Synthesis of pyridine- and 2-oxazoline-functionalized vinyl polymers by alane-based frustrated lewis pairs

Article abstract of DOI:10.1055/s-0033-1341248

Reported herein is the first example of polymerization of polar vinyl monomers bearing the C=C-C=N functionality by Frustrated Lewis pairs (FLPs). In particular, FLPs based on Al(C₆F₅)₃ and N-heterocyclic carbenes rapidly

Full text of DOI:10.1055/s-0033-1341248

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Synthesis of Pyridine- and 2-Oxazoline-Functionalized Vinyl Polymers by
Alane-Based Frustrated Lewis Pairs
FLPs for N-Functionalized Vinyl Polymer Synthesis
Jianghua He,a Yuetao Zhang,a,b Eugene Y.-X. Chen*a
a
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA
Fax +1(970)4911801; E-mail: eugene.chen@colostate.edu
b
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. of China
Received: 16.02.2014; Accepted after revision: 04.04.2014
FLPs into medium to high molecular weight, pyridine-
Abstract: Reported herein is the first example of polymerization of
and 2-oxazoline-functionalized vinyl polymers.
polar vinyl monomers bearing the C=C–C=N functionality by Frus-
trated Lewis pairs (FLPs). In particular, FLPs based on Al(C6F5)3
and N-heterocyclic carbenes rapidly convert 2-vinyl pyridine and 2-
isopropenyl-2-oxazoline into medium to high molecular weight, N-
functionalized vinyl polymers. Activated monomer-alane adduct 1
and initiated zwitterionic intermediate 2 have been isolated and
structurally characterized, providing strong evidence for the pro-
posed bimolecular, activated monomer polymerization mechanism.
As a dual-functional monomer (M), iPOx has been exten-
sively investigated through cationic ring-opening polym-
erization of the oxazoline moiety to yield polymers
exhibiting a variety of application in biomedicine and life
sciences.8 On the other hand, vinyl addition (co)polymer-
ization of iPOx by free radical,9 anionic,10 and metal-me-
diated coordination11 polymerization methods leads to the
formation of a vinyl polymer, poly(2-isopropenyl-2-oxa-
zoline) (PiPOx), with the pendant 2-oxazoline ring being
connected to a vinyl polymer backbone, which provides a
versatile venue for post-polymerization functionalization.
Like iPOx, 2-VP also contains a C=C–C=N functionality,
which could be polymerized through a conjugated addi-
tion across the C=C double bond and the imine C=N bond.
Indeed, controlled polymerization of 2-VP has been
achieved.11,12 However, to date there has been no report on
the polymerization of 2-VP and iPOx by FLPs. Accord-
ingly, we disclose herein our results on the polymerization
of 2-VP, iPOx, and 4-methyl-2-isopropenyl-2-oxazoline
(MiPOx) by FLPs and CLPs consisting of LAs [including
Al(C6F5)3, B(C6F5)3, MeAl(BHT)2 (BHT = 4-Me-2,6-
t-Bu2C6H3O)13 and AlEt3] and LBs [including phosphines
t-Bu3P, Mes3P (Mes = 2,4,6-Me3C6H2) and Ph3P], and
N-heterocyclic carbene (NHC) bases [including 1,3-di-
tert-butylimidazol-2-ylidene (It-Bu), 1,3-di(2,4,6-tri-
methylphenyl)imidazol-2-ylidene (IMes), and 1,3,4-tri-
phenyl-4,5-dihydro-1H-1,2,4-trizazol-5-ylidene (TPT)]
(Scheme 1). The fundamental difference between the pre-
vious work and the current work is that the previously re-
Key words: frustrated Lewis pairs, polymers, pyridines, nitrogen,
monomers, polymerization
Much of the emerging ‘frustrated Lewis pair’ (FLP)
chemistry1 has focused on small molecules, on their acti-
vation, on stoichiometric or catalytic transformations, and
on new reactivity or novel reaction development.2 A new
direction has been to veer toward polymerization catalysis
mediated by Lewis pairs (LPs), either FLPs or classical
Lewis pairs (CLPs), for the synthesis of macromolecules
as useful polymeric materials.3 In this type of polymeriza-
tion, the Lewis acid (LA) and Lewis base (LB) of a LP
work cooperatively to activate the monomer substrate and
participate in chain initiation or chain propagation and ter-
mination/transfer events. We recently reported the chain-
growth addition polymerization of conjugated polar al-
kenes, such as linear and cyclic acrylic monomers, partic-
ularly methacrylates, acrylamides, and biorenewable α-
methylene-γ-butyrolactones, by bulky, strongly Lewis
acidic Al(C6F5)3-based FLPs.4 Bourissou et al. recently
communicated the ring-opening (co)polymerization of
heterocyclic monomers such as lactide and lactones by
Zn(C6F5)2-based LPs.5 Erker et al. described the radical
polymerization of styrene mediated by the persistent FLP-
NO aminoxyl radical derived from N,N-cycloaddition of
a cyclohexylene-bridged intramolecular P→B FLP to ni-
tric oxide.6 We have reported the addition polymerization
of biorenewable α-methylene-γ-butyrolactones by borane
B(C6F5)2-based intramolecular, interacting FLPs and
B(C6F5)3-based CLPs.7 Herein we communicate the first
example of polymerization of 2-vinyl pyridine (2-VP) and
2-isopropenyl-2-oxazoline (iPOx) by Al(C6F5)3-based
n
n
LA/LB
N
N
N
O
N
O
or
or
R
R
R = H, iPOx
2-VP
R = Me, MiPOx
LA:
LB:
Al(C6F5)3, B(C6F5)3, MeAl(BHT)2, AlEt3
t-Bu3P, Mes3P, Ph3P
Ph
N
N
N
N
N
N
N
Mes
Ph
Mes
Ph
It-Bu
IMes
TPT
SYNLETT 2014, 25, 1534–1538
Advanced online publication: 28.04.2014
DOI: 10.1055/s-0033-1341248; Art ID: st-2014-b0127-c
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
Scheme 1 N-Functionalized monomers as well as LAs and LBs,
which construct FLPs or CLPs, employed in the current study

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FLPs for N-Functionalized Vinyl Polymer Synthesis
1535
ported FLP polymerization occurs through the conjugate very effective for such polymerizations, the results of
addition across the C=C–C=O functionality, while the which are summarized in Table 1. A typical polymeriza-
present work utilizes FLPs to promote conjugate addition tion procedure involved premixing of the LA with mono-
across the C=C–C=N functionality.
mer, followed by addition of the LB solution to start the
polymerization, but the monomer can also be added last.14
Control runs for the polymerization of 2-VP and iPOx by
all the individual LAs and LBs listed in Scheme 1 in tolu- Polymerization of 2-VP (200 equivalents) by
ene, CH2Cl2, or DMF, with a [M]/[LA] or [M]/[LB] ratio 2Al(C6F5)3/It-Bu (or 0.5 mol% FLP loading) in toluene
of 200 at room temperature (ca. 25 °C) inside an inert was rapid at room temperature, achieving 96% monomer
glovebox, afforded no monomer conversion after up to 24 conversation in 20 minutes (run 1), which corresponds to
hours. Furthermore, in contrast to the polymerization of a turn-over frequency (TOF) of 580 h–1. The resulting
conjugated polar vinyl monomers bearing the C=C–C=O polymer, P(2-VP), exhibited a Mn of 8.02 × 104 g/mol and
functionality, FLPs based on a combination of Al(C6F5)3 a molecular weight distribution of Đ = 1.60. The calculat-
(2 equiv) and a phosphine (t-Bu3P, Mes3P, or Ph3P) are in- ed initiator efficiency [I* = Mn (calcd)/Mn (exptl), where
active for polymerization of 2-VP and iPOx bearing the Mn (calcd) = MW(M) × [M]/[FLP] × conversion (%) +
C=C–C=N functionality, regardless of the mixing se- MW (chain-end groups)] was only 26%. This polymeriza-
quence, highlighting the large difference in the reactivity tion was significantly accelerated by increasing the
of these two classes of conjugated polar alkenes towards amount of the LA added, indicative of the activated-
FLPs or CLPs. The inability of phosphine nucleophiles to monomer propagation for the FLP-mediated polymeriza-
initiate this polymerization is due to their inability to per- tion. Thus, the TOF was increased to 790 h–1 (run 2), 1050
form the formal Michael addition reaction between a h–1 (run 3), and 1490 h–1 (run 4), when the amount of the
phosphine such as t-Bu3P and a monomer–LA adduct LA was increased to three, four, and five equivalents, re-
such as (2-VP)·Al(C6F5)3, as shown by NMR experi- spectively. The Mn of the resulting polymer was actually
ments. On the other hand, FLPs based on an Al(C6F5)3 decreased, attributed to the increased initiator efficiency
(two equivalents)–NHC combination were found to be in the presence of a higher concentration of the LA (e.g.,
Table 1 Selected Results of Polymerization of 2-VP and iPOx by Al(C6F5)3/Base FLPsa
Run
M
LA
LB
[LA]/[LB] [M]/[LB]
Time (min) Conversion (%)b Mn (kg/mol)c Đ (Mw/Mn)c
1
2
2-VP
2-VP
2-VP
2-VP
2-VP
2-VP
2-VP
2-VP
2-VP
2-VP
2-VP
2-VP
2-VP
iPOx
iPOx
iPOx
iPOx
Al(C6F5)3
It-Bu
It-Bu
It-Bu
It-Bu
It-Bu
IMes
It-Bu
It-Bu
It-Bu
It-Bu
It-Bu
It-Bu
It-Bu
It-Bu
IMes
IMes
IMes
2:1
3:1
4:1
5:1
2:1
2:1
2:1
2:1
2:1
2:1
3:1
4:1
5:1
2:1
2:1
2:1
2:1
200
200
200
200
1200
200
200
200
200
200
200
200
200
200
200
400
200
20
15
96.3
98.6
95.9
99.2
80.2d
49.2d
44.3d
38.4
315
10.3
48.9
–
1.60
1.67
1.70
1.59
2.04
4.71
5.39
–
Al(C6F5)3
3
Al(C6F5)3
11
4
Al(C6F5)3
8
5
Al(C6F5)3
1440
1440
1440
1440
1440
25
100
6
Al(C6F5)3
100
100
0
7
MeAl(BHT)2
AlEt3
8
9
B(C6F5)3
0
–
–
10
11
12
13
14
15
16
17
M·Al(C6F5)3
M·Al(C6F5)3
M·Al(C6F5)3
M·Al(C6F5)3
M·Al(C6F5)3
M·Al(C6F5)3
M·Al(C6F5)3
B(C6F5)3
98.5
74.5
54.8d
49.4d
45.3d
73.7
15.0
14.9
–
1.55
1.60
1.58
1.59
2.98
2.90
3.24
–
15
96.4
94.8
97.4
81.4
11
10
1440
1440
1440
1440
>99
>99
0
a Reaction conditions: reactions were carried out at ambient temperature (ca. 25 °C) in toluene with 5 mL of the total solution volume. The alane
LA was used as an adduct of toluene, (toluene)0.5·Al(C6F5)3, or the monomer, M·Al(C6F5)3.
b Monomer conversion measured by 1H NMR spectroscopy with an internal standard.
c Number-average molecular weight (Mn) and molecular weight distribution (Đ) determined by GPC in DMF relative to PMMA standards.
d A minor (0.1–0.7%) high MW shoulder peak was present.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1534–1538

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J. He et al.
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I* = 55%, run 4). On the other hand, employing a high tween iPOx and MiPOx towards FLPs can be attributed to
[M]/[FLP] ratio of 1200 afforded a high molecular-weight the steric hindrance at the nitrogen atoms and the electron-
polymer with Mn = 3.15 × 105 g/mol and Đ = 2.04 (run 5). ic richness of MiPOx relative to iPOx, thus rendering
Replacing It-Bu with IMes yielded a much less active sys- MiPOx more resistant toward nucleophile attack.
tem (run 6 vs. run 1), now requiring 24 hours to achieve
To probe a possible chain initiation and propagation
quantitative monomer conversion, whereas TPT was
mechanism, we preformed the alane monomer adduct, (2-
completely inactive. Substituting the strong Lewis acid
VP)·Al(C6F5)3 (1), and structurally characterized it by sin-
gle-crystal X-ray diffraction analysis. The structure fea-
Al(C6F5)3 with MeAl(BHT)2 also resulted in a much less
active polymerization system (run 7 vs. run 1). More dras-
tures strong activation of 2-VP by the LA Al(C6F5)3
tically, replacing Al(C6F5)3 with B(C6F5)3 or AlEt3 led to
through N-coordination of the monomer to the Al center
a completely inactive polymerization system (runs 8 and
of the LA [Al–N = 1.987(1) Å, Figure 1].16 Next,
9). Polymerizations employing the preformed monomer-
addition of the base It-Bu to adduct 1 cleanly formed a
zwitterionic species, imidazolium pyridylaluminate
It-BuCH2CH=(C5H4N)Al(C6F5)3 (2), which has also been
LA adduct, (2-VP)·Al(C6F5)3, afforded the results similar
to those by the toluene adduct, (toluene)0.5·Al(C6F5)3
(runs 10–13 vs. runs 1–14). Also noteworthy is that, when
structurally characterized by single-crystal X-ray diffrac-
carried out in CH2Cl2 or DMF, the above polymerizations
tion analysis (Figure 2).17 As a consequence of conjugate
were much less effective or completely shut down, attrib-
addition of It-Bu to adduct 1, now a double bond is formed
15
utable to rapid deactivation of Al(C6F5)3 in CH2Cl2 or
between C(5) and C(6) in 2 with a bond length of 1.359(4)
impeding of the monomer activation by virtue of strong
Å, whereas a single bond is formed between C(6) and
coordination of DMF to Al(C6F5)3.
N(3) on the pyridyl ring with a bond length of 1.425(3) Å
The Al(C6F5)3/It-Bu FLP is also active for polymerization [which is comparable to a double-bond-like distance of
of iPOx, but the polymerization activity was much lower N(1)–C(1) = 1.366(2) Å in adduct 1].
than that for the polymerization of 2-VP, with a low TOF
of ca. 7 h–1 (run 14). The polymer produced had a medium
high molecular weight of Mn = 7.37 × 104 g/mol and a rel-
atively broad molecular weight distribution of Đ = 2.98.
Replacing It-Bu with IMes enabled quantitative monomer
conversion for the polymerization of either 200 or 400
equivalents of iPOx (runs 15 and 16). As in the case of the
polymerization of 2-VP, the use of B(C6F5)3, in place of
Al(C6F5)3, yielded a completely inactive system (run 17).
Interestingly, all the FLPs investigated herein failed to po-
lymerize MiPOx. This sharp contrast in reactivity be-
Figure 2 X-ray structure of zwitterionic intermediate 2·CH2Cl2 de-
rived from the activation of 2-VP by the Al(C6F5)3/It-Bu FLP. Hydro-
gen atoms and CH2Cl2 have been omitted for clarity and ellipsoids
drawn at 50% probability. Selected bond lengths [Å] and angles [º]:
Al(1)–N(3) 1.906(2), Al(1)–C(31) 2.011(3), Al(1)–C(19) 2.010(3),
Al(1)–C(25) 2.027(3), C(6)–C(5) 1.359(4), C(5)–C(4) 1.521(4),
C(4)–C(1) 1.500(4), N(3)–C(6) 1.425(3), N(3)–Al(1)–C(31)
114.85(11)°, N(3)–Al(1)–C(19) 105.02(11)°, C(31)–Al(1)–C(19)
113.21(11)°, N(3)–Al(1)–C(25) 109.59(11)°, C(31)–Al(1)–C(25)
100.09(11)°, C(19)–Al(1)–C(25) 114.36(11)°.
To test whether zwitterionic species 2 itself is active for
the polymerization of 2-VP, we added an excess amount
of the monomer 2-VP (200 equivalents) to a solution of 2,
but we observed no polymer formation after up to 24
Figure 1 X-ray structure of (2-VP)·Al(C6F5)3 (1). Hydrogen atoms
hours. On the other hand, addition of a small amount of
the LA Al(C6F5)3 (0.2 equivalents) to the abovementioned
solution immediately started the polymerization and com-
pletely converted the monomer into polymer. Switching
to the IMes base, the Al(C6F5)3/IMes-based zwitterionic
intermediate, IMes-CH2CH=(C5H4N)Al(C6F5)3 (3), was
have been omitted for clarity and ellipsoids drawn at 50% probability.
Selected bond lengths [Å] and angles [º]: Al(1)–N(1) 1.9873(11),
Al(1)–C(14) 1.9894(13), Al(1)–C(8) 1.9971(13), Al(1)–C(20)
2.0010(13), N(1)–C(1) 1.3656(17), N(1)–Al(1)–C(14) 110.62(5)°,
N(1)–Al(1)–C(8) 103.36(5)°, C(14)–Al(1)–C(8) 113.91(6)°, N(1)–
Al(1)–C(20) 105.96(5)°, C(14)–Al(1)–C(20) 107.99(5)°, C(8)–
Al(1)–C(20) 114.63(6)°.
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FLPs for N-Functionalized Vinyl Polymer Synthesis
1537
generated in the same fashion, which was found to behave
similarly to zwitterion 2 in the above mechanistic probing
reaction sequences. Overall, these results showed that an
excess amount of the LA is required, in addition to the
zwitterionic intermediate, to generate an effective polym-
erization, which is consistent with the bimolecular, acti-
vated monomer propagation mechanism previously
proposed for the conjugate-addition polymerization by
LPs.4 In this specific case, a propagating step is proposed
to involve nucleophilic attack of the activated monomer
(i.e., adduct 1) by zwitterion 2 and its homologues; the LA
attached to the antepenultimate monomer unit after each
addition step is recaptured by the incoming monomer, and
the repeated conjugate addition in such a propagating cy-
cle produces the high molecular-weight polymer (Scheme
2).
References and Notes
(1) For selected reviews, see: (a) Frustrated Lewis Pairs I & II,
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D. W.; Erker, G., Eds.; Springer: New York, 2013. (b) Erker,
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Bannwarth, C.; Garon, C. N.; Caputo, C. B.; Grimme, S.;
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(b) Ménard, G.; Hatnean, J. A.; Cowley, H. J.; Lough, A. J.;
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N
N
LB
LA
Al(C6F5)3 (LA)
N
N
LA
It-Bu (LB)
N
(3) Chen, E. Y.-X. Top. Curr. Chem. 2013, 334, 239.
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Y.-X. Angew. Chem. Int. Ed. 2010, 49, 10158.
2
N
N
N
2-VP
n
LA
LA
N
LA
(5) Piedra-Arroni, E.; Ladavière, C.; Amgoune, A.; Bourissou,
D. J. Am. Chem. Soc. 2013, 135, 13306.
N
N
N
N
(6) Sajid, M.; Stute, A.; Cardenas, A. J. P.; Culotta, B. J.;
Hepperle, J. A. M.; Warren, T. H.; Schirmer, B.; Grimme, S.;
Studer, A.; Daniliuc, C. G.; Fröhlich, R.; Petersen, J. L.;
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1
2
Scheme 2 Proposed bimolecular, activated monomer propagation
mechanism for the polymerization of 2-VP by the Al(C6F5)3-based
FLP
In summary, FLPs based on Al(C6F5)3 and NHCs have
been found to effectively polymerize 2-VP and iPOx to
medium to high molecular-weight, N-functionalized vinyl
polymers, representing the first example of Lewis pair po-
lymerization of conjugated polar vinyl monomers carry-
ing the C=C–C=N functionality. Preliminary mechanistic
efforts have isolated and structurally characterized the ac-
tivated monomer-LA adduct 1 and the initiated zwitter-
ionic intermediate 2, and the results obtained to date point
to a bimolecular, activated monomer polymerization
mechanism.
Acknowledgment
(9) (a) Weber, C.; Neuwirth, T.; Kempe, K.; Ozkahraman, B.;
Tamahkar, E.; Mert, H.; Becer, C. R.; Schubert, U. S.
Macromolecules 2012, 45, 20. (b) Zhang, N.; Huber, S.;
Schulz, A.; Luxenhofer, R.; Jordan, R. Macromolecules
2009, 42, 2215.
This work was supported by the US National Science Foundation
(CHE-1150792). We thank Boulder Scientific Co. for the generous
donation of B(C6F5)3 for research.
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661.
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Chem. Soc. 2013, 135, 8810.
(12) Kaneko, H.; Nagae, H.; Tsurugi, H.; Mashima, K. J. Am.
Chem. Soc. 2011, 133, 19626.
Supporting Information for this article is available online
at
http://www.thieme-connect.com/products/ejournals/journal/
10.1055/s-00000083.SunpfgIpi
o
o
nr
i
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1534–1538

1538
J. He et al.
CLUSTER
(13) Inoue and co-workers first reported the concept of
acceleration in anionic polymerization of MMA via steric
separation of the nucleophile and the bulky aluminum Lewis
acid. See: Kuroki, M.; Watanabe, T.; Aida, T.; Inoue, S.
J. Am. Chem. Soc. 1991, 113, 6903.
(14) See the Supporting Information for experimental and
characterization details.
(15) Chakraborty, D.; Chen, E. Y.-X. Inorg. Chem. Commun.
2002, 5, 698.
2364.84(15) Å3, Z = 4, Dcalcd = 1.779 Mg/m3, GOF = 1.051,
R1 = 0.0243 [I >2σ(I)], wR2 = 0.0659. CCDC 972837
contains the supplementary crystallographic data for this
structure.
(17) Selected crystallographic data for 2·CH2Cl2:
C37H29AlCl2F15N3, monoclinic, space group P2(1)/c, a =
11.6507(7) Å, b = 16.8033(10) Å, c = 19.1096(12) Å, β =
94.747(3)°, V = 3728.3 (4) Å3, Z = 4, Dcalcd = 1.601 Mg/m3,
GOF = 1.060, R1 = 0.0512 [I >2σ(I)], wR2 = 0.1482. CCDC
972835 contains the supplementary crystallographic data for
this structure.
(16) Selected crystallographic data for 1: C25H7AlF15N,
monoclinic, space group P2(1)/c, a = 10.6644(4) Å, b =
14.0645(5) Å, c = 16.5182(6) Å, β = 107.349(2)°, V =
Synlett 2014, 25, 1534–1538
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