A Sulfur-sulfur cross-linked polymer synthesized from a polymerizable dithiocarbamate as a source of dormant radicals

Article abstract of DOI:10.1002/anie.200906676

(Figure Presented) Cross on UV light: A polymer bearing dithiocarbamate ester groups, linked to the polymer backbone through the dithiocarbamate nitrogen atom is capable of reversible cross-linking and de-linking through photochemical rearrangement of the pendant groups (see scheme).

Full text of DOI:10.1002/anie.200906676

Angewandte
Chemie
DOI: 10.1002/anie.200906676
Polymer Cross-Linking/De-Linking
À
A Sulfur Sulfur Cross-Linked Polymer Synthesized from a
Polymerizable Dithiocarbamate as a Source of Dormant Radicals**
Luis Miguel Garcꢀa-Con, Michael J. Whitcombe,* Elena V. Piletska, and Sergey A. Piletsky
The dithiocarbamates (DTCs) have a special place within
polymer chemistry due to their versatile role as thermal and
photochemical initiators.[1] In particular the role of dithiocar-
bamate esters as “iniferters” in “living” radical polymeri-
zations has practical significance.[2] The living nature of DTC
ester-initiated photopolymerization derives from the extreme
difference in reactivity of the radicals created by homolytic
cleavage of the DTC ester bond. This gives rise to a reactive
carbon-centered radical and a much less reactive, or dormant,
dithiocarbamate radical, in which the unpaired electron is
delocalized over the DTC structure. The chemistry of the
dithiocarbamate radical is dominated by recombination to
reform covalent bonds. It is the ability of dormant radicals to
recapture the carbon-centered radical after addition of
monomer to form a new DTC ester that imparts living
character to DTC ester-initiated photopolymerization. Com-
pounds of this type have been used in the synthesis of
telechelic polymers,[3] block,[4,5] or graft copolymers[5,6] and
hyperbranched polymers.[7] In the latter context, Otsu et al.[8]
synthesized the first example of a DTC ester of a polymer-
izable derivative, the 4-vinylbenzyl ester of N,N-diethyldi-
thiocarbamic acid, creating “inimer”—a compound that
combines iniferter and monomer functionalities. Inimer
could be polymerized photochemically to form hyper-
branched polymers or by conventional thermal initiation to
form linear polymers with pendant DTC ester groups. These
polymers are macro-iniferters, which allow the photochemical
grafting of a range of other monomers. As well as the inimer
of Otsu et al.,[8] similar compounds, based on methacrylates,
have been reported.[9]
system and the active radical is the low molar mass fragment.
A likely scenario under these circumstances is that during
prolonged irradiation, the more readily diffusible carbon-
centered radicals will dimerize, forming stable low-molar
mass by-products, leading to an excess of dithiocarbamyl
radicals attached to the polymer. This will result in the
creation of a relatively stable population of polymeric
dormant radicals. The physics and chemistry of immobilized
dithiocarbamyl dormant radicals is expected to be an exciting
field of investigation, since materials containing stable
radicals may possess useful photoresponsive magnetic,
mechanical, and electrical properties. Polymers with other
stable radical functionalities, such as nitroxy radicals, have
been reported as novel battery materials[10] and alkoxamines
in the side chain[11] or main chain[12] and have been used as
thermally activated systems for the reorganization of poly-
meric materials through radical crossover reactions. Similarly,
reversible “de-linking” to radical fragments has been shown
to occur in response to light or pressure in the case of
triphenylimidazole dimers present as cross-links on a meth-
acrylate backbone.[13]
In order to develop new materials to explore the proper-
ties of polymeric dithiocarbamyl radicals, we designed a
monomer, superficially similar to inimer,[8] but reversing the
orientation of the molecule, as shown in Scheme 1, such that
the dithiocarbamyl residue is the polymerizable part of the
molecule.
In polymers of these compounds, irradiation will generate
active macroradicals and dormant dithiocarbamyl radicals as
low molar mass fragments. The macroradicals will react
rapidly with monomer, oxygen, or with dithiocarbamyl radical
to reform DTC esters. The situation will be different,
however, if the dithiocarbamyl radical is part of the polymeric
Scheme 1. The structure of Otsu’s “inimer”[8] (left) and of the mono-
mer used in this work: benzyl N-butyl-N-(4-vinylbenzyl) dithiocarba-
mate (1), or “reversed inimer” (right), showing the differences between
the two structures with respect to the active and dormant radical
precursor sites and the position of the polymerizable functionality.
[*] L. M. Garcꢀa-Con, Dr. M. J. Whitcombe, Dr. E. V. Piletska,
Prof. S. A. Piletsky
Cranfield Health, Cranfield University
Vincent Building, Cranfield, Bedfordshire, MK43 0AL (UK)
Fax: (+44)1234 758380
E-mail: m.whitcombe@cranfield.ac.uk
Homepage: http://www.cranfield.ac.uk/health/researchareas/
smartmaterials/index.jsp
The monomer, 1 (Scheme 1), was prepared by reaction of
CS2 with amine in the presence of base,[14] following the one-
pot method proposed by Azizi et al.[15] A nominally linear
polymer, 2, was prepared in toluene by AIBN-initiated
thermal polymerization in the dark.
[**] We thank Cranfield Health for funding and Bruker Biospin
(Germany) for providing the EPR data.
Supporting information for this article, including the synthesis of all
compounds and polymers and of the irradiation experiments, is
available on the WWW under http://dx.doi.org/10.1002/anie.
200906676.
UV irradiation of solutions of 2 is expected to result in
À
cleavage of the C S bond, creating benzyl radicals and
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polymeric sulfur radicals. While recombination of the radical
pair will reform the initial DTC ester groups, recombination
of “like” radicals will result in cross-linking due to the
The kinetics of radical formation during irradiation of 1
and 2 was investigated by EPR spectroscopy (Figure 1a). In
these two cases the DTC ester group is intact at the start of
the experiment. The intensity of the EPR signal was seen to
increase linearly as a function of irradiation time over the first
2000 s, starting from zero intensity at time t = 0 s. This is a
characteristic of a system in which the formation of active
species is fast compared to other processes (such as polymer
propagation) and where the rate of termination can be
considered to be negligible until relatively high conver-
sions.[16] The auto cross-linked polymer, 3, showed different
behavior, possessing a significant EPR signal before irradi-
ation due to the presence of isolated dormant radical species
(Figure 1b). Since this sample had been prepared more than
2 weeks previously, it shows the remarkable longevity of
dormant radicals in this system. The generation of additional
radical species is expected to occur as a result of dissociation
of thiuram groups, present as cross-links between polymer
chains. The rate of development of the EPR signal in the case
of 3 was not linear with time. The signal did not increase after
ca. 1500 s, suggesting that the rates of radical formation and
loss were approximately equal.
À
formation of S S bonds and, as frustrations due to cross-
linking increase, isolated sulfur radicals, which are expected
to be relatively long-lived. This process will be aided by
benzyl radical recombination to form 1,2-diphenylethane (4)
as by-product (Scheme 2). Irradiation of 2 was performed in
In order to investigate the early stages of photochemical
cross-linking, samples of 2 were irradiated for 1, 3, or 5 min
only and the solutions examined by GPC. The non-irradiated
2 (Figure 2a, curve A) showed a monomodal peak centered at
Scheme 2. Photocross-linking of 2 to 3, with elimination of 1,2-
diphenylethane (4).
chloroform solution. While this solvent is not ideal for
performing photochemical studies, due to the possibility of
side-reactions from photolysis of the solvent, nevertheless it
was used on account of the greater solubility of the polymer in
CHCl3 than in other solvents (such as acetonitrile) which do
not suffer from these problems.
Figure 2. GPC curves for: a) 2 (A) and 3 supernatant (B); b) samples
taken from the initial stages of the photochemical cross-linking
reaction.
Figure 1. Intensity of the EPR signal against irradiation time for: a) 1
and 2; and b) 3.
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a molecular weight (MW) of 18000. During irradiation, the
solution viscosity increased rapidly due to increases in molar
mass. The average MW of the polymeric fraction produced
after 5 min irradiation was 20000 (Figure 2b, upper line). The
solution also contained a growing fraction with a much higher
MW, estimated to be > 150000, which is most likely formed
during the initial stages of gel formation.
UV irradiation for around 4 h was required to create 3
which was no longer soluble in either CHCl3 or THF and
appeared as a swollen gel. Precipitation into methanol was
performed to separate the gel fraction from soluble material
and the supernatant analyzed by GPC. The result (Figure 2a,
curve B) showed complete disappearance of 2 (MW= 18000)
and formation of both high and low molecular weight
products, proving that cross-linking had occurred between
polymer chains. The by-product of active radical recombina-
tion, 1,2-diphenylethane (4), was also detected, the peak in
the GPC having the same retention time as an authentic
sample. Identification of the by-product was confirmed by
HPLC-MS and NMR spectroscopy. Evidence for the pro-
posed mechanism of cross-linking can also be seen in the FT-
IR spectrum. The spectrum of 3 (Figure 3, upper curve)
Figure 4. GPC of 5, showing re-formation of soluble polymeric prod-
ucts.
As a final demonstration that the cross-links in 3 were due
À
to reversible formation of S S bonds, 3 was irradiated with an
excess of tetraethyl dithiuram disulfide (6) (Scheme 3). Under
these conditions the polymer was rendered completely
soluble after 6 h irradiation, followed by 42 h in the dark,
the GPC (Figure 4) and NMR spectra (Supporting Informa-
tion, Figure S2) being consistent with the change.
We have presented an efficient approach to the synthesis
of a new range of photoresponsive DTC-containing polymeric
materials. We believe that the ability to generate a high
concentration of stable radicals under controlled conditions
has the potential to be used in many application areas. These
may include engineering adhesives, UV filters, separation
materials, sensors for radicals, trapping of reactive oxygen
species and as antioxidants, thermoplastic vulcanizates, and as
new materials for microelectronics.
Received: November 26, 2009
Revised: February 17, 2010
Published online: April 29, 2010
Figure 3. IR spectra of the linear (2) and cross-linked polymers (3),
À
showing the appearance of a new band due to S S bond formation.
Keywords: dithiocarbamate ester · reversible cross-linking ·
photochemistry · radical reactions · thiuram
.
À1 [17]
À
showed a strong peak due to S S stretching at 750 cm
which was absent from the spectra of 1 and 2 (Figure 3, lower
,
=
À
curve). Signals due to C S and C N stretching vibrations, at
1174 and 1477 cmÀ1 respectively, were found in the spectra of
all three compounds.
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