Polymerization of Allenes by Using an Iron(II) β-Diketiminate Pre-Catalyst to Generate High Mn Polymers

Article abstract of DOI:10.1002/chem.202101078

Herein, we report an iron(II)-catalyzed polymerization of arylallenes. This reaction proceeds rapidly at room temperature in the presence of a hydride co-catalyst to generate polymers of weight up to M_n=189 000 Da. We have determined the polymer structure and chain length for a range of monomers through a combination of NMR, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) analysis. Mechanistically, we postulate that the co-catalyst does not react to form an iron(II) hydride in situ, but instead the chain growth is proceeding via a reactive Fe(III) species. We have also performed kinetic and isotopic experiments to further our understanding. The formation of a highly unusual 1,3-substituted cyclobutane side-product is also investigated.

Full text of DOI:10.1002/chem.202101078

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Chemistry—A European Journal
Polymerization of Allenes by Using an Iron(II) β-
Diketiminate Pre-Catalyst to Generate High Mn Polymers
Callum R. Woof,[a] Derek J. Durand,[b] and Ruth L. Webster*[a]
Abstract: Herein, we report an iron(II)-catalyzed polymer-
ization of arylallenes. This reaction proceeds rapidly at room
temperature in the presence of a hydride co-catalyst to
generate polymers of weight up to Mn =189 000 Da. We have
determined the polymer structure and chain length for a
range of monomers through a combination of NMR, differ-
ential scanning calorimetry (DSC) and gel permeation
chromatography (GPC) analysis. Mechanistically, we postulate
that the co-catalyst does not react to form an iron(II) hydride
in situ, but instead the chain growth is proceeding via a
reactive Fe(III) species. We have also performed kinetic and
isotopic experiments to further our understanding. The
formation of a highly unusual 1,3-substituted cyclobutane
side-product is also investigated.
Introduction
considerably longer in chain length and has analyzed the
physical properties of the polymers (Scheme 1).[12] Endo and
Tomita have also reported examples of living polymerization
of phenylallene using a nickel catalyst,[13] as well as multi-
component coupling polymerizations.[14] In this study, we
report the first polymerization of phenylallene using an iron
pre-catalyst and perform mechanistic and analytical studies to
further understand how the reaction proceeds and investigate
the physical properties of the polymers.
Allenes are emerging as a powerful and unique building block
in molecular synthesis, owing to their facile reactivity, range of
functionalities and unusual stereochemistry.[1] There are many
reported studies that use allenes as coupling partners in
annulation and addition reactions,[2] as building blocks in
organic synthesis,[3] and as structural features in molecular
materials.[4] In comparison to these, there are relatively few
studies into polymerization reactions involving allenes, the
first being by Wotiz using Co2(CO)8.[5] These polymers have
attracted interest due to their unusual structure, opportunities Results and Discussion
for further functionalization[6] and development of complex
molecular architecture.[7] For example, Li has reported hyper-
branched RAFT polymerization using allene-derived
monomers.[8]
Owing to its reactivity and ease of handling and synthesis,
phenylallene has been used as a key model substrate for these
reactions. Radical polymerization of phenylallene leading to
low Mn and high Ð polymers were first reported by Anderson[9]
and further investigated by Takeshi.[10] Barrett has reported
that polymeric products are observed when phenylallene is
treated with Grubbs catalyst,[11] while Cui has demonstrated
that rare-earth metal catalysts can generate polymers that are
We have reported numerous alkene functionalization reactions
catalyzed by an iron(II) β-diketiminate complex [Fe],[15] and
were interested in applying our previously reported hydro-
boration chemistry[16] to allene substrates. To our surprise,
rather than undergoing an expected hydroboration reaction
with phenylallene (1), we instead generate a polymeric
[a] C. R. Woof, Dr. R. L. Webster
Department of Chemistry
University of Bath
Claverton Down, Bath, BA2 7AY (UK)
E-mail: r.l.webster@bath.ac.uk
[b] D. J. Durand
School of Chemistry
University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202101078
© 2021 The Authors. Chemistry - A European Journal published by Wiley-
VCH GmbH. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and re-
production in any medium, provided the original work is properly cited.
Scheme 1. Examples of previously reported phenylallene polymerization
catalysts, and the reaction reported herein.
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substance as the primary product; iron β-diketiminate catalysts
have previously been shown to competent for some forms of
coordination polymerization, for example for polylactide[17]
and polyethylene synthesis.[18] Following our serendipitous
discovery, we proceeded to optimize the reaction. Phenyl-
allene polymerization proceeds with one equivalent of HBpin
relative to substrate (Table 1, Entry 2) and with a co-catalytic
amount of HBpin (Table 1, Entry 3). Stoichiometric loading of
HBpin leads to lower yield and lower Mn polymer (53 versus
152 kDa), presumably due to the propensity for chain-termi-
nating reactions with HBpin in solution. Other hydride sources,
such as H3N·BH3, Me2HN·BH3 and HSi(OEt)3 also give high
conversions (Table 1, Entries 4 to 6). H3N·BH3 gives the highest
Mn polymer while maintaining narrow Ð (189 kDa, Ð = 1.19),
which may be linked to both the lack of reactivity H3N·BH3
shows in dehydropolymerization chemistry,[15c] therefore chain
termination or side-reactions are minimized, along with the
potential role of H3N·BH3 in pre-catalyst activation (see below).
1H and 13C{1H}[19] NMR analysis indicate the terminal (2,3)-
substituted polymer (P12,3) is the primary structural product.
Investigating the structure of P12,3 in greater detail, the
NMR spectra further show that there are three distinct proton
environments (1.7:1.1:1 ratio) around the main chain, which
we believe relates to the different conformations the polymer
can take. NOE NMR spectroscopy indicates that these environ-
ments are conformationally distinct from each other (See
Supporting Information). Diad structures would be expected
to correlate through NOE and 2D NOESY NMR experiments. In
both cases, no correlation is observed between the different
methylene proton environments, leading to us to conclude
that each proton is primarily adjacent to protons in the same
environment. Although we cannot rule out a random arrange-
ment of diads, based on the data we propose longer blocks of
conformers. Using oligomers containing seven repeat units,
Density Functional Theory (DFT) calculations (B3LYP-D3/6-
31G* (SCF=Benzene), 298 K) support the notion that there are
three distinct conformers present in the product (one major
species and two minor species, which matches observations
by NMR spectroscopy), with the ground state species
(0.0 kcalmolÀ 1), followed by
a
conformer at ΔG= +
2.7 kcalmolÀ 1 and one at ΔG= +6.5 kcalmolÀ 1 (Figure 1). A
fourth higher energy conformer appears at ΔG= +
8.9 kcalmolÀ 1 [20]
Population distributions calculated using
.
Maxwell-Boltzmann distribution further supports the exper-
°
imental data; at raised temperatures (up to 80 C) the signals
remain distinct and do not appear to interconvert on an NMR
timescale. Furthermore, heating the polymer for an extended
°
length of time (80 C, 3 days) does not lead to a change of
intensities or other changes to the NMR spectrum (See
Supporting Information). This appears to rule out structure
interconversion.
As H3N·BH3 gives the highest conversion, Mn, a narrow Ð
and a reasonably high ratio of the two observed isomers (P12,3
and P11,2), further reaction optimization was carried out using
this hydride source (Table 2). Whilst there is relatively little
difference between stirred and unstirred flasks (Table 2,
Entry 2), there is
a significant increase in Mn in more
concentrated conditions, albeit at the expense of conversion
to polymer (Table 2, Entries 3 and 4). In particular, solvent-free
conditions (Table 2, Entry 4) lead to a dramatic reduction in
conversion, which we attribute to lack of stabilization of
activated pre-catalyst (see below). Raising the temperature has
a modest effect on Mn but increases polydispersity significantly
(Table 2, Entry 5). Quenching the reaction with methanol
before full monomer conversion enables smaller molecular
Table 1. Effect of hydride source on polymerization.
Entry Hydride
Conversion
[%]
Mn
[kDa]
Ð
–
P12,3
P11,2
:
DP
1
None
3
–
–
–
2[a]
3
HBpin
HBpin
H3N·BH3
82
91
93
53
152
189
10.5
62
–
1.12 8:1
1.36 5:1
1.19 7:1
6.25 5:1
1.18 6:1
460
1310
1630
91
530
–
4
5
Me2HN·BH3 89
HSi(OEt)3
H3N·BH3
6
92
0
7[b]
–
–
Conditions: 600 μL C6D6, 0.5 mmol 1, 1 mol% [Fe], 5 mol% hydride, 16 h,
RT. Conversion determined by 1H NMR spectroscopy. [a] 1 eq. of HBpin
relative to allene (0.5 mmol), 7% conversion to hydroborated product
°
observed. [b] No [Fe], 60 C, 72 h.
Figure 1. Oligomer structures optimized by DFT.
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We do not observe any polymerization occurring when using
Table 2. Variation of reaction conditions.
non-aryl allene substrates, such as cyclohexylallene or meth-
oxyallene. Presumably aryl substituents provide the electronic
effects necessary to enable reactivity to proceed and has been
highlighted by other researchers investigating the reactivity
and functionalization of allenes.[21] Furthermore, although
disappointing, the lack of reactivity with non-aryl allenes is not
surprising given previous reports using the pre-catalyst, which
has been dominated by functionalization of aryl
substrates[15c,22] or intramolecular cyclizations.[15a,23]
To understand the nature of the polymerization, some
mechanistic investigations were performed. Kinetic analysis
indicates the reaction is first order with respect to the catalyst,
and PMe3 poisoning experiments indicate the reaction is not
nanoparticle mediated. Addition of a radical clock does not
significantly alter rate. Application of deuterium-labelled
phenylallene (1-d2, Scheme 2a) in polymerization under
standard conditions leads to clean isotopic retention in the
product, as indicated by protic signals at 3.46 ppm (methylene
protons) that integrate to the quantity of residual protons in
1-d2, with the protic signal at 6.24 ppm (α proton) integrating
to 1H. If scrambling had occurred we would expect the
6.24 ppm integral to be less than 1H. Furthermore, the 2H NMR
spectrum, albeit weak, only shows peaks at 3.70-3.45 ppm.
Using deuterated hydride sources, such as Me2DN·BH3 or
Me2HN·BD3, and quenching the reaction to ensure low Mn
polymer is formed, give no apparent D-incorporation in the
polymer (Scheme 2b). We have previously reported that the
bridged iron hydride dimer ([Fe]H)2 is an on-cycle species
Entry Deviation from standard condi-
tions
Conversion
[%]
Mn
[kDa]
Ð
1
2
none
93
91
92
71
96
19
96
189
83.9
–
12.6
132
10.5
–
1.19
1.38
–
1.75
1.85
1.33
–
no stirring
100 μl solvent
no solvent
heated at 80 C
quenched after 5 minutes
20 mol% catalyst loading
3[a]
4[a]
5[b]
6
°
7[a]
Conversion determined by 1H NMR spectroscopy. [a] Under these
conditions, polymer length was greater than 250 kDa and beyond the
resolution of the GPC column, data reported in Supporting Information. 1H
NMR indicates the same polymer structure is forming. [b] 90% polymer,
6% dimer, see below for further discussion.
weight polymers to be isolated (Table 2, Entry 6). MALDI-ToF
analysis shows repeat units of 116 Da and end groups are
proteo/allenyl in nature i.e. neither CH2SiMe3, BH2 nor NH2
fragments are incorporated into the polymer chain. Increasing
the catalyst loading does not reduce the Mn (>250 kDa,
Table 2, Entry 7), but as observed in Table 1, varying the
quantity of hydride source does appear to give more
controlled polymerization. The optimized conditions for the
synthesis of polymer P1 are [Fe] (1 mol%), H3N·BH3 (5 mol%),
C6D6 (0.6 mL), RT, 16 h; the resultant 189 kDa polymer is
around seven times that reported by Cui[12] and over twenty
times that reported by Endo.[13a] Importantly, polymerization
catalyzed by [Fe] is not at the expense of Ð (1.2 compared to
1.6–2.1 and 1.1 from Cui and Endo respectively).
DSC analysis of the resultant polyphenylallene (P1)
°
indicates a Tg of 64.8 C, which is similar to the range reported
[12]
°
by Cui (Tg =61.3–61.8 C). We do not observe any melting
transitions, either through DSC analysis or visual observations,
indicating that the P1 is amorphous rather than crystalline.
The structural nature of the polymer was investigated using
WAXD studies (See Supporting Information). Analysis of a
polymer fiber gave no Bragg peaks or 2D orientation,
indicating that the polymer generated is amorphous.
We have applied our optimized polymerization conditions
to several arylallene substrates (Table 3). All substrates lead to
high conversions, high Mn and narrow Ð, although the reaction
with para-fluoro reagent
6 and para-chloro 7 (Table 3,
Entries 5 and 6) require higher catalyst loading and have lower
conversion. The Mn for P6 is particularly low compared to the
other polymers produced. As expected, a larger Mn tends to
lead to a wider Ð (Table 3, Entries 1, 4 and 6), whereas a low
Mn gives a narrow Ð (e.g. Table 3, Entries 2 and 5). It is difficult
to link the P2,3 :P1,2 ratio to Mn or Ð, for example, it could be
assumed that reactions that have less control over polymer-
ization and give high Mn and broad Ð may also be less
selective, but this is not necessarily the case here (compare
Table 3, Entries 4 and 5). Ortho-methyl substrate 3 gives poor
P
2,3 :P1,2 ratio, which may be linked to a lack of isomer
selectivity due to steric interference. In all cases there are
distinct chain proton environments with similar NOE correlat-
ing peaks as observed with P1 (See Supporting Information).
Scheme 2. Mechanistic studies.
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Table 3. Scope and properties of functionalized arylallene polymers.[a]
°
Entry
1
Major isomer
Conversion [%]
94
P
2,3 :P1,2
Mn [kDa]
Ð
DP
Tg [ C]
P22,3
8:1
7:1
229
1.63
1760
86.4
2
P32,3
96
72
1.17
550
63.0
3
P42,3
P52,3
P62,3
P72,3
91
98
77
74
5:1
185
174
53
1.62
2.05
1.12
1.67
1420
1190
400
75.4
81.6
57.0
97.1
4
11:1
13:1
6:1
5[b]
6[b]
166
1110
[a] Conditions: 600 μL C6D6, 0.5 mmol allene, 1 mol% [Fe], 5 mol% H3N·BH3, 16 h, RT. Conversion determined by 1H NMR spectroscopy. [b] 5 mol% [Fe].
some dehydrocoupling reactions.[24] However, ([Fe]H)2 is a
poor catalyst for this transformation (5% conversion, 16 h, RT)
in the absence of H3N·BH3. Performing the reaction with [Fe]
under a hydrogen atmosphere does not lead to reasonable
conversion; consequently, we do not believe this particular
iron hydride dimer is an active catalyst. We have recently
reported evidence for Fe(I) species forming from the reaction
of [Fe] with the same hydride sources employed in this
reaction. In this previous study, DFT studies supported the
reaction proceeding via oxidative addition of CÀ H bonds to
generate Fe(III) intermediates.[25] In light of this, we postulate
that the polymerization proceeds in
a similar manner
(Scheme 3): an initial pre-catalyst activation that involves
Scheme 3. Postulated catalyst activation step and subsequent polymer-
ization process.
reduction to an Fe(I) species, I. We anticipate that formation of
I
proceeds via an η6-arene stabilized intermediate, the
formation of which is necessary to efficiently access
I
(supported by the drop-off in conversion in the absence of
oxidative addition of a terminal CÀ H bond (II), consistent with
solvent, Table 2). The η2-supported allene then undergoes
no deuterium incorporation when using Me2HN·BD3, followed
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Acknowledgements
The EPSRC Centre for Doctoral Training in Catalysis (EP/
L016443/1) is thanked for funding (CRW, DJD). The authors
would like to thank the MC2 facility at the University of Bath, in
particular Dr. Martin Levere and Dr. Rémi Castaing, for
assistance. Many thanks are extended to Dr. Thom McGuire and
Dr. Antoine Buchard for helpful discussions and to Dr. Danila
Gasperini for preparation of the Fe(I) species. Dr. Adam Squires
is thanked for WAXD analysis.
Scheme 4. [2+2] cycloaddition of phenylallene.
by insertion of monomer into the FeÀ C bond (III), which is the
key chain propagation process. Given the high degree of
polymerization (DP) values across all substrates and condi-
tions, it is likely that only a relative small amount of metal Conflict of Interest
centers are active species. In situ wide sweep NMR spectro-
scopy confirms that a significant amount of pre-catalyst
remains unreacted even after full conversion of the substrate.
Preparation of an analogous Fe(I)-η6-arene species, [Fe](I),
leads to similar reactivity without a hydride source present
(90% conversion to P1; 6:1 ratio P12,3 :P11,2, Mn =190 kDa, Ð=
1.51) supporting our hypothesis.
The authors declare no conflict of interest.
Keywords: allenes · cycloaddition · homogeneous catalysis ·
iron · polymers
Interestingly, when the polymerization is performed at
80 C, an unusual side-reaction is observed (Scheme 4). 1,3-
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°
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°
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Manuscript received: March 25, 2021
Accepted manuscript online: June 18, 2021
Version of record online: ■■■, ■■■■
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FULL PAPER
A rare example of transition metal-
catalyzed polymerization of aryl
allenes is presented. Reactions are
mild and generate very high Mn
polymers with narrow polydispersity
by using a well-defined Fe(II) pre-
catalyst and a hydride additive. [2+2]
cycloaddition to form cyclobutanes is
also viable using the same catalytic
system.
C. R. Woof, D. J. Durand, Dr. R. L.
Webster*
1 – 7
Polymerization of Allenes by Using
an Iron(II) β-Diketiminate Pre-
Catalyst to Generate High Mn
Polymers