Photo-induced ligation of acrylonitrile-butadiene rubber: Selective tetrazole-ene coupling of chain-end-functionalized copolymers of 1,3-butadiene

Article abstract of DOI:10.1021/ma401154k

A highly selective photo-induced nitrile imine mediated tetrazole-ene coupling (NITEC) of chain-end-functionalized nitrile-butadiene rubber (NBR) is reported, providing nitrile rubbers with molar masses of up to 48 000 g·mol^-1. NBR was obtained via the reversible addition-fragmentation chain transfer (RAFT) mediated copolymerization of acrylonitrile and 1,3-butadiene employing a novel photoreactive tetrazole-functionalized trithiocarbonate. The herein reported tetrazole-functionalized trithiocarbonate represents - to the best of our knowledge - the first ever reported photoreactive RAFT agent capable of undergoing light-induced ligations with enes. Molar masses of the tetrazole-functionalized NBRs were in the range of 1000 to 38 000 g·mol^-1 with dispersities between 1.1 to 1.6. By an appropriate choice of the tetrazole substituents, a reaction of the in situ formed enophile with the double bonds or the nitrile moieties of the incorporated monomer units within the polymer backbone - present in high excess relative to the dipolarophile linker molecule - was not observed. Underpinned by DFT calculations, the selectivity was identified to originate from a reduced LUMO energy level of the maleimide linker compared to the nonactivated backbone olefins when employing a nitrile-imine of moderate reactivity.

Full text of DOI:10.1021/ma401154k

Article
pubs.acs.org/Macromolecules
Photo-Induced Ligation of Acrylonitrile-Butadiene Rubber: Selective
Tetrazole−Ene Coupling of Chain-End-Functionalized Copolymers of
1,3-Butadiene
Christoph J. Durr,†,‡ Paul Lederhose,†,‡ Lebohang Hlalele,†,‡ Doris Abt,†,‡ Andreas Kaiser,§
̈
Sven Brandau,§ and Christopher Barner-Kowollik*,†,‡
†Preparative Macromolecular Chemistry, Institut fur Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology
̈
(KIT), Engesserstrasse 18, 76128 Karlsruhe, Germany
‡Institut fur Biologische Grenzflachen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz Platz 1, 76344
̈
̈
Eggenstein-Leopoldshafen, Germany
§Lanxess Emulsion Rubber, BP 7−Z.I. Rue du Ried, 67610 La Wantzenau, France
S
* Supporting Information
ABSTRACT: A highly selective photo-induced nitrile imine
mediated tetrazole−ene coupling (NITEC) of chain-end-
functionalized nitrile−butadiene rubber (NBR) is reported,
providing nitrile rubbers with molar masses of up to 48 000
g·mol−1. NBR was obtained via the reversible addition−
fragmentation chain transfer (RAFT) mediated copolymerization
of acrylonitrile and 1,3-butadiene employing a novel photo-
reactive tetrazole-functionalized trithiocarbonate. The herein
reported tetrazole-functionalized trithiocarbonate representsto
the best of our knowledgethe first ever reported photoreactive
RAFT agent capable of undergoing light-induced ligations with
enes. Molar masses of the tetrazole-functionalized NBRs were in
the range of 1000 to 38 000 g·mol−1 with dispersities between 1.1
to 1.6. By an appropriate choice of the tetrazole substituents, a reaction of the in situ formed enophile with the double bonds or
the nitrile moieties of the incorporated monomer units within the polymer backbonepresent in high excess relative to the
dipolarophile linker moleculewas not observed. Underpinned by DFT calculations, the selectivity was identified to originate
from a reduced LUMO energy level of the maleimide linker compared to the nonactivated backbone olefins when employing a
nitrile−imine of moderate reactivity.
were proposed. In addition, several other thermally,24,25
INTRODUCTION
■
chemically26−29 or photochemically30−34 triggered ligation
Within the past decade, orthogonal ligation techniques have
protocols have been developed. An outstanding example of
found substantial interest in the field of polymer chemistry.1−4
Their advent originates from the introduction of reversible-
deactivation polymerization techniques such as nitroxide
an orthogonal ligation chemistry is the UV-light triggered nitrile
imine mediated tetrazole−ene coupling (NITEC).35−38 First
described in 1967 by Huisgen and co-workers,39 the method is
mediated polymerization (NMP),5 atom transfer radical
mainly applied in the field of biochemistry40−42 and was used
polymerization (ATRP)6 and reversible addition−fragmenta-
for bio-orthogonal conjugation of proteins in vitro43 and in
tion chain transfer (RAFT) polymerization7−12 allowing the
vivo.44,45 Recently, polymer scientists have found interest in this
synthesis of polymers with well-defined chain-end function-
ality.13 Orthogonal ligation is applied in all fields of modern
technique free of catalysts and additives for the modification of
polymersomes46 or the spatially controlled immobilization of
polymers on silicon or cellulose.47 Mechanistically, the
irreversible light triggered release of molecular nitrogen from
a 2,5-tetrazole provides a nitrile imine dipole that reacts in situ
with olefinseither activated or unactivatedvia the for-
mation of a pyrazoline structure.
polymer science2,14 and has been employed for chain-end
functionalization, dendrimer-,15,16 block- and star (co)-
polymer17 synthesis and the immobilization of polymers on
solid (bio)substrates.18,19 The most prominent ligation
technique, the copper mediated azide−alkyne cycloaddition
(CuAAC), was reinvented by Kolb, Finn, and Sharpless20 with
the introduction of the “click” concept in 2001 and since then
continuously refined. In recent years, strategies to improve
catalysts in efficiency21 and nature22 were developed and
catalyst-free strategies employing ring-strained cycloalkynes23
Received: June 4, 2013
Revised: June 27, 2013
© XXXX American Chemical Society
A
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Scheme 1. Overall Strategy of the UV-Induced Modular Coupling of NBR Building Blocks to Obtain High Molecular Weight
Nitrile Rubber
We recently reported the reversible-deactivation radical
copolymerization of acrylonitrile (AN) and 1,3-butadiene
(BD) to obtain nitrile−butadiene rubber (NBR) of low
dispersity under controlled molecular weight conditions.48,49
The RAFT polymerization protocol was further exploited for
the utilization of chain-end functionality in successive modular
strategies for the synthesis of block- and miktoarm star
copolymers via hetero-Diels−Alder cyclizations50 andby
employing a CuAAC reaction pathwayfor the accessibility
of molar masses above those that can be obtained in sequential
RAFT polymerizations.48,51
In the current contribution, a photo-induced approach for
the synthesis of linear high molecular weight acrylonitrile−
butadiene rubber (NBR) via the NITEC is presented. Such an
approach is highly desirable, since linear high molecular weight
polymer of the technically important synthetic rubber cannot
be obtained in a sequential RAFT process. With respect to an
industrial application, the NITEC technique overcomes the
limitations of other ligation techniques derived from the need
of metal catalysts, often exhibiting negative effects on NBR
aging properties.52 The presented photochemical tetrazole−ene
coupling technique reveals a high selectivity and solely proceeds
with the specific linker molecule. A coupling of the tetrazole
chain-ends with the “enes” present in high concentrations
within the NBR backbone is not observed.
synthesized from 4-(2-phenyl-2H-tetrazol-5-yl)benzoic acid (2)
in two consecutive esterification steps and employed in the
RAFT mediated radical copolymerization of AN and BD. In the
presence of a small molecule linker, bis(maleimido)hexane (5),
the RAFT based photoreactive polymers (4) were irradiated
with UV-light (λmax = 254 nm) to form the diaryl nitrile imine
enophile in situ. The nitrile imine and the dipolarophile
maleimide subsequently react to give the coupled NBR (6).
EXPERIMENTAL SECTION
■
Materials. Acrylonitrile (AN, > 99%, Acros), 1,3-butadiene (BD, >
99.5%, Air Liquide), N,N′-dicyclohexylcarbodiimide (DCC, 99%,
Acros), 4-(dimethylamino)pyridine (DMAP, 99%, Aldrich), propane-
1,3-diol (98%, Aldrich), benzenesulfonyl hydrazide (98%, ABCR), 4-
formylbenzoic acid (97%, Aldrich), acetonitrile (Rotisolv, HPLC
grade), pyridine, (99%, ABCR), aniline (>99.5%, Aldrich), sodium
nitrite (97+%, Acros), hydrochloric acid (37%, Roth), 1,1′-azobis-
(cyclohexane-1-carbonitrile) (98%, Aldrich), 1-ethylpiperidine hypo-
phosphite (95%, Aldrich), 1-octene (98%, Aldrich), trans-3-octene
(97%, ABCR) and chlorobenzene (Acros, 99+%) were used without
further purification. 1,6-Bis(maleimido)hexane (5) was obtained from
BASF SE, Ludwigshafen, Germany. Other solvents (synthesis grade)
were obtained from VWR and used as received. 2-((Dodecylsulfanyl)-
carbonothioyl)sulfanyl propanoic acid (DoPAT) was obtained from
Lanxess Deutschland GmbH. 4-(2-Phenyl-2H-tetrazol-5-yl)benzoic
acid was synthesized according to the literature.35
Synthesis of Tetrazole-Functional RAFT Agent (3). DoPAT
(2.000 g, 5.7 mmol), DMAP (0.014 g, 0.1 mmol), and 1,3-propanediol
(1.299 g, 17.1 mmol) were dissolved in 10 mL of THF. The solution
was cooled to 0 °C and DCC (1.290 g, 6.3 mmol) was added. The
cooling bath was removed and the reaction mixture was stirred at
ambient temperature for 16 h. THF was removed under reduced
Our overall strategy of NBR coupling is depicted in Scheme
1. A novel tetrazole-functionalized controlling agent 3
representing, to the best of our knowledge, the first ever
reported photoreactive tetrazole-functionalized RAFT agent
capable of undergoing light-induced ligations with eneswas
B
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pressure. The obtained solid was dissolved in Et2O, extracted with 1 M
hydrochloric acid (4 × 100 mL) and washed with saturated NaHCO3
solution (100 mL). The organic layer was dried over MgSO4 and Et2O
was removed under reduced pressure. The crude product was purified
via column chromatography on silica gel using hexane/ethyl acetate
(3:1, v/v, Rf 0.36) as the eluent. After drying under high vacuum
compound 1 was obtained as a yellow oil (1.591 g, 68%).
1H NMR (400 MHz, CDCl3): δ 4.83 (q, J = 7.4 Hz, 1H,
CH(CH3)), 4.31−4.28 (m, 2H, C(O)O−CH2), 3.69 (t, J = 7.4 Hz,
2H, CH2−OH), 3.34 (t, J = 7.4 Hz, 2H, S−CH2), 1.90−1.86 (m, 2H,
O−CH2−CH2−CH2−OH), 1.70−1.66 (m, 2H, S−CH2−CH2), 1.59
(d, J = 7.4 Hz, 3H, CH(CH3)), 1.42−1.38 (m, 2H, S−CH2−CH2−
CH2), 1.26−1.21 (m, 16H, CH3−(CH2)8), 0.86 (t, J = 7.4 Hz, 3H,
CH2−CH3).
polystyrene standards (Polymer Standard Service) ranging from 160 to
6 × 106 g·mol−1. Samples were injected from solutions in THF (2
mg·mL−1) and molecular weight was evaluated with the Mark−
Houwink−Kuhn−Sakurada parameters of NBR.53
ESI-MS spectra were recorded on a LXQ mass spectrometer
(ThermoFisher Scientific, San Jose, CA) equipped with an
atmospheric pressure ionization source operating in the nebulizer
assisted electrospray mode. The system was calibrated with a standard
containing caffeine, Met-Arg-Phe-Ala acetate, and a mixture of
fluorinated phosphazenes (Ultramark 1621), purchased from Aldrich.
A spray voltage of 4.5 kV, a dimensionless sweep gas flow rate of 2 and
a dimensionless sheath gas flow-rate of 12 were applied. The capillary
voltage, the tube lens offset voltage and the capillary temperature were
set to 60 V, 110 V and 275 °C, respectively. For SEC−ESI−MS
measurements the mass spectrometer was coupled to a Series 1200
HPLC-system (Agilent, Santa Clara, CA) with THF as the eluent in
accordance to a setup described earlier.54 Polymer samples were
dissolved in THF at 2 mg·mL−1 and injected onto the HPLC system.
Fluorescence emission spectra were recorded in quartz cuvettes
loaded with a sample volume of 230 μL on a Varian Cary Eclipse
Fluorescence Spectrometer. UV absorption spectra were recorded on a
Varian Cary 300 UV−vis spectrometer.
13C NMR (101 MHz, CDCl3): δ 222.27 (C(S)−S), 171.28 (C(O)−
O), 62.00, 48.16, 37.37, 32.05, 29.76, 29.75, 29.68, 29.56, 29.47, 29.22,
29.03, 28.02, 22.82, 17.09, 14.25, 14.22 (CH2−CH3).
The hydroxyl-functional RAFT agent 1 (3.800 g, 9.3 mmol), DMAP
(0.026 g, 0.02 mmol) and diaryl tetrazole 2 (2.890 g, 10.9 mmol) were
dissolved in 20 mL THF. The solution was cooled to 0 °C and DCC
(2.450 g, 11.9 mmol) was added. After stirring the reaction mixture for
16 h at ambient temperature THF was removed under reduced
pressure. The obtained solid was dissolved in Et2O, extracted with 1 M
hydrochloric acid (4 × 200 mL) and washed with saturated NaHCO3
solution (200 mL). The organic layer was dried over MgSO4 and Et2O
was removed under reduced pressure. The crude product was purified
via column chromatography on silica gel using hexane/ethyl acetate
(3:1, v/v Rf 0.47) as the eluent. After drying under high vacuum the
title compound 3 was obtained as a yellow oil (2.760 g, 45%).
1H NMR (400 MHz, CDCl3): δ 8.28 (d, J = 7.6 Hz, 2H, Hd), 8.17−
8.10 (m, 4H, Hd′,c), 7.53 (t, J = 7.3 Hz, 2H, C6H5-meta-H), 7.46 (t J =
7.4 Hz, 1H, C6H5-para-H), 4.81 (q, J = 7.1 Hz, 1H, CH(CH3)), 4.45
(t, J = 7.9 Hz, 2H, aryl−C(O)O−CH2), 4.33 (t, J = 6.8 Hz, 2H,
CH(CH3)−C(O)O−CH2), 3.32 (t, J = 7.2 Hz, 2H, S−CH2), 2.19−
2.16 (m, 2H, O−CH2−CH2−CH2−O), 1.66−1.60 (m, J = 7.1 Hz, 2H,
S−CH2−CH2), 1.59 (d, J = 7.2 Hz, 3H, CH(CH3)), 1.38−1.34 (m,
2H, S−CH2−CH2−CH2), 1.38−1.33 (m, 16H, CH3−(CH2)8), 0.86
(t, J = 7.2 Hz, 3H, CH2−CH3).
RESULTS AND DISCUSSION
■
Cyclization of various diaryl nitrile imines with nonactivated
olefins was reported to proceed within seconds.44 In NBR
polymerizations, incorporation of BD into the growing polymer
chain predominantly occurs in a 1,4-mode resulting in the
formation of internal double bonds distributed along the
polymer backbone. These double bonds are potential
“dipolarophiles” able to react with the nitrile imine dipole.
Close to 10% of the BD incorporation occurs in a 1,2-mode
leading to pendant double bonds,55 less sterically hindered and
more readily available for cyclization than the internal ones. In
1972, Stille et al. investigated [4 + 2] cyclizations of nitrile
imines and residual double bonds of styrene−butadiene type or
natural rubber type elastomers for their thermally induced
cross-linking.56 In their work, tetrazole-functionalized styrene
monomers were synthesized and copolymerized into the
elastomers as pending nitrile imine dipole precursors. At
elevated temperatures, cross-linked rubbers were obtained and
characterized by solubility studies. Nitrogen release was
observed at temperatures above 150 °C with tetrazole half-
lives in the range of several minutes. Since the reaction kinetics
and cyclization affinities of diaryl nitrile imines toward olefins
strongly depend on the substitution patterns of the aromates, a
careful choice of substituents on the aromatic rings of the diary
tetrazole is crucial to prevent cross-linking of the tetrazole-
functionalized NBRs and to allow for a selective coupling of the
tetrazole-functionalized NBRs with the small molecular linker
5. In the present study, a tetrazole was thus chosen exhibiting a
cylization reactivity sufficiently low to not proceed with the
double bonds of the NBR backbone, yet sufficiently high to
proceed to high conversion within a reasonable time frame
when adding the maleimide linker 5.
13C NMR (101 MHz, CDCl3): δ 222.19 (C(S)−S), 171.33
(CH(CH3)−C(O)−O), 165.97 (aryl−C(O)−-O), 164.46 (C(N−
R)−N), 136.92, 131.88, 131.47, 130.38, 130.03, 129.88, 127.14,
120.06, 62.55, 61.83, 47.96, 37.44, 32.04, 29.76, 29.74, 29.68, 29.57,
29.47, 29.22, 29.05, 28.12, 27.99, 22.82, 16.87, 14.25 (CH2−CH3).
Polymerizations. RAFT-mediated copolymerizations of AN and
BD were performed in a pressure stable glass reactor in a setup
described earlier.48 Samples were taken after preset time intervals and
precipitated in cold ethanol. Experimental details are provided in Table
2.
Coupling of Tetrazole-Functionalized NBR via NITEC. In a 50
mL round-bottom quartz glass flask, diaryl tetrazole-functionalized
NBR was dissolved in acetonitrile. Then 0.5 equiv of bis(maleimido)-
hexane linker was added from a stock solution in acetonitrile. The
solution was irradiated with UV-light of 254 nm (Lamag TLC lamp,
electric power 8 W, lamp-flask distance at the position of the largest
flask diameter ∼5 cm) for 3 h. The solvent was removed under
reduced pressure and the resulting polymers were analyzed without
further purification.
DFT Calculations. The HOMO and LUMO energies were
calculated using the B3LYP/6-31G//B3LYP/6-31G model chemistry
with GAMESS.
Orthogonality of the nitrile imine to the double bonds of the
1
Instrumentation. H NMR and 13C NMR spectra were recorded
1
polymer backbone was proven experimentally in a H NMR
model investigation (for 1H NMR spectra, see Figure S1 in the
Supporting Information). Solutions of tetrazole 3 in acetonitrile
were irradiated with UV light at λmax = 254 nm in the presence
of 1-octene or trans-3-octene, chosen as model compounds
mimicking the pendant and internal double bonds of the
polymer backbone, respectively. While after irradiation in the
presence of either 1-octene or trans-3-octene the absence of
aromatic resonances of the diaryl tetrazole protons at 8.4 ppm
at ambient temperature on a Bruker Advance 400 NMR spectrometer
or a Bruker AM 250 NMR spectrometer and referenced to the residual
solvent signal.
Molecular weight determination was performed on a SEC system
(PL-GPC 50 Plus, Polymer Laboratories) consisting of an auto
injector, a guard column (PLgel Mixed C, 50 × 7.5 mm), three linear
columns (PLgel Mixed C, 300 × 7.5 mm, 5 μm bead-size) and a
differential refractive index detector using THF as the eluent at 35 °C
and a flow rate of 1 mL·min−1. The system was calibrated using narrow
C
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indicates a quantitative release of nitrogen, formation of the
respective pyrazoline compounds is not observed and
deactivation of the nitrile imine occurs via alternative pathways.
In contrast, irradiation of 3 in the presence of maleimide
quantitatively gives the pyrazoline adduct, as characterized by
multiplets at 5.2 and 4.9 ppm. When irradiating 3 in the
presence of both, trans-3-octene and maleimide, sole formation
of the maleimide cycloadduct was observed. In 1,3-dipolar
cycloaddition reactions of type I (according to Sustmann’s
classification57) the reaction rate of the cycloaddition reaction
increases with a decrease in the free-energy gap between the
highest occupied molecular orbital (HOMO) of the enophile
and the lowest unoccupied molecular orbital (LUMO) of the
dipolarophile.58 Lin and co-workers were able to exploit the
interrelation of the reaction rate and the free-energy gap by
lifting the HOMO energy levels of the intermediate diaryl
nitrile imines to reduce reaction times by tuning the aryl
substitution pattern.44 In analogy, the selectivity of the 1,3-
dipolar cycloaddition toward maleimide observed in our study
can be explained by the molecular orbital (MO) energy levels
of the reactants. The LUMO energy levels of 1-octene, trans-3-
octene and maleimide were accessed by DFT calculations and
are reported in Table 1, entries 2−4. When comparing the
discussion, since due to the distance of the trithiocarbonate and
the ester moiety to the diaryl tetrazole moiety in compound 3,
the dissimilar substitution pattern will not have a pronounced
effect on the obtained energy levels. The difference of the
HOMO energy level of the nitrile imine intermediate of
tetrazole 3′ obtained in the current study and the energy level
earlier reported by Wang et al.,44 is a result of the difference in
the model chemistries (HF/3-21G//AM1 vs B3LYP/6-31G//
B3LYP/6-31G) used. The LUMO energy level of the nitrile
moietya potential ‘ene’ present in both the solvent and the
NBR backboneis positioned above the one of 1-octene.
Nevertheless, light-induced coupling of tetrazole functional
polymers with acetonitrile was observed when a plain solution
of tetrazole functional polymer was irradiated with UV (see
Figures S3 and S4) and can be explained with the high excess of
solvent molecules relative to the polymer.
A high density of the chain-end functionality of polymers is a
prerequisite for effective orthogonal ligation reactions on
polymers. When orthogonal to the polymerization process,
the utilization of functionalized chain transfer agents in RAFT
polymerizations allows for the synthesis of chain-end-function-
alized polymers with a high functional density without the need
for postpolymerization modifications.8 Because of a low
propagation rate coefficient,59 the solution based reversible-
deactivation radical copolymerization of AN and BD requires
elevated temperatures to obtain moderate conversions. The
thermal stability of 3 in solution was evidenced by comparison
Table 1. Molecular Orbital Energies of the Dipole and the
a
Dipolarophile Model Compounds
entry
compound
3′
EHOMO (eV)
ELUMO (eV)
1
of the H NMR spectra before and after heating the diaryl
b
1
−5.249
−
tetrazole at 100 °C for 8 h in DMSO-d6 (Figure S5), allowing
the direct synthesis of tetrazole functional polymers via the
RAFT technique. The trithiocarbonate 3 was thus investigated
toward its ability to control the free radical copolymerization of
AN and BD. Polymerizations were performed at 100 °C in the
absence of light. The obtained polymers 4a−4f range from
oligomeric species to polymers with molar masses of up to
38 000 g·mol−1, while dispersities below 1.5 were observed. A
slightly higher dispersity of 1.6 is observed for NBR 4f of much
higher molecular weight as a result of the prolonged reaction
time and the therewith associated accumulation of inactive
polymer chains. Experimental details for each individual
polymerization are provided in Table 2. Molecular weights in
the current study are reported as actual molar masses obtained
from SEC via universal calibration employing the Mark−
Houwink−Kuhn−Sakurada (MHKS) parameters of NBR,53
recently determined by us. When comparing the molar masses
obtained in the current study with those obtained under
identical polymerization conditions reported in previous studies
2
3
4
5
maleimide
−
−
−
−
−2.996
0.659
0.727
0.947
1-octene
trans-3-octene
acetonitrile
a
Energies of the molecular orbitals are obtained via DFT calculations
(B3LYP/6-31G//B3LYP/6-31G) and are given as zero-point
corrected values. The HOMO energy level of the nitrile imine
intermediate of tetrazole 3′ is reported. The chemical structure of 3′ is
provided in Figure S2.
b
LUMO energy levels of the olefins with the HOMO energy
level of the nitrile imine intermediate of diaryl tetrazole 3′
(Table 1, entry 1), the free-energy gap between the MOs of the
dipole and the dipolarophile increases in the order of
maleimide < 1-octene < trans-3-octene, in line with the high
reactivity observed for maleimide. To keep computational costs
low, the HOMO energy level of the nitrile imine intermediate
of tetrazole 3′ (see Figure S2) instead of 3 was used for
a
Table 2. Experimental Details of the RAFT-Mediated Copolymerizations of AN/BD
b
c
c
entry
RAFT agent
[RAFT]0 (mM)
[Ini]0 (mM)
t (h)
p
(%)
Mn (g·mol−1
)
Đ
d
f
1
3
3
3
3
3
3
91.1
27.9
21.0
8.4
5.5
1.7
2.6
1.0
0.3
0.2
3
5.5
5
37.2
4a
4b
4c
4d
4e
4f
700
1.2
1.2
1.3
1.4
1.5
1.6
e
2
18.5
18.0
12.0
10.8
8.9
1800
3100
3
4
5
6
7
4700
3.5
8
9400
0.8
22
38 000
a
Conditions: NBR was synthesized with an overall monomer concentration of 9.5 mol·L−1 in chlorobenzene at 100 °C employing 1,1′-
b
azobis(cyclohexane-1-carbonitrile) as a thermal initiator. Polymers were recovered by precipitation in ethanol, except otherwise stated. Conversion
was determined gravimetrically. Obtained from SEC via universal calibration employing the MHKS parameters of NBR.53 Polymerization was
c
d
e
f
performed in acetone; polymer was recovered by solvent evaporation. Polymer was recovered by precipitation of polymer in cold hexane/Et2O. As
a result of the low molecular weight, a strong deviation of molar mass obtained via universal calibration from the effective molar mass is observed.
ESI-MS of polymer 4a shows signals with the highest intensities at around 1100 Da.
D
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1
Figure 1. H NMR characterization of (A) tetrazole-functionalized NBR 4b of 1800 g·mol−1 and B) coupled NBR 6b of 3200 g·mol−1, both
measured in CDCl3.
a decrease by a factor of 2 in the values is observed. The
decrease is a result of the higher K value of NBR relative to
polystyrene (49.5 × 10−5 dL·g−1 vs 14.1 × 10−5 dL·g−1), since
tha α values are almost identical for NBR and polystyrene
(0.689 vs 0.70).60
the ester moieties of the propyl linkage, f−g, are located at 4.4
and 4.2 ppm, respectively. Resonances at 0.9, 1.2, and 3.3 ppm
indicate the presence of the Z-group dodecyl mercaptan chain-
end as described in more detail for tetrazole functional NBR 4a
in Figure S8 in the Supporting Information. Identical chain-end
resonances are observed when analyzing polymers of higher
molecular weight.
In the copolymerizations employing the photoreactive
trithiocarbonate 3, a linear relation of molar mass with
conversion is observed as depicted in Figure S6, indicating
living characteristics. The deviation from the linear behavior at
the early polymerization stages is due to a hybrid effect as
previously described.61 The high end-group functionality of the
obtained polymers 4 was proven by SEC-ESI mass spectrom-
etry analysis. A magnified view into the region of 700 to 1400
Da of the low molecular weight NBR 4a is provided in Figure
S7 in the Supporting Information section. The main signals are
assigned to the sodium adducts [4m+n + Na]+ of the tetrazole-
functional RAFT polymer, indicating formation of tetrazole-
functionalized chains. Nevertheless, a small fraction of NBR 7
free of tetrazole functionality and unable to undergo further
ligation is observed. These species are formed by the
recombination of two propagating copolymer chains that
were both initiated by the azo initiator fragment and are an
inevitable side product of the RAFT process.9 The observed
signal pattern (see inset, Figure S7) is a result of the masses of
the BD and AN repeat units differing in 1 Da only. Each
individual signal can be assigned to the superposition of the
isotope pattern of polymer chains with a similar degree of
polymerization, DP = m + n, yet different ratios of the AN to
BD contents, m/n. 1H NMR analysis of the tetrazole-
functionalized NBR 4b is provided in Figure 1A and further
confirms the formation of functionalized chains. The multiplet
resonances between 8.4 and 7.1 ppm are assigned to the
aromatic diaryl tetrazole chain-end protons a−e of the polymer.
Moreover, the signals of the methylene protons in α-position to
In previous studies on photo-induced tetrazole−ene
coupling, irradiation of tetrazoles to induce nitrogen release
was performed at wavelengths ranging from 254 to 365
nm.35−38,40−47 To choose an appropriate irradiation wavelength
for our experiments, a UV absorption spectrum of the tetrazole
functional chain transfer agent 3 was recorded. As depicted in
Figure S9, the UV absorption spectrum of RAFT agent 3 (solid
line) features a broad absorption band exhibiting its maximum
absorption at 286 nm and is similar to the UV absorption
spectrum of tetrazole functional NBR, exemplarily provided for
NBR 4a (dashed line). Nevertheless, the absorption of 3 (and
4) is the result of the superposition of the absorption profiles of
both, the tetrazole R-group and the trithiocarbonate chain-end.
Contributions of the two individual components were
investigated with model compounds possessing either the
trithiocarbonate or the diaryl tetrazole feature. The latter is
represented by 3-hydroxypropyl 4-(2-phenyl-2H-tetrazol-5-
yl)benzoate (9, see Figure S10), obtained from tetrazole 2 by
esterification with 1,3-propanediol, and exhibits an absorption
maximum of 275 nm (dotted line). The esterified compound
was used instead of 2 to preclude a batho- or hypsochrome
influence of the carboxylate on the light absorption properties
of the chromophore. Absorption of the trithiocarbonate is
represented by ethyl 2-(((dodecylthio)carbonothioyl)thio)-
propanoate (8, see Figure S10). The compound possesses a
propanoate ester moiety and a dodecyl chain in vicinity to the
trithiocarbonateidentical to the direct environment of the
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Figure 2. SEC traces of NBR coupling via the NITEC approach. SEC traces of coupled NBRs 6 are represented by solid lines, traces of tetrazole
functional polymers 4 are depicted as dashed lines. (A) Coupling of NBR 4b of 1800 g·mol−1 giving NBR 6b of 3200 g·mol−1, (B) coupling of NBR
4e of 9400 g·mol−1 giving NBR 6e of 17 000 g·mol−1, and (C) coupling of NBR 4f of 38 000 g·mol−1 giving NBR 6f of 48 000 g·mol−1. SEC data
was evaluated employing the MHKS parameters of NBR.53
trithiocarbonate of controlling agent 3and shows its
maximum absorption at 305 nm assigned to the π−π*
transition of the trithiocarbonate (dash-dotted line). The
polymerization via the RAFT process leads to an alteration of
the direct environment of the trithiocarbonate, since the
incorporation of monomers occurs in between the sulfur and
the tertiary propanoate carbon after homolytic dissociation of
the C−S single bond. Skrabania et al. reported a strong
influence of the substitution patterns on the π−π* and n−π*
transition of trithocarbonates.62,63 Nevertheless, since 3 and 4a
do not show a significant disparity in their absorption, insights
obtained from the trithiocarbonate 8 can be used to describe
the absorption of the tetrazole-functionalized polymers 4a−f.
Supported by these results, in the current work nitrogen release
from the tetrazoles to allow ligation of the nitrile rubbers is
triggered by irradiation with a UV source of λmax = 254 nm to
facilitate absorption of the tetrazole while minimizing the
interference with the trithiocarbonate.
4.9 ppm (Figure S12−S13). The conjugation further induces a
shifting of the aromatic resonances a−c and f in comparison to
those of tetrazole 3. Moreover, resonances of the propanoate
protons k and j in vicinity to the trithiocarbonate indicate that
coupling is complete before decomposition of the trithiocar-
bonate occurs. However, prolonged irradiation is resulting in
photobleaching of 10. 1H NMR spectra after irradiation of 3 in
the presence of linker 5 for 15, 60, and 180 min is provided in
Figure S14. While pure 10 is obtained after 15 and 60 min of
irradiation, several aromatic signals are observed after 180 min
of irradiation and indicate a decomposition of the pyrazoline
coupling moiety.
In analogy to the UV-induced coupling of tetrazole
functional controlling agent 3 giving the model compound
10, NBRs 4 were subjected to UV irradiation in the presence of
0.5 equiv of linker 5. In contrast to the small molecule
couplings described above, a prolonged irradiation time of 180
min was necessary to obtain the coupled polymers. The
necessity of prolonged reaction time is hypothesized to be
caused by a deceleration of the cyclization step of the in situ
formed nitrile imine and the maleimide. Cyclization of the
polymeric nitrile imine with the maleimide is diffusion
controlled and determines reaction rates, since polymer
unfolding and the convergence of the nitrile imine and the
maleimide functionality is a prerequisite for the cyclization. In
contrast, the UV-induced nitrogen release does not require the
vicinity of the reactants and therefore may not exhibit a
dependency on the dimension of the reactants.
The behavior of trithiocarbonate RAFT agents under UV
irradiation has been subject to intensive investigations. Driven
by the search for UV initiated controlled polymerization
strategies, the radical decomposition pathways of trithiocar-
bonates have been elucidated.64−66 In line with these
experiments, the irradiation of trithiocarbonate 8 with UV
light of 254 nm leads to a complete decomposition within 3 h.
As depicted in Figure S11, the decomposition is evidenced in
1H NMR spectroscopy by the loss of the propanoate methin
and methyl proton resonances c and d at 4.8 and 1.6 ppm,
respectively. Nevertheless, the UV induced coupling of the pure
controlling agent 3 upon addition of 0.5 equiv of 5
quantitatively gave the respective pyrazoline 10 within 15
min without observing any decomposition of the trithiocar-
The SEC traces of the tetrazole-functionalized NBR
precursors before and after the UV induced ligation are
depicted in Figure 2. Traces of the coupled NBRs 6 (solid
lines) exhibit a distinct shift to lower retention time and thus
higher molecular weight when compared to those of the
respective tetrazole-functionalized NBRs 4 (dashed lines). The
SEC traces of the coupling of NBR 4b is provided in Figure 2A.
1
bonate. Formation of 10 is evidenced via H NMR spectros-
copy by the formation of multiplets of the methin protons d
and e in α-position to the imide carbonyls, resonating at 5.2 and
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Figure 3. (A) Illustration of the UV absorption spectrum (dash-dotted line) and the fluorescence emission spectra at λexc of 254 nm (dotted line),
300 nm (dashed line) and 384 nm (solid line) of the model compound 10 coupled via NITEC. (B) Fluorescence emission spectra at λexc of 254 nm
of tetrazole functional polymer 4d in presence of 0.5 equiv of linker 5 measured at various irradiation times between the mixing of the starting
materials (blue line) and 70 min of irradiation (red line). (C) Evolution of the FI with irradiation time at an emission wavelength λem of 497 nm and
an excitation wavelength λexc of 254 nm.
The peak retention time shifts from 23.8 to 22.8 min and
accounts for an increase of the molar mass from 1800 g·mol−1
to 3200 g·mol−1 (for details see Table S2, entry 1). The
discrepancy of the theoretically expected molar mass Mnexp(6b)
of 3900 g·mol−1 (calculated from the sum of 2 × Mn(4b) +
M(5)) and the experimentally observed molar mass can in part
be explained with the commonly observed inaccuracy of molar
masses in the low molecular weight region when calculated via
universal calibration.67 Nevertheless, the monomodal distribu-
tion of 6b exhibits a slight tailing on the low molecular weight
side. The tailing is a result of the presence of the
nonfunctionalized polymer chains 7 unable to undergo
conjugation.
maxima around 250 nm (I), 300 nm (II), and 380 nm (III).
The fluorescence emission spectra of 10 exhibit increasing
relative fluorescence intensities (FI) at excitation wavelengths
λexc of 250 nm (dotted line), 300 nm (dashed line) and 384 nm
(solid line) in the order of II < I < III and allow the assignment
of the absorption maxima I and III to the fluorescent
chromophore. Absorption at 300 nm is caused by the
trithiocarbonate moiety yet the occurrence of fluorescence
most probably arises from the spectral overlap with absorption
bands I and III. The feature at 500 nm observed in the emission
spectrum at a λexc of 250 nm is a result of higher order
transmissions.69 Fluorescence emission spectra of the coupled
NBRs 6 (not shown) exhibit features identical to 10, proving
the diaryl pyrazolines to be the moiety responsible for
polymer−polymer coupling. The coincidence of the absorption
wavelength of the fluorescent chromophore moiety with the
wavelength employed for inducing the tetrazole−ene coupling
of NBR building blocks, allows an in situ tracking of the
cyclization progress with common fluorescence spectrometer
equipment. Figure 3B shows the evolution of the fluorescence
emission of a 28 μM solution of polymer 4d in acetonitrile in
the presence of 0.5 equiv of the maleimide linker 5 over the first
70 min of the reaction, excited with a λexc of 254 nm. A steady
increase of the FI is observed, yet the increase slows down with
the progress of the reaction. A plot of the FI of the emission
spectra at an emission wavelength λem of 497 nm versus time,
provided in Figure 3C, illustrates the plateauing of the FI at
approximately 150 min of irradiation time. The time-dependent
tracking of the reaction progress clearly evidence the necessity
to perform the irradiation of polymeric tetrazoles on a longer
time scale than required for small molecular analogues.
Coupling of NBR 4e of 9400 g·mol−1, depicted in Figure 2B
provides NBR 6e of 17 000 g·mol−1 (Table S2, entry 2).
Irradiation of NBR 4f in the presence of 5 yields coupled NBR
6f of 48 000 g·mol−1 (Figure 2C), exhibiting a dispersitiy of 1.6
(Table S2, entry 3). In analogy to the coupling of 4b and 4e,
here a decrease in dispersity is not observed during conjugation.
Since polymer coupling is a convolution procedure, in an ideal
ligation reaction a doubling of molar mass along with a decrease
in dispersity is expected.68 However, simulations of RAFT-
mediated copolymerizations of AN and BD theoretically
confirmed the formation of up to 7 weight-% of non-
functionalized polymers during polymerization.50 These poly-
mer species do not possess the targeted RAFT R-group
functionality and lead to molar masses of the coupled polymers
below the theoretically expected values, even in cases where a
full conversion of the functional chain-ends occurs. As a
consequence, a broadening of the molecular weight distribu-
tions is observed when nonfunctionalized polymer species are
present in polymer−polymer coupling reactions.
1H NMR spectroscopy of coupled polymer 6b is provided
exemplarily in Figure 1B and exhibits resonances of the
pyrazoline methin protons h and i in α-position to the
carbonyls of the imide, resonating at 5.2 and 4.9 ppm and in
part overlapping with resonances of the NBR backbone olefins
(see inset). In analogy to the small molecule studies,
Since the NITEC is a pro-fluorescent reaction, fluorescence
spectroscopy allows the investigation of the polymer coupling
reactions. A UV−vis absorption spectrum of the fluorescent
model compound 10 is provided in Figure 3A as a dashed-
dotted line and reveals the presence of three local absorption
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transformation of the diaryl tetrazole 4b into the pyrazoline
structure of 6b induces a shift of the protons of the aryl
substituents from 8.3, 8.2, 7.6, and 7.5 ppm (observed in the
spectrum of the tetrazole functional polymer 4b, Figure 1A,
signals a−e) to 8.1, 7.6, 7.3, and 7.0 ppm (Figure 1B, signals j−
n). Full conversion of the tetrazole functional chain-ends is
evidenced by the absence of the resonances a−e in the
spectrum of the coupled NBR 6b. The small resonances in the
aromatic region are traced back to bleaching effects as described
for the conjugation model reaction of tetrazole functional
controlling agent 3 (vide supra). The measurements confirm
that the ligation of the tetrazole-functional NBRs exclusively
occurs with the reactive maleimide linker, without attacking
olefins within the polymer backbone.
ACKNOWLEDGMENTS
■
The authors thank Lanxess Deutschland GmbH for supporting
the current project and the excellent collaboration. C.B.-K.
acknowledges additional funding from the Karlsruhe Institute
of Technology (KIT) via its Excellence and Helmholtz
programs as well as the German Research Council (DFG)
and the Ministry of Arts and Science of the State of Baden-
Wurttemberg via their large equipment schemes. The authors
̈
thank Astrid Hirschbiel (KIT) for measuring the SEC ESI−MS
spectra. Lukas Stolzer (KIT) is acknowledged for his help with
the fluorescence measurements, while we are grateful to Dr.
Thomas Wolf (KIT) for useful discussions reagarding the DFT
calculations.
ABBREVIATIONS
CONCLUSION
■
■
AN, Acrylonitrile; BD, 1,3-butadiene; NITEC, nitrile imine
mediated tetrazole−ene coupling; NBR, nitrile−butadiene
rubber; RAFT, reversible addition−fragmentation chain trans-
fer; FI, fluorescene intensity
In conclusion, the nitrile imine mediated tetrazole−ene
coupling was demonstrated to be a fast and efficient method
for the coupling of industrially relevant NBR building blocks to
obtain nitrile rubber of high molecular weight. The method
provided is an extraordinarily pure example of NBR
conjugation since it is free of catalysts, additives or chemical
stimuli and a postpolymerization transformation of the
preformed polymer building blocks is not required prior to
ligation. An appropriate choice of the dipolarophile linker and
the nitrile imine precursor allowed the NITEC reaction to
selectively proceed with the olefin linker. A reaction of the in
situ formed enophile with the double bonds or the nitrile
moieties of the incorporated monomer units within the
polymer backbonepresent in high excess relative to the
dipolarophile linker moleculewas not observed. Underpinned
by DFT calculations, the selectivity was explained by the
reduced LUMO energy level of the maleimide linker compared
to the nonactivated backbone olefins when employing a nitrile
imine of moderate reactivity. An in-depth analysis of the
coupled polymers revealed the maleimide derived fluorescent
pyrazoline to be the element responsible for polymer−polymer
coupling. The presented method is not limited to the coupling
of linear NBR chains. By alteration of the architecture of the
maleimide, block and starpolymers and polymer brushes can be
accessed. In addition, the molecular weight, crucial for the
performance of rubber products, may be further increased by
utilizing α,ω-tetrazole bisfunctional NBR building blocks in a
photo-induced step-growth polymerization process. Moreover,
the modification of surfaces with maleimide might allow for the
immobilization of NBR on solids opening up a wide field of
technical applications.
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