Utilization of a Meldrum's acid towards functionalized fluoropolymers possessing dual reactivity for thermal crosslinking and post-polymerization modification

Article abstract of DOI:10.1039/c5cc02382c

New thermally cross-linkable and/or post-functionalizable perfluorocyclobutyl (PFCB) polymers containing Meldrum's acid moieties have been successfully prepared via the thermal cyclopolymerization of a new Meldrum's acid functionalized aromatic trifluorov

Full text of DOI:10.1039/c5cc02382c

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Utilization of a Meldrum’s acid towards functionalized
fluoropolymers possessing dual reactivity for thermal
crosslinking and post-polymerization modification†
Cite this:DOI: 10.1039/c5cc02382c
Received 21st March 2015,
Accepted 29th April 2015
a
b
a
a
Jingbo Wu, Scott T. Iacono,* Gregory T. McCandless, Dennis W. Smith Jr. and
a
Bruce M. Novak
DOI: 10.1039/c5cc02382c
www.rsc.org/chemcomm
New thermally cross-linkable and/or post-functionalizable perfluoro-
cyclobutyl (PFCB) polymers containing Meldrum’s acid moieties have
been successfully prepared via the thermal cyclopolymerization of
a new Meldrum’s acid functionalized aromatic trifluorovinyl ether
(TFVE) monomer.
Perfluorocyclobutyl (PFCB) aryl ether polymers are a class of
high value-added, tailorable fluoropolymers with a combination
of many desired properties, such as processability with common
solvents, excellent thermal and chemical stability, low dielectric
1
values, and high fidelity micro-moulding replication. Due to
these desired properties, PFCB aryl ether polymers have demon-
2
strated many applications such as photonics, electro-optic
3
4
5,6
materials, proton exchange membranes, hybrid composites,
7
,8
and hole-transport materials.
Typically, PFCB aryl ether polymers can be easily prepared as
thermoplastics or thermosets by heating the aryl trifluorovinyl
ether (TFVE) di- or tri-functional monomers or comonomers in
the bulk or in solution above 150 1C via catalyst-free, step-
growth cyclopolymerization processes. By simply optimizing toleration of functional groups during polymerization processes,
time and temperature parameters, the molecular weights of alternative strategies are highly desired in order to expand the pool
PFCB aryl ether polymers can be easily controlled which is a key PFCB aryl ether polymers.
Scheme 1 Preparation of PFCB aryl ether polymers from two common
step-growth routes for functional group (FG) inclusion.
1
,9
feature to bulk material fabrication. Introduction of function-
alities into the PFCB aryl ether polymer backbone is used to tailor synthesis for decades.
Meldrum’s acid chemistry has been widely used in organic
1
0–13
Recently, Meldrum’s acid derivatives
their applications. Generally, functional PFCB aryl ether polymers have been successfully incorporated into a variety of polymers
can be prepared by direct polymerization of functional aryl TFVE as both synthetic building blocks and thermolytic precursors to
1
4–18
monomers via radical [2+2] cycloaddition reaction (route 1) or PFCB dialkyl ketenes.
Thermal treatment of polymers containing
aryl ether monomers with functional monomers via condensation 5,5-disubstituted Meldrum’s acid groups resulted in the loss of
reactions (route 2) as shown in Scheme 1. Due to the difficulty in acetone and carbon dioxide, thereby yielding dialkyl ketenes
the synthesis of complex functional aryl TFVE monomers and which were used to cross-link the polymer chain via [2+2]
dimerization and/or introduce various functional groups to
1
9
a
the polymer side chain via nucleophilic addition. Meldrum’s
acid and the resulting ketene would provide a novel and effective
strategy to tandemly post-functionalize and/or cross-link PFCB
aryl ether polymers thereby further expanding their applications. The
Meldrum’s acid functionalized aryl TFVE monomer 5 (Scheme 2)
and subsequent PFCB aryl ether polymer/copolymer 6/7 under-
goes thermoset 9 formation or affords a post-functionalized
Department of Chemistry and the Alan G. MacDiarmid NanoTech Institute,
The University of Texas at Dallas, Richardson, Texas, 75080, USA
Department of Chemistry and Chemistry Research Center, United States Air Force
Academy, Colorado Springs, Colorado 80840, USA.
b
E-mail: scott.iacono@usafa.edu
†
Electronic supplementary information (ESI) available: Spectroscopic characterization,
13 19
1
H, C, F NMR, and X-ray crystallographic information. CCDC 1053394. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc02382c
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Scheme 2 Synthesis of Meldrum’s acid functionalized aryl TFVE 5 (with
ORTEP structure, as shown 50% probability) and corresponding linear
PFCB aryl ether homopolymer 6.
Fig. 1 ATR-FTIR spectra of monomer 5 and polymers 6 and 9.
with polymer 6 in ATR-FTIR spectra is shown in Fig. 1 (top). After
polymerization, a broad, strong absorbance of the PFCB ring
Scheme 3 Synthesis of PFCB aryl ether polymers 6, 7, 9, and 10 derived
from monomer 5.
À1
breathing mode appeared at 961 cm confirming chain exten-
À1
sion while the characteristic absorbance at 1738 cm corre-
sponding to Meldrum’s acid groups remained intact. Heating
thermoplastic 10 (Scheme 3). To our knowledge, this dual reac- monomer 5 at elevated temperature of 180 1C for 8 h yielded
tivity is the first demonstrated example for a fluoropolymer and insoluble material due to the formation of ketene via thermolysis
can be easily applied to other momomeric motifs.
The newly designed Meldrum’s acid functionalized aryl (Fig. S13, ESI,† vida infra).
TFVE monomer 5 was synthesized in a good isolated yield ATR-FTIR analysis (Fig. 1, bottom) after heating polymer 6 at
80%) by simple alkylation of Meldrum’s acid with 4-trifluoro- elevated temperature of 220 1C resulted in the formation of the
vinyl benzyl bromide 4 by following standard procedure in intermediate PFCB aryl ether polymer 8 enriched with ketene.
of dialkyl Meldrum’s acid followed by subsequent crosslinking
(
2
0
Scheme 2. A single crystal X-ray diffraction study was carried The resulting network polymer 9 via 8 outlined in Scheme 3
À1
out on monomer 5 and the molecular structure is also shown showed residual absorbance at 2108 cm corresponding to the
1
8
Scheme 2 inset. Experimental parameters as well as packing typical absorbance for ketenes, which indicates the unreacted
views are provided in the ESI.† PFCB aryl ether homopolymer 6 ketene intermediates were very stable for a long period of time
À1
can be prepared via the thermal cyclopolymerization of mono- at air atmosphere, and a weak absorbance at 1813 cm
,
mer 5 at 160 1C. The polymerization temperature is a critical corresponding to the typical absorbance for cyclobutane-1,3-
parameter in order to achieve PFCB aryl ether polymer containing diones formed from dialkyl ketenes via [2+2] dimerization
1
8
Meldrum’s acid which should be high enough to activate the procedure. This observation indicated a crosslinked PFCB
polymerization process but low enough to avoid the thermolysis aryl ether polymer 9 was achieved which was insoluble with no
of Meldrum’s acid groups. Heating monomer 5 at 160 1C for 8 h, observable glass transition temperature (T
g
) was observed below
the soluble oligomer of 6 was achieved with a number-average 260 1C due to the high cross-linking density. In order to produce
1
9
molecular weight (M
(
n
) estimated by F NMR end-group analysis processable higher molecular weight polymer with the Meldrum’s
À1
GPC analysis) of 4100 (3600) g mol . A comparison of monomer 5 acid functional groups intact, monomer 5 was copolymerized neat
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with commercially available biphenyl aryl trifluorovinyl ether fluoropolymers, PFCB aryl ether polymer. Meldrum’s acid func-
monomer (BP-TFVE) at 170 1C for 24 h with a molar feed ratio tionalized aryl TFVE monomer has been designed and synthe-
3
:50 (5:BP-TFVE). Polymerization produced a soluble, film-forming sized by direct alkylation of Meldrum’s acid with bromomethyl
À1
copolymer 7 with an acceptable M of 12 000 g mol (GPC).
aryl TFVE monomer in a good yield utilizing operationally simple
n
Post-polymerization modification (PPM) becomes an attractive transformations. Meldrum’s acid functionalized PFCB aryl ether
approach to effectively incorporate diverse functional groups into homopolymer and copolymers have been successfully prepared
2
1
a single polymer precursor and very few functionalized PFCB via the standard thermal [2+2] cyclodimerization of aryl TFVE
22
aryl polymers have been prepared by PPM. In order to demon- groups at programmable temperatures. Transformation of
strate PPM utility of this system, we simply heated a mixture of an Meldrum’s acid to ketene intermediates successfully occurred by
O-nucleophile,
N-ethyl-N-(2-hydroxyethyl)-4-(4-nitrophenylazo) thermolysis above 200 1C. Post-polymerization modification via
aniline (Disperse Red 1, denoted DR-1) and Meldrum’s acid the ketene intermediate was demonstrated by the successful
functionalized copolymer 7 at 200 1C for 3 h which resulted in incorporation of DR-1 into PFCB polymer matrix to form a side-
the electro-optically active functionalized polymer 10. The suc- chain electro-optic system. The cross-linkable and/or functionaliz-
cessful incorporation of DR-1 into PFCB aryl ether polymer matrix able Meldrum’s acid groups containing PFCB aryl ether polymers
was clearly visible by the light-red colour of polymer 10 and could be potentially useful in various active technology sectors,
1
further validated by H NMR spectrum whereby the disappear- such as electro-optic devices and proton exchange membranes
ance of the H-signals of methyl groups in the Meldrum’s acid (PEMs) for fuel cells.
moiety at 0.65 ppm and the appearance of the characteristic
peaks of three methylene–CH2 groups in DR-1 chromophore Foundation (Grant AT-0041), the Alan G. MacDiarmid Nano-
Fig. S11, ESI†). ATR-FTIR spectrum of 10 further confirmed the Tech Institute, the National Science Foundation through the
formation of ester bond (Fig. S18, ESI†). Center for Energy Harvesting Materials and Systems (NSF-I/
Differential scanning calorimetry (DSC) studies on the pre- UCRC, grant 1035024), the NMR facility of chemistry depart-
pared polymers in this study reveal well-defined T s from their ment (NSF, grant CHE-1126177), and the University of Texas at
corresponding thermal responses of the first heating cycle. Dallas for their support. STI acknowledges the Air Force Office
As showed in Fig. S19 (ESI†) (top), oligomer 6 exhibits a T of of Scientific Research (AFOSR) for financial support.
0 1C, which is lower than the T of optimized PFCB aryl ether
polymers. Polymer 10 exhibits a T of 120 1C, which is slightly
We would like to thank the Intel Corporation, the Welch
(
g
g
9
g
Notes and references
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K
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1
1
1
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