Unsaturated Alcohols as Chain-Transfer Agents in Olefin Polymerization: Synthesis of Aldehyde End-Capped Oligomers and Polymers

Article abstract of DOI:10.1021/jacs.0c06644

Palladium diimine-catalyzed polymerization of olefins using unsaturated alcohols as chain-transfer agents has been demonstrated. The reaction affords aldehyde end-capped polymers whose molecular weight can be tuned by varying the ratio of olefin/chain-transfer agent. Notably, >95% efficient end capping with aldehyde can be achieved under optimized conditions. This end-capping procedure is a rare example of introducing a highly reactive and versatile terminal functionality in polyolefin chains using a functional group-tolerant late metal catalyst. The reactivity of these end-capped polymers is illustrated here via functionalization with dyes to yield colored, hydrocarbon-soluble polyolefin derivatives.

Full text of DOI:10.1021/jacs.0c06644

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Unsaturated Alcohols as Chain-Transfer Agents in Olefin
Polymerization: Synthesis of Aldehyde End-Capped Oligomers and
Polymers
Xing-Wang Han, Olafs Daugulis,* and Maurice Brookhart*
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ABSTRACT: Palladium diimine-catalyzed polymerization of olefins using unsaturated alcohols as chain-transfer agents has been
demonstrated. The reaction affords aldehyde end-capped polymers whose molecular weight can be tuned by varying the ratio of
olefin/chain-transfer agent. Notably, >95% efficient end capping with aldehyde can be achieved under optimized conditions. This
end-capping procedure is a rare example of introducing a highly reactive and versatile terminal functionality in polyolefin chains
using a functional group-tolerant late metal catalyst. The reactivity of these end-capped polymers is illustrated here via
functionalization with dyes to yield colored, hydrocarbon-soluble polyolefin derivatives.
1. INTRODUCTION
Terminally functionalized oligomers and polymers generated
from simple olefins are attractive materials that have a variety
of applications such as detergents, oil additives, and
plasticizers, as well as precursors to block copolymers. A
straightforward approach to such materials can be achieved via
chain-transfer polymerization wherein the chain-transfer (CT)
reaction deposits a functional group on the terminal position of
the hydrocarbon chain.1 There are two attractive features of
this approach: multiple functionalized chains are produced per
metal atom, and the polymer/oligomer molecular weight can
be controlled by the ratio of monomer to CT agent. There are
numerous examples of alkyl and aryl silicon, boron, aluminum,
zinc, and magnesium compounds used as CT agents in
combination with early transition and lanthanide metal
Figure 1. Synthesis of End-functionalized Polyethylenes via Chain
Transfer Using Late Transition Metal Catalysts
catalysts resulting in formation of −MRn end groups.2,3
These CT agents generally bear nonpolar substituents (alkyl
or aryl groups) due to the incompatibility of early metals with
reported using silanes as CT agents in both Pd- and Co-
most functional groups; however, such metal-terminated
catalyzed polymerizations of ethylene.4b−d In these systems,
polymer chains can be converted in a second step to a
the R3SiH CT agent cleaves the growing chain to generate a
number of polar organic functionalities. Highly hindered
LnM−SiR3 species which reinitiates chain growth with the
secondary alkyl or aryl phosphines or amines are viable CT
propagating chain bearing a −SiR3 end group. Cleavage of this
agents for use with lanthanide catalysts and result in formation
chain by HSiR3 releases the silyl end-capped polyethylene,
of polyolefins capped with bulky amine or phosphine groups.3
Late transition metal catalysts are more tolerant of polar
functionality than early metal catalysts, but their use in chain-
Received: June 24, 2020
transfer reactions for incorporation of terminal polar
functionality into polyolefins has seen limited attention.4
Examples are summarized in Figure 1. Mecking reported a
remarkable reaction employing vinyl furan as the CT agent
which results in the telechelic polymers shown.4a Guironnet
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CH3(CH2CH2)nSiR3, and reforms LnM−SiR3 closing the
catalytic cycle. Guironnet showed that polar trialkoxysilanes,
(RO)3SiH, were also effective CT agents. Nozaki recently
reported use of the diazo diester, (CH3CO2)2CN2, as a CT
agent to yield polymers with −CHC(CO2CH3)2 end
groups.4c
We report here a method to introduce an aldehyde as a
terminal functionality into polyolefins. The aldehyde group is a
highly desirable functionality in that it is exceptionally reactive
and can be readily converted to a wide variety of other
functional groups.5 It is an excellent precursor for preparing
polyolefin diblock copolymers.6
these diimine palladium complexes makes such a process
viable. Further validating this approach are reports of “redox
relay” Heck reactions in which the Heck product is terminated
by Pd(II) migration along an alkyl chain via a series of β-
hydride elimination/readdition reactions to a remote alcohol
to form ketone or aldehyde products11a−d and the observation
that bis-phosphine Pd(II) complexes catalyze isomerization of
unsaturated alcohols to ketones or aldehydes.11e
β-Hydride elimination and formation of unsaturated
homopolymer will be in competition with insertion of
unsaturated alcohol and aldehyde formation, so for maximum
yields of aldehyde-functionalized chains, catalysts yielding
narrow dispersities in homopolymerizations should yield the
highest fraction of aldehyde-functionalized polymer. These
considerations have allowed development of the chain-transfer
reaction employing palladium diimine catalysts as described in
Scheme 1 for preparation of polyolefins of variable molecular
weight bearing end-capped aldehyde groups.
2. RESULTS AND DISCUSSION
2.1. Reaction Design Considerations. Bulky aryl-
substituted diimine−palladium complexes have been exten-
sively studied as catalysts for polymerization of ethylene and
other α-olefins to high molecular weight materials.7 These
catalysts produce uniquely branched polymers by virtue of a
“chain-walking” process in which the palladium “walks” along
the chain via a series of β-hydride elimination/readdition
reactions. Importantly, mechanistic studies show palladium
walks, on average, over several hundred carbons prior to
insertion and chain transfer. These palladium diimine systems
show good functional group tolerance in copolymerizations
with a variety of polar comonomers.8 Furthermore, modified
diimine complexes are efficient catalysts for olefin isomer-
ization and isomerization of unsaturated silyl-protected
alcohols to silyl enol ethers.9 Consequently, we explored the
possibility that incorporation of an unsaturated alcohol CT
agent into a growing polymer chain followed by isomerization
and β-hydride elimination could lead to an aldehyde end-
capped oligomer or polymer and regenerate an active catalyst.
The basic process envisioned is shown in Scheme 1 for end-
functionalizing polyethylene using 3-buten-1-ol as the CT
2.2. Catalyst and Chain-Transfer Reagent Screening.
We initially chose to examine the well-studied tetraisopropyl-
substituted aryldiimine catalyst 1 shown in Figure 2. 1-Hexene
Figure 2. Diimine−palladium catalysts used in this study.
was selected as the polymerizable olefin, while 5-hexen-1-ol
was used as the CT agent. The choice of 5-hexen-1-ol in which
the α-olefin is well isolated from the hydroxyl functionality
ensures that its insertion rate should be essentially identical to
that of 1-hexene. Thus, both olefins will be consumed at the
same rate, and the degree of polymerization of the aldehyde-
capped polymer should not vary with conversion and reflect
the ratio of the two monomers. Scheme 2 shows a typical result
Scheme 1. General Method for Synthesizing Polyolefins
Bearing a Terminal Aldehyde Functionality
Scheme 2. Initial Chain-Transfer Screening Using 1
employing catalyst 1 (10 μmol) with 10 mmol of 1-hexene and
0.3 mmol of 5-hexen-1-ol as CT agent in CH2Cl2 over 20 h.
The polymer yield was 800 mg (ca. 91%), indicating ca. 920
reagent. When the butenol inserts into the growing poly-
ethylene chain, the palladium “walks” to the end of the chain
and undergoes β-elimination to yield polyethylene bearing an
aldehyde end group and a palladium hydride that initiates a
new chain.10 (The β-elimination reaction is shown here to
occur from C1; mechanistic details of the CT reaction will be
addressed below.)
High yields of aldehyde formation following unsaturated
alcohol insertion rely on chain walking to the end of the chain
occurring faster than chain transfer via β-hydride elimination to
yield a polymer bearing a hydroxyl functionality and an internal
olefinic group. As noted above, rapid, efficient chain walking in
1
turnovers. The DP estimated by H NMR spectroscopy was
32, corresponding approximately to the ratio of hexene:hex-
enol. No RCH2OH functionalities were detected in the
isolated polymer, indicating that all the −OH groups were
converted to aldehyde. The hexene and hexenol clearly react at
the same rate since the DP at 6 h (18% yield) is essentially the
same as the DP at 20 h (91% yield). Furthermore, monitoring
a chain-transfer reaction using 8 mmol of 1-hexene:0.2 mmol
of 7-octen-1-ol shows that the ratio of unreacted hexene to
octenol remains constant throughout the course of the reaction
(see Supporting Information, Figure S1).
B
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a
Table 1. Screening Unsaturated Alcohols as Chain-Transfer Agents in Polymerization of 1-Hexene
b
c
d
e
e
entry
CT
yield/g
trace
branches/1000 C
product ratio
Mn (g/mol)
Mw/Mn
f
1
C3
C4
C5
C6
C8
C8
C8
2
3
4
5
0.33 (37%)
0.73 (81%)
0.45 (50%)
0.33 (37%)
0.66 (73%)
0.63 (70%)
0.29 (34%)
0.57 (68%)
101
108
111
103
96
115
104
102
1/0.38
1/0.12
1/0.10
1/0.06
1/0.06
1/0.10
8500
6600
5900
5800
4800
8600
27 100
27 600
1.4
1.4
1.4
1.3
1.5
1.5
1.5
1.6
g
6
h
7
8
i
j
9
a
Conditions: V (total) = 5 mL, 10 μmol of Pd catalyst 2; the solution was stirred under N2 in a 10 mL Schlenk tube at room temperature for 20 h.
b
c
d
1
Yields of isolated products after precipitation with MeOH given in parentheses. Determined using H NMR spectroscopy. Ratio of aldehyde
e
end-capped polymer to monounsaturated polymer determined using 1H NMR spectroscopy. Mw and Mn values determined by GPC analysis with
f
g
h
samples run in THF at 40 °C relative to polystyrene standards. Palladium allyl complexes are formed. A 20 mmol amount of catalyst. A 0.25
mmol amount of CT agent. No CT agent. No CT agent, 48 h.
i
j
a
Table 2. End-Capping Reactions Using Different Ratios of 1-Hexene:7-Octen-1-ol
b
c
d
c
c
e
e
entry
1-hexene (mmol)
yield/g
branches/1000 C
product ratio
DP
Mn (g/mol)
Mn (g/mol)
Mw/Mn
1
2
3
4
5
6
20
16
12
8
4
2
0.82 (49%)
0.79 (60%)
0.83 (82%)
0.62 (92%)
0.33 (97%)
0.14 (82%)
0.11 (63%)
0.05 (3%)
0.09 (5%)
0.66 (39%)
0.59 (35%)
108
100
106
111
100
103
103
99
1/0.25
1/0.20
1/0.16
1/0.11
1/0.05
1/0.04
1/0.02
115.0
79.8
69.0
46.6
19.8
14.8
7.0
9800
6800
5900
4000
1800
1400
700
10 700
9800
8500
6800
3200
2400
1400
9100
1.4
1.4
1.4
1.4
1.7
1.6
1.6
1.1
1.1
1.5
f
7
2
g
8
h
20
20
20
20
9
99
99
100
16 200
27 500
27 300
i
10
11
27 300
j
1.5
a
b
Conditions: V(total) = 5 mL, 10 μmol of Pd catalyst; solution was stirred under N2 in a 10 mL Schlenk tube at room temperature for 20 h. Yields
c
d
1
of isolated products after precipitation with MeOH given in parentheses. Determined using H NMR spectroscopy. Ratio of aldehyde end-
capped polymer to monounsaturated polymer determined using H NMR spectroscopy. Mw and Mn values determined by GPC analysis with
samples run in THF at 40 °C relative to polystyrene standards. Hexene (2.0 mmol), CT (0.4 mmol). No CT agent, 30 min. No CT agent, 60
min. Hexene (20 mmol), 1-octanol (0.2 mmol), no CT agent. Hexene (20 mmol), no CT agent.
e
1
f
g
h
i
j
Since there was a significant quantity of monounsaturated
homopolymer (ca. 25%) formed from normal β-hydride
elimination/chain transfer we screened other ortho-disubsti-
tuted aryl diimine catalysts and determined that unsymmetri-
cally substituted diimine catalyst 2 (see Figure 2) was superior
to 1 in terms of the fraction of homopolymer generated. This
catalyst was used to screen the chain-transfer reaction using 1-
hexene against several primary unsaturated alcohols as shown
in Table 1. A catalyst loading of 10 μmol was used for
polymerization of 10 mmol of hexene with 0.5 mmol of
unsaturated alcohol as the CT agent. While allyl alcohol (entry
1) was completely inefficient, CT agents with longer separation
between the terminal hydroxyl and the alkene functionalities
afforded good results. Entry 5 shows that the C8 alcohol, 7-
octen-1-ol, provided the highest ratio of aldehyde relative to
unsaturated homopolymer. Yields could be increased using a
higher catalyst loading (entry 6) or a lower loading of the CT
agent (entry 7, 0.25 mmol), the latter result suggesting that the
CT agent may give rise to a catalyst decomposition pathway
(see below). Entries 8 and 9 illustrate the behavior of this
catalyst in the absence of the CT agent. Polymer yields are
generally lower, which can be ascribed to increasingly lower
rates of insertion as the polymer chain grows.12
2.3. Chain Transfer in 1-Hexene Polymerization. On
the basis of these results we prepared a series of aldehyde end-
capped poly(1-hexene)s of variable molecular weights employ-
ing 1-hexene in combination with 7-octen-1-ol as the chain-
transfer reagent. Results are summarized in Table 2, and Figure
1
3 shows a typical H NMR spectrum of an aldehyde end-
capped polymer. The concentration of octenol was held at 0.2
mmol, and that of hexene was varied from 20 to 2 mmol. The
catalyst loading of 10 μmol increased correspondingly from
0.05 to 0.5 mol % relative to hexene. Yields generally increased
as the hexene concentration decreased as a consequence of an
increase in the mol % of catalyst loading relative to hexene.
The degrees of polymerization determined by 1H NMR
spectroscopy correspond reasonably closely to the ratio of
hexene:octenol, indicating these two olefins exhibit equal rates
C
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control for entry 1. The higher polymer yield for entry 1 vs
entry 10 suggests that the rate of insertion decreases with chain
length and is consistent with earlier observations.12 The similar
yields of entries 10 and 11 suggest the hydroxyl functionality
has no significant influence on the polymerization reaction.
2.4. Chain Transfer in Ethylene Polymerization. Next,
chain transfer employing ethylene in combination with 7-
octen-1-ol was investigated (Table 3). A reaction employing 15
μmol of catalyst 2 and 0.125 mmol of octenol under 1 atm
ethylene gave 0.47 g of oligomer with a ratio of end-capped
polymer/homopolyethylene of 1/0.18 in 6 h. End-capped
oligomer was obtained with a Mn of 15.3 × 103, 79 branches/
1000 carbons, and low polydispersity of 1.4 (entry 1).
Increasing the octenol concentration led to the expected
decrease of product molecular weight (entry 2), and increased
reaction time led to higher yields (entry 3). Further increase of
octenol concentration led to an excellent 1/0.04 ratio of
desired aldehyde end-capped polymer to undesired homopoly-
ethylene (entry 4); however, the yield was somewhat
decreased, pointing to the possibility that a higher concen-
tration of chain-transfer agent may lead to catalyst decom-
position. Entry 6 shows that at 3.4 atm ethylene pressure a high
proportion of unfunctionalized polyethylene is obtained. Entry
7 is the control run without octenol. Polyethylene with a
molecular weight of about 82 000 is obtained.
2.5. Mechanistic Considerations. The mechanistic
details of the chain-transfer reaction and formation of aldehyde
were investigated by examining the catalytic isomerization of 3-
buten-1-ol to butyraldehyde (Scheme 3). Quantitative isomer-
ization (50 turnovers) is observed upon treating 0.5 mmol of
3-buten-1-ol 3 with 10 μmol of catalyst 2 in CD2Cl2 over 3 h
(the first turnover yields pentanal 7, which is not detected). To
probe the mechanistic details of the isomerization, the
deuterated alcohol 3-d1, CH2CHCH2−CH2OD, was
employed. The isomerization results in formation of
butyraldehyde with ca. 75% D at C4 (5) and 25% at C3 (6).
No deuterium is detected at C1 or C2. This result is consistent
with the general catalytic cycle shown in Scheme 3. Beginning
from palladium deuteride 8, 3-d1 can undergo either 2,1
insertion to place D at C4 (75%) or 1,2 insertion to place D at
C3 (25%). Chain walking follows insertion to generate species
9 with palladium bound α to the hydroxyl group. β-Elimination
then occurs from 9 to yield the labeled aldehyde products 5
1
Figure 3. H NMR spectrum of the isolated aldehyde end-capped
polymer from Table 2, entry 6.
of insertion as established above for catalyst 1. For example, in
entry 1, an average DP of 100 is predicted vs a measured DP of
115. In entry 4, a DP of 40 is predicted versus a measured DP
of 46.6. In entry 6, a DP of 10 is predicted but a DP of 14.7 is
observed. This high observed DP seems likely due to loss of
low molecular weight aldehydes during isolation. Entry 7
shows that using 0.4 mmol of octenol, oligomers with DP as
low as 7 were obtained in moderate yields. The Mn values
estimated by gpc are consistently higher than the Mn values
calculated from the DP values determined by 1H NMR
spectroscopy. The ratio of aldehyde end-capped polymer:-
homopolymer decreases as expected as the ratio of
hexene:octenol increases and more β-elimination to yield
homopolymer occurs prior to insertion of octenol and chain
transfer. Entries 8 and 9 are homopolymerizations of 1-hexene
run to DPs comparable to those for the chain-transfer
experiments in entries 1 and 2. The molecular weight
distributions of the homopolymerizations are quite narrow
(ca. 1.1), indicating little chain transfer. In comparison, the
molecular weight distributions of polymers generated from the
chain-transfer reactions (entries 1−7) are significantly greater
than 1.1 as expected. Homopolymerizations run to a much
higher DP show, again as expected, increased dispersity due to
increasing chain transfer (entries 10 and 11). Entry 10 using a
saturated alcohol, 1-octanol, in place of octenol serves as a
a
Table 3. Chain-Transfer Reactions Employing Ethylene with 7-Octen-1-ol as Chain-Transfer Agent
b
c
d
c
e
e
entry
C2H4 (atm)
CT (mmol)
yield/g
branches/1000 C
product ratio
Mn (g/mol)
Mn (g/mol)
Mw/Mn
1
2
3
1
1
1
1
1
3.4
3.4
0.125
0.25
0.25
0.50
0.50
0.50
0
0.47
0.43
0.62
0.35
0.45
0.45
0.68
79
76
65
65
65
60
60
1/0.18
1/0.01
1/0.10
1/0.04
1/0.09
1/0.42
9100
6400
8200
3600
4700
41 700
11 200
7500
10 600
5400
7400
41 100
82 000
1.4
1.4
1.3
1.7
1.4
1.7
1.7
f
4
f
5
g
6
h
7
a
Conditions: V(total) = 5 mL, 15 μmol of Pd catalyst; the solution was stirred under N2 in a 10 mL Schlenk tube at room temperature for 6 h.
b
c
d
Yields of isolated products after precipitation with MeOH given in parentheses. Determined by 1H NMR spectroscopy. Ratio of aldehyde end-
e
capped polymer to monounsaturated polyethylene determined by 1H NMR spectroscopy. Mw and Mn determined by GPC analysis with samples
f
g
h
run in THF at 40 °C relative to polystyrene standards. The solution was stirred for 10 h. C2H4 (3.4 atm), reacted for 1.0 h. C2H4 (3.4 atm), no
CT agent.
D
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Scheme 3. Chain-Transfer Reaction with D-Labeled 3-
Butenol
Scheme 5. Possible β-Elimination Mechanisms of Metal α-
Hydroxyalkyl Complexes
Scheme 6. Examples of Functionalization of Aldehyde End-
Capped Polymers
and 6 and regenerate the Pd−D species 8 which closes the
catalytic cycle.13
These results rule out β-elimination from species 10 to form
an enol 11 followed by isomerization which would place D at
C2 (Scheme 4), giving exclusively 12. β-Elimination from a
palladium alkoxide 13, which would result in partial
deuteration at C1, affording 14 and 4 is also excluded.
Scheme 4. Alternate Modes of Aldehyde Formation Ruled
out by Labeling Studies
irradiation. Further investigations of reactions of these
aldehyde end-capped polymers and oligomers are currently
in progress.
DFT calculations by Goddard and co-workers in connection
with the Wacker oxidation process suggested that aldehyde
formation from a Pd(II)(chloro)(α-hydroxyalkyl) species is
likely not a simple β-elimination process and that direct
transfer of the hydroxyl hydrogen to Pd(II) through a standard
four-center transition state is energetically unattractive.14 DFT
calculations point to transfer of the proton of the OH group to
the bound chloride in a five-membered transition state as
shown in Scheme 5, followed by release of aldehyde, HCl, and
Pd(0), which is essentially a reductive elimination process.
Reoxidation of Pd(0) closes the catalytic cycle.
Applying this concept to the α-hydroxy species 9 suggests
that proton transfer may occur to an external alcohol as in 16
or to a bound alkoxide as in 17 to form aldehyde and a Pd(0)
species, which upon deuteration (oxidation) would form
LnPd(II)-D 8. While the net result corresponds to a standard
β-elimination process, it proceeds by reductive elimination
followed by reoxidation (protonation of the Pd(0) species).
2.6. End-Capped Polymer Functionalization. Examples
of the functionalization of aldehyde end-capped polymers are
shown in Scheme 6. The oligomer (polymer) can be
quantitatively oxidized to a surfactant carboxylic acid 18.
Reaction with 2,4-dinitrophenylhydrazine gives a deep-yellow
hydrazone 19 in 88% yield. Furthermore, the aldehyde moiety
was coupled with dansyl hydrazine, giving end-capped polymer
20, which has a strong fluorescence emission upon 365 nm UV
3. SUMMARY
In conclusion, we have described a convenient method of
preparing aldehyde end-functionalized polyolefins using an
unsaturated alcohol as the chain-transfer agent and functional
group-tolerant palladium diimine catalysts. The molecular
weight of the end-capped polymers can be controlled by the
ratio of unsaturated alcohol:polymerizable olefin. The method
relies on the efficient chain-walking feature of these catalysts
which can move the metal to the carbon α to the hydroxyl
functionality where formal β-elimination occurs to yield the
aldehyde. The “β-elimination” reaction likely occurs by an E2-
type mechanism to form aldehyde and a Pd(0) species;
protonation of Pd(0) regenerates active catalyst. Aldehydes are
highly reactive and versatile functional groups that can be
converted to numerous other functionalities. We have
illustrated such conversions here including synthesis of
polyolefins end-functionalized with dyes via coupling with
−RNHNH2 groups.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
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Experimental procedures, characterization data, and
spectra of new compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Olafs Daugulis − Center for Polymer Chemistry, Department of
Chemistry, University of Houston, Houston, Texas 77204-5003,
United States; orcid.org/0000-0003-2642-2992;
Email: olafs@uh.edu
Maurice Brookhart − Center for Polymer Chemistry,
Department of Chemistry, University of Houston, Houston,
Texas 77204-5003, United States; Email: mbrookha@
central.uh.edu
Author
Xing-Wang Han − Center for Polymer Chemistry, Department
of Chemistry, University of Houston, Houston, Texas 77204-
5003, United States; orcid.org/0000-0003-1301-9029
Complete contact information is available at:
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(3) (a) Amin, S. B.; Marks, T. J. Organolanthanide-Catalyzed
Synthesis of Amine-Capped Polyethylenes. J. Am. Chem. Soc. 2007,
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lanthanide-Catalyzed Synthesis of Phosphine-Terminated Polyethy-
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This manuscript is dedicated to Prof. Pierre Dixneuf for his
outstanding contributions to organometallic chemistry and for
his vigorous advocacy of international associations and
collaborations. This research was supported by the Welch
Foundation (Chair No. E-0044 to O.D. and Grant No. E-1893
to M.B.). We thank Prof. Oleg Ozerov for insights concerning
the mechanism of the β-elimination reaction to form aldehyde.
The GPC system used in polymer analysis was funded by the
Welch Foundation (Grant No. H-E-0041) through the UH
Center of Excellence in Polymer Chemistry.
́
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