chemical
synthesis related to drug discovery, chemical catalysis, biocompa-
tible medical implants and prostheses, diagnostic devices, and
surface-modified fabrics, amongst many others. Whilst there are a
number of existing physical (e.g. corona or plasma discharge, ion
beam irradiation1) and chemical (e.g. silanisation, oxidation,
chlorination, acylation and quaternisation2–4) methods for the
surface modification of polymers, the frequent requirement for
significant infrastructure, harsh reaction conditions, and limitation
to specific polymer types (e.g. polybutadiene5), which must possess
suitable chemical functionality capable of direct modification, led
us to consider alternative chemical methods. Of interest to us was
an alternative that would be amenable to a large range of
polymers, permitting direct chemical modification under mild
conditions and using inexpensive reagents. Although reports of
polymer modifications mediated by carbene insertion have been
made,6–14 limitations to that methodology include the need for
{ Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/cc/b4/b414856h/
*mark.moloney@chem.ox.ac.uk
Scheme 1
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Application of this sequence to ketones 1c–e, readily prepared
from 4-bromomethylbenzophenone 1b19 and the alkoxides derived
from either 3,4-dimethoxyphenol, 3-N,N-diethylaminophenol, or
2-N-ethylanilinoethanol, gave diazo compounds 2b–d in excellent
yield and as intensely coloured oils; these materials could be safely
and conveniently stored at 0 uC, and without significant
decomposition over extended periods.
(Hybond2); thus, each polymer was subjected to the successful
three-step colouration process (initial adsorption, heating, and
treatment with diazonium salt 5) either without any carbene
precursor, with benzophenone 1e, or with diazo compound 2d.
The results from these experiments indicated that maximum
colouration required the combination of carbene insertion derived
from 2d and diazonium reagent 5, consistent with initial carbene
surface insertion onto the polymer followed by diazonium
coupling to generate a diazo chromophore.{ The intensity of
colouration which was achieved was both a function of the nature
of the substrate, the efficiency of carbene insertion, and the UV
chromophore which is generated in the diazo coupling step
(Figs. 1, 2 and 3). The method is particularly effective for the
highly adsorbant polystyrene XAD substrate due to its high
surface area (725 m2/g), but silica and nylon also gave intense
colouration; cotton however proved to be less receptive to this
approach than the synthetic polymers. These results suggest that
the method is likely to be broadly applicable across a range of
polymeric substrates. The colouration process routinely used an
extensive washing process (involving sequential washes with hot
water, acetone, alkali and acid), but in order to ensure that the dye
was fixed irreversibly, samples of cotton, nylon and polystyrene
were extracted using a Soxhlet apparatus with acetone and
aqueous solutions of pH 1 and 15; high level retention of colour
was observed in all cases.
The suitability of compounds 2b–d for the modification of
several substrates (mercerised woven cotton, Nylon membrane
(Hybond M, ex Amersham International, Cat. No. RPN203N),
polystyrene (Amberlite XAD-4 non-ionic polymeric adsorbent
beads), Controlled Pore Glass (CPG) and silica (Sorbsil2 C60H
(40–60 mm) silica gel)) was then investigated. Thus, each substrate
was treated with the diazo compounds 2b–d in ether solution and
the solvent allowed to evaporate slowly. The coloured substrate
was then heated, both to decompose the diazo compound and to
cause reaction of the generated carbene with the polymer surface;
this process led to rapid decolourisation of the substrate, consistent
with the decomposition of the diazo compound. Polymeric
substrates 3a,b and 4, activated with an electron-rich aromatic
residue (Scheme 1) were thereby obtained.{
Application of this approach to other polymeric substrates and
other diazonium salts indicates that the method is likely to be
general. Thus, treatment of polythene, silica, and controlled
pore glass with diazo compound 2d and diazonium salts 5, 6
and/or 7 gave the coloured products shown in Figs. 2 and 3; the
Reaction of modified polystyrenes 3a,b and 4 with diazonium
salt 5 gave intense colouration, arising from the expected diazo
coupling (Fig. 1a, b and c); the dimethoxy substituted system of
substrate 3a gave less intense colouration than the more strongly
activated systems present in 3b and 4. The intense colouration
which was achieved was consistent with initial modification of
the base polymer with the carbene derived from diazo compounds
2b–d followed by diazo coupling with diazonium salt 5. In further
experiments, nine controls were run simultaneously to verify
that the resulting colouration of the polymer was not due merely
to physical adsorption of dye for the substrates mercerized
woven cotton, polystyrene (Amberlite XAD-4), silica and nylon
Fig. 2 Silica (L to R): unmodified; modified with precursor 2d followed
by diazonium salt 5; modified with precursor 2d followed by coupling with
diazonium salt 6 under alkaline conditions; modified with precursor 2d
followed by diazonium salt 6 with acid wash; modified with precursor 2d
followed by diazonium salt 7.
Fig. 1 Polystyrene XAD: (a) modified with precursor 2b followed by
diazonium salt 5; (b) modified with precursor 2c followed by diazonium
salt 5; (c) modified with precursor 2d followed by diazonium salt 5.
Fig. 3 Polythene (L to R): unmodified; modified with precursor 2d
followed by diazonium salt 5; modified with precursor 2d followed by
diazonium salt 6.
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Notes and references
{ Preliminary experiments using XPS on polystyrene and polythene
modified with the carbene derived from 4,49-dichloro(diazo)diarylmethane
revealed a well-defined Cl 2p peak associated with aryl chlorine atoms.
For the polythene substrate, the Cl2p/C1s intensity ratio had a value of
about 1/100 (R. H. Bowdler, R. G. Egdell, and M. G. Moloney,
unpublished results).
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Fig. 4 PTFE (L to R): unmodified; modified with precursor 2d followed
by diazonium salt 6; modified with precursor 2d followed by diazonium
salt 5.
chromophore of products derived from the coupling with Disperse
Yellow 7 depended on whether the diazo coupling made use of
alkaline or acidic conditions.4 Surprisingly, polytetrafluoroethylene
gave some colouration, although it was of very low intensity
(Fig. 4).
By using the colouration of polymers as a diagnostic step, we
have established the proof of the principle of the modification of
polymers using carbenes. However, this method is not restricted
to colouration of polymers, and is potentially applicable to
the introduction of diverse functionality with desirable chemical
and/or physical properties in a chemical coupling step which
makes use of the electron-rich aromatic ring introduced in the
carbene coupling step. Additionally, the method allows function-
alisation of diverse polymer types, and offers the potential for
the convenient incorporation of chemical functionality after the
polymer fabrication process;20,21 investigations in that regard are
currently in progress and will be reported in due course.
We gratefully acknowledge Zeneca Specialities and BASF for a
CASE studentship to K.A., and Dr Karl Harrison (Department of
Chemistry, University of Oxford) for the photographs of the
coloured polymers.
Karim M. Awenat,a Philip J. Davis,a Mark G. Moloney*a and
Warren Ebenezerb
aUniversity of Oxford, Department of Chemistry, Chemistry Research
Laboratory, Mansfield Road, Oxford, UK OX1 3TA.
E-mail: mark.moloney@chem.ox.ac.uk
bBASF plc, P.O. Box 4, Earl Rd, Cheadle Hulme, Cheadle, Cheshire,
UK SK8 6QG
992 | Chem. Commun., 2005, 990–992
This journal is ß The Royal Society of Chemistry 2005