Chain-growth cycloaddition polymerization via a catalytic alkyne [2 + 2 + 2] cyclotrimerization reaction and its application to one-shot spontaneous block copolymerization

Article abstract of DOI:10.1021/ja203584c

A cobalt-catalyzed alkyne [2 + 2 + 2] cycloaddition reaction has been applied to polymerizations yielding linear polymers via selective cross-cyclotrimerization of yne-diyne monomers, which occurs in a chain-growth manner. Additionally, through control of the alkyne reactivity of the two monomers, this method was efficiently applied to the spontaneous block copolymerization of their mixture. Here we present the proposed mechanism of the catalyst transfer process of this cycloaddition polymerization.

Full text of DOI:10.1021/ja203584c

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Chain-Growth Cycloaddition Polymerization via a Catalytic Alkyne
[2 þ 2 þ 2] Cyclotrimerization Reaction and Its Application to
One-Shot Spontaneous Block Copolymerization
Yu-ki Sugiyama, Rei Kato, Tetsuya Sakurada, and Sentaro Okamoto*
Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
S Supporting Information
b
spontaneous block copolymerization using two monomers hav-
ABSTRACT: A cobalt-catalyzed alkyne [2 þ 2 þ 2]
ing different reactivities.
cycloaddition reaction has been applied to polymerizations
yielding linear polymers via selective cross-cyclotrimerization
of yneꢀdiyne monomers, which occurs in a chain-growth
manner. Additionally, through control of the alkyne reactivity
of the two monomers, this method was efficiently applied to
the spontaneous block copolymerization of their mixture.
Here we present the proposed mechanism of the catalyst
transfer process of this cycloaddition polymerization.
As an effective catalyst for alkyne cycloaddition reactions, we
developed dipimp/CoCl2 6H2O/Zn [dipimp = 2-(2,6-diiso-
3
propylphenyl)iminomethylpyridine], which catalyzes the cy-
cloaddition of a variety of alkynes to the corresponding sub-
stituted benzenes under mild reaction conditions.4 To apply this
catalysis to the synthesis of linear polymers, we had to control the
reactivity of the alkynes to promote selective cross-coupling
(path a in Scheme 1). The reaction of simple terminal alkynes
such as phenylacetylene and 1-hexyne yielded the corresponding
trisubstituted benzenes quantitatively: the reaction of 1,6-diynes
containing terminal alkynes proceeded along a self-cycloaddition
pathway to yield dimers and trimers [see the Supporting
Information (SI) for more details].4 In contrast, the reaction of
a 1:1 mixture of 1,6-diyne 1 and propargyl alcohol 2a or ether 2b
containing an internal alkyne proceeded smoothly to furnish the
cross-coupled product 3 quantitatively without homocoupling of
1 (Scheme 2). The internal alkyne structure prevented self-
addition, and the fact that 4-octyne (2c) failed to react indicated
the importance of the participation of the propargylic oxygen
with 2a and 2b.
ycloaddition polymerization reactions are a versatile means
C
for synthesizing unique polymers, as exemplified particularly
in recent utilizations of [4 þ 2] cycloadditions between alkynes
and dicyclopentadienones and [3 þ 2] cycloadditions of alkynes
and azides (H€uisgen reaction).1 These processes proceed using a
variety of monomer systems such as AB and A2/B2, generating
various macromolecules. Their propagation occurs in a step-
growth manner to yield polymers with relatively high polydis-
persity having uncontrolled molecular weights.
On the basis of these results, we designed yneꢀdiyne 4a as a
type-I prototype monomer, expecting it to undergo selective
cross-cycloaddition (Scheme 3). Table 1 and Figure 1 summarize
the results of cobalt-catalyzed cycloaddition reactions of yneꢀ
The alkyne [2 þ 2 þ 2] cycloaddition reaction occurring in
the presence of an appropriate transition-metal catalyst has
attracted interest as a straightforward means of synthesizing
substituted benzenes.2 In macromolecule synthesis, it has been
reported that homocycloaddition (cyclotrimerization) can poly-
merize diyne compounds such as 1,8-nonadiyne in a step-growth
manner. This process with polymerization of AB2-type mono-
mers yields highly branched macromolecules.3a,b (Homo)-
cyclotrimerization of alkynes has also been utilized for construc-
tion of benzene-core dendrimers.3c However, more controlled
polymerizations that produce linear polymers via the reaction of
AB-type monomers I comprising monoalkyne and diyne moi-
eties (Scheme 1) have not been reported to date. This is largely
because of the difficulty in directing alkyne reactivities toward
selective cross-coupling. The selective cross-coupling between
the monoalkyne and diyne parts (path a) yields linear polymers.
Alternatively, polymerization involving a self-coupling reaction
(path b and/or c) forms linear and branched structures in a
noncontrolled manner. Herein we report the first realization of
the selective formation of linear polymers from yneꢀdiyne
monomer I via a catalytic alkyne cycloaddition reaction. In
addition, polymerization proceeded in a chain-growth manner,
achieving control over the molecular weights and polydispersity
of the resultant polymers. The method was applied to one-shot
diyne 4a. For this reaction, a mixture of 4a, dipimp/CoCl2 6
3
H2O (5 mol %), and Zn powder (15 mol %) in N-methyl-
2-pyrrolidone (NMP) was stirred at 50 °C in either the presence
and absence of the alkyne additive as an activator. Samples of the
reaction mixture extracted using a syringe at the indicated times
were analyzed by gel-permeation chromatography (GPC) [eluent,
THF; calibration, polystyrene (PS) standards]. Figure 1a depicts
the results of the reaction in run 4 of Table 1.
In all cases, the catalysis produced polymers with Mn in the
range (4.0ꢀ8.2) ꢁ 103 and having a unimodal profile on GPC
charts. The presence of an alkyne additive made the polydisper-
sity (PDI = Mw/Mn) narrower than when the reaction was
performed in its absence. Thus, the addition of a catalytic amount
of MeO2CCtCCO2Me (DMAD) or triyne 5 (5ꢀ15 mol %) as
an activator led to a PDI as narrow as 1.22ꢀ1.28 (runs 2ꢀ5). As
shown in Figure 1b, plots of Mn versus conversion in the absence
of an alkyne additive were nonlinear over 50% of the conversion
(run 1; also see SI-Figures 1 and 2). In contrast, the addition of an
Received: April 19, 2011
Published: June 02, 2011
r
2011 American Chemical Society
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Scheme 1. [2 þ 2 þ 2] Cycloaddition Reactions of
YneꢀDiyne I
Scheme 2. Model Reactions for Designing Monomers
Scheme 3. Polymerization of Triyne Monomer 4a
Figure 1. (a) GPC charts of poly-4a obtained from the reaction of run 4
in Table 1. (b) Plots of Mn vs conversion for runs 1, 2 and 4. (c) Plots of
Mn (9) and PDI (b) vs conversion for run 4. (d) Plots of ln [M]0/[M]
(9) and PDI (b) vs time for run 4. (e) MALDIꢀTOF mass spectra of
poly-4 obtained by sampling from the reaction of run 4.
Table 1. Cobalt-Catalyzed Cycloaddition Polymerization of 4a
b
mixture of polymers having two types of terminal structures,
Aand B. Meanwhile, the reactions activated by triyne 5(runs4and5)
proceeded smoothly without participation of 5 in the poly-
merization and yielded polymers of type A having a narrow PDI
when 5 was converted to 6. As revealed by the MALDIꢀTOF
mass spectra shown in Figure 1e (also see SI-Figure 12), the two
series of major peaks were in agreement with values calculated
using the formulas 302.2n þ 23.0 (b) and 302.2n þ 23.0 ꢀ 69.0
(O), where 302.2 isthe massof the monomer unit, 23.0 is the mass
of Naþ, and 69.0 is the mass of a terminal propargyl moiety
(HOCH2CtCCH2) resulting from poly-4a fragmentation under
the analytical conditions. Plots of Mn and PDI values for the
polymersversus conversionover the entire polymerizationprocess
were linear, with PDI e 1.26 (Figure 1a,c). Values of Mn
determined by GPC were in good agreement with those deter-
mined by NMR analysis (SI-Figure 13). The semilogarithmic
kinetic plot of the polymerization was linear up to 92% conversion
(Figure 1d). These results clearly indicate that polymerization
proceeded in a chain-growth manner.
run
additive (mol %)
time (h)
conv. (%)a
10ꢀ3Mn
PDIb
1
2
3
4
5
none
2
2
4
3
1
75
95
90
92
96
8.19
6.76
4.00
6.28
6.46
1.43
1.28
1.22
1.26
1.35
DMAD (15)c
DMAD (5)c
5 (15)
5 (10)
a Determined by 1H NMR analysis of samples taken from the reaction
mixture. b PDI = Mw/Mn, as determined by GPC analyses in THF using
PS standards. c Terminal structures of the polymers:
Next, we applied the method to the polymerization of a
mixture of two monomers with different reactivities. The model
study revealed that electron-deficient alkynes such as alkynyl
esters react with diynes much more rapidly than propargylic
alcohols (Scheme 2; also see the SI). On the basis of these results,
yneꢀdiyne 4b was designed as a second monomer, and the
polymerization of a mixture of 4a and 4b in a 0.06/0.05/0.15/
electron-deficient alkyne [DMAD or triyne 5, each of which is a
very active alkyne in the (homo)cyclotrimerization] activated the
catalysis to start the polymerization quickly, and the increase in
Mn was proportional to the monomer conversion (also see SI-
Figures 3ꢀ6). The mass spectrometry and NMR analyses of the
polymers obtained from the DMAD-activated reactions (runs 2
and 3; SI-Figure 11) indicated that the product comprised a
0.15 dipimp/CoCl2 6H2O/Zn/5 catalyst system was investigated
3
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Scheme 4. Polymerization of a Mixture of Monomers 4a and
4b
immortal polymerization mechanism cannot be ruled out. The
cobalt catalyst may be intramolecularly transferred from the
[4 þ 2] cycloaddition intermediate to the diyne termini simulta-
neously with its reductive elimination when a ligand (dipimp) may
not participate any longer in the propagation process (see SI-
Figure 14 for a more detailed discussion). At the initiation stage,
concurrent beginning of the reaction is essential to attain con-
trolled polymerization. Thus, the addition of a highly reactive
electron-deficient alkyne such as DMAD or triyne 5, which
undergoes cyclotrimerization much more rapidly than the
monomer(s) employed, might enable a quick, simultaneous
generation of an active catalyst in a short period.
In conclusion, we have demonstrated that the alkyne [2 þ 2 þ 2]
cycloaddition reaction catalyzed by dipimp/CoCl2 6H2O/Zn is
3
applicable to polymerization, yielding linear polymers via a
selective cross-cyclotrimerization reaction that occurs in a chain-
growth manner. To the best of our knowledge, this is the first
example of chain-growth cycloaddition polymerization. The utili-
zation of this method for one-shot spontaneous block copolym-
erization of a mixture of two monomers has also been
demonstrated. On the basis of the high functional group compat-
ibility with the catalysis,4 this method may be useful in preparing
diverse functionalized polymers in a controlled manner. Further
investigation of the reaction mechanism and applications is
underway.
’ ASSOCIATED CONTENT
Figure 2. Polymerization behavior of a mixture of 4a and 4b. (a) Plots
of conversion (b, 4a; 9, 4b) and ln [M]0/[M] (O, 4a; 0, 4b) vs time.
(b) GPC profile of the resulting polymer (after 90 min).
S
Supporting Information. Experimental procedures and
b
characterization of products. This material is available free of
charge via the Internet at http://pubs.acs.org.
Scheme 5. Proposed Mechanism of the Propagation Steps
’ AUTHOR INFORMATION
Corresponding Author
okamos10@kanagawa-u.ac.jp
’ ACKNOWLEDGMENT
We thank the Scientific Frontier Research Project from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT), Japan, for financial support.
(Scheme 4). The polymerization behavior as shown by plots of
conversion and ln [M]0/[M] versus time for each monomer is
presented in Figure 2a (also see SI-Figures 8 and 9).
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