Bicyclic guanidinium tetraphenylborate: A photobase generator and a photocatalyst for living anionic ring-opening polymerization and cross-linking of polymeric materials containing ester and hydroxy groups

Article abstract of DOI:10.1021/ja802816g

,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) with pK_a of 26.03 in acetonitrile can be effectively released by photolysis of TBD?HBPh₄ salt, which represents a new family of short-wave UV photobase generators. This photobase generator enables the photoinduced living ring-opening polymerization of cyclic esters and the photoinduced cross-linking of various polymeric materials containing the hydroxyl-ester groups. Copyright

Full text of DOI:10.1021/ja802816g

Published on Web 06/04/2008
Bicyclic Guanidinium Tetraphenylborate: A Photobase Generator and A
Photocatalyst for Living Anionic Ring-Opening Polymerization and
Cross-Linking of Polymeric Materials Containing Ester and Hydroxy Groups
Xun Sun, Jian Ping Gao, and Zhi Yuan Wang*
Department of Chemistry, Carleton UniVersity, 1125 Colonel By DriVe, Ottawa, Ontario, Canada K1S 5B6
Received April 16, 2008; E-mail: wayne_wang@carleton.ca
Photobase generators (PBGs) are compounds capable of in situ
generating basic species by irradiation (e.g., mostly by UV-vis
irradiation). As a photocatalyst for polymer cross-linking and
modification, PBGs are widely used in coatings and microelectron-
ics industry, such as photo curing of epoxy resins and photoinduced
imidization of poly(amic acid).1 Thus far, all the reported PBGs
produce amines and amidines with pKa of 10-24 in acetonitrile
(AN).2 These photogenerated bases, including 1,8-diazabicyclo-
Figure 1. (a) UV-vis absorption spectra of TBD•HBPh4 and TBD (2.0
undec-7-ene (DBU, ANpKa ) 24.3), are not powerful enough to
× 10-5 M) in AN. (b) Changes of UV-vis spectra of Phenol Red solution
initiate a living ring-opening polymerization (ROP). In comparison,
a bicyclic guanidine, namely 1,5,7-triaza-bicyclo [4.4.0]dec-5-ene
(TBD), is a stronger base with ANpKa of 26.03 and has been proved
to be a very effective bifunctional catalyst for trans-esterification
and ROP of cyclic monomers such as δ-valerolactone (VL) and
ꢀ-caprolactone (CL). It can simultaneously activate the ester through
acyl transfer and trigger the alcohol with hydrogen bonding.3 TBD
also catalyzes the reactions between isocyanates with alcohols to
form urethanes and the Michael addition of acetoacetates to
acrylates,3d which are all suitable base-catalyzed reactions for
polymer curing. Therefore, it would be both fundamentally interest-
ing and practically useful to realize PBGs that can generate TBD
and other stronger bases for photoinduced living polymerizations
and polymer curing.
upon addition of a solution of TBD•HBPh4 being irradiated over time.
Figure 2. 1H NMR spectra (DMSO-d6) of TBD•HBPh4 (a) before and (b)
after irradiation at 254 nm with a dose of 92 × 10-6 Einstein, and as well
of (c) a mixture of Ph3B and TBD and (d) TBD.
Herein we propose the photogeneration of TBD from its
tetraphenylborate salt (TBD•HBPh4) via a photoinduced proton
transfer reaction. It is known that irradiation of sodium tetra-
phenylborate (NaBPh4) can effectively abstract the acidic protons
from other molecules, such as water and alcohols.4 By comparing
pKa values of TBD, or the acidity of the conjugated acid (TBD•H+),
with methanol and ethanol (waterpKa ) 13.6 vs 15.5-16.1),2a it is
clear that TBD•H+ is a much better proton donor than alcohols.
Therefore, TBD•HBPh4 should be photochemically reactive and
able to liberate TBD upon irradiation even in the presence of
alcohols and water.
103 M-1cm-1 at 250 nm) and this light source is readily available.6
Upon addition of irradiated solution of TBD•HBPh4 to the aqueous
solution of Phenol Red, a new band at 560 nm appeared, assigned
to the deprotonated Phenol Red after reaction with the released
base, and its intensity increased with an increase of irradiation time
(Figure 1b). In comparison, TBD or its HCl salt is inert to the
photolysis at 254 nm. The photoactivity of TBD•HBPh4 remains
unchanged for at least six months when being stored at room
temperature in the dark.
TBD•HBPh4 was obtained by simply mixing hydrochloric acid
(HCl), TBD, and NaBPh4 in water, followed by filtration and
recrystalliztion.5 It appears as white crystalline solids with a melting
point at 223 °C and begins to decompose at 243 °C. TBD•HBPh4
absorbs within the ultraviolet C region (100-280 nm), with an
intense peak at 195 nm (ꢀ195nm ) 1.26 × 105 M-1cm-1) and a
weak broad peak in 220-280 nm (ꢀ254nm ) 5.05 × 103 M-1cm-1).
To determine the structures of generated basic species, the course
of photolysis of TBD•HBPh4 with a different amount of dose was
monitored by 1H NMR spectroscopy (see Supporting Information,
SI). Protons Hd (3.26 ppm) and He (3.16 ppm, Figure 2a) adjacent
to the nitrogen atoms in TBD•H+ were gradually disappearing and
shifting to 3.08 ppm (Figure 2b) after irradiation at 254 nm. In
comparison, the same characteristic methylene protons in TBD
appeared at 3.01 ppm (Figure 2d). However, since triarylborane is
the main product in photolysis of NaBPh4,4a the in situ formation
of an ate-complex between TBD and triarylborane during the
photolysis of TBD•HBPh4 should be expected. Apparently, the ex-
situ formed ate-complex displays the same protons at 3.08 ppm
(Figure 2c), identical to the in situ formed TBD-Ph3B complex
(Figure 2b). Therefore, the NMR studies confirmed the liberation
The target base TBD absorbs weakly at 190-230 nm (ꢀ193nm
6.98 × 103 M-1cm-1) (Figure 1a).
)
The base-generating ability of TBD•HBPh4 was first tested by
irradiation of its degassed AN solution at 254 nm, followed by
analysis using Phenol Red indicator. The choice of the irradiation
wavelength was made for the fact that its absorption coefficient is
comparable to those of some widely used photoacid generators,
such as arylonium salts and arylsulfonium salts (e.g., 3.0-7.0 ×
9
8130 J. AM. CHEM. SOC. 2008, 130, 8130–8131
10.1021/ja802816g CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Proposed Mechanism for the Photo-Generation of TBD
Further utilization of TBD•HBPh4 for photoinduced polymer
cross-linking can be envisioned, based on the TBD-catalyzed trans-
esterification reaction between an ester and an alcohol. Thus, upon
addition of the irradiated AN solution of TBD•HBPh4 to a mixture
at a 1:1 (wt/wt) ratio of poly(methyl methacrylate) (PMMA) and
2-hydroxyethyl cellulose (HEC) in N,N-dimethylformamide, a gel
was formed in 30 min at room temperature, indicating polymer
cross-linking. This photoinduced polymer cross-linking can also
proceed at the interface of the two polymer films. Upon irradiation
of a thick film of HEC coated with a thin layer of PMMA containing
5 mol % TBD•HBPh4, only PMMA on the irradiated areas were
left on HEC after washing off the top layer, as evident by a large
change in the contact angle of water from 45° (for HEC) to 86°
(for PMMA) (SI, Figure S6). Furthermore, a self-photo-cross-linking
polymer system can be realized, by introducing a hydroxy-ester
moiety in polymer. One example demonstrated herein is the use of
hydroxypropyl acrylate (HPA) as a self-cross-linkable unit in the
copolymers with methyl methacrylate (MMA) and N-vinylpyrroli-
done (VP), respectively. Films of the two copolymers, P(MMA-
co-HPA) (1:1) and P(VP-co-HPA) (1:1), containing 5 mol %
TBD•HBPh4 were irradiated at 254 nm and then baked at 100 °C
for 3 min. Both polymer films became insoluble in organic solvents
due to polymer cross-linking, as a result of trans-esterification and
subsequent loss of methanol or/and 1,3-propylenediol (SI, Figure
S8). In comparison, P(MMA-co-HPA) could not be cross-linked
using 5 mol% Ph3B. Thus, polymers or blends containing hydroxyl
and ester groups can be photocross-linked with a catalytic amount
of TBD•HBPh4.
from TBD•HBPh4
of TBD from TBD•HBPh4. Furthermore, by plotting the area of
the new peak at 3.08 ppm as a function of absorbed photons, the
quantum yield (Φ) for TBD generation is calculated to be 0.18
(SI). A higher efficiency of photolysis of TBD•HBPh4 can be
expected at a shorter wavelength, due to its much higher absorption
coefficient.
To investigate the photobase-generating mechanism, the
kinetics of photolysis of TBD•HBPh4 was studied by UV-vis
spectroscopy, with reference to the known photochemistry of
NaBPh4 (SI, Figure S9).4 Both compounds show nearly identical
absorption, suggesting that BPh4- serves as a chromophore rather
than the TBD•H+ cation. By monitoring the spectral changes
during the photolysis of NaBPh4 and TBD•HBPh4 under same
conditions, the following were observed: (1) a drastic decrease
in absorbance at 200 nm and an increase in absorbance at 250
nm with the two tight isosbestic points (192 and 239 nm) for
both salts and (2) virtually identical rate constants of photodis-
sociation of 0.0368((0.002) s-1 and 0.0370((0.002) s-1 for
NaBPh4 and TBD•HBPh4, respectively (SI). The results strongly
suggest that photolysis of TBD•HBPh4 proceeds in the same
-
In conclusion, TBD•HBPh4 represents a new family of short-
wave UV PBGs and is able to generate a base 100 times more
basic than the strongest base (DBU) generated among all the
previously reported PBGs. This PBG enables the photoinduced
living ROP of cyclic esters and the photocross-linking of polymeric
materials containing the hydroxyl-ester groups.
pathway as NaBPh4. Accordingly, the excited BPh4 ion
rearranges and then abstracts a proton from the neighboring
TBD•H+ cation to release TBD (Scheme 1). The resulting
trivalent arylborane further decomposes to aromatic fragments.
Considering the higher absorption coefficients in the deep UV
region of BPh4- (Figure 1a), TBD can be photogenerated below
200 nm, for example, at 193 nm from ArF excimer laser, in a
higher quantum yield than being done at 254 nm. There are clear
advantages of using sub-200 nm light in photolithography and
other photoinduced processes.7
This photobase-generating mechanism implies that other stronger
organic bases can be converted to photobases as their HBPh4 salts.
Indeed, photolysis of t-BuP1(dma)•HBPh4 at 254 nm generated a
P1 base, t-butyliminotris(dimethylamino)phosphorane (ANpKa )
26.9), as verified by 31P NMR spectroscopy and pH indicator (SI,
Figures S11-S12).
Because TBD can still be released in the presence of alcohols
(methanol and ethanol), due to its lower pKa than alcohols,
TBD•HBPh4 can be utilized as a photocatalyst for some base-
catalyzed reactions such as the anionic ROP of cyclic esters. Thus,
bulk polymerization of CL was attempted using 1 mol % of
TBD•HBPh4 in the presence of 1 mol% of n-hexanol as an initiator.
After irradiation for 5 min at 254 nm in argon, polymerization then
proceeded for 8 h at 60 °C. Poly(CL) in a molecular weight (Mn)
of 16 537 g mol-1 or DP of 144 was obtained with ∼70% monomer
conversion. The ROP rate increased with longer irradiation time
(SI, Figure S5a), indicating the release of more base catalysts. The
living nature of this ROP is evident by the observed linear
relationship between the monomer conversion and molecular weight
of poly(CL) (SI, Figure S5b, c). No poly(CL) was formed using
NaBPh4 or Ph3B, thus eliminating their possible roles as a
photocatalyst or in situ generated Lewis catalyst for ROP. It should
also be noted that DBU generated from reported PBGs is insufficient
to trigger the ROP of CL.3b
Acknowledgment. We thank the Natural Sciences and Engi-
neering Research Council of Canada for financial support.
Supporting Information Available: Synthesis, characterization,
kinetic studies, polymerization, and cross-linking. This material is
available free of charge via the Internet at http://pubs.acs.org.
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