Dual Polymerization Pathway for Polyolefin-Polar Block Copolymer Synthesis via MILRad: Mechanism and Scope

Article abstract of DOI:10.1021/jacs.0c10588

This work explores the mechanism whereby a cationic diimine Pd(II) complex combines coordination insertion and radical polymerization to form polyolefin-polar block copolymers. The initial requirement involves the insertion of a single acrylate monomer into the Pd(II)-polyolefin intermediates, which generate a stable polymeric chelate through a chain-walking mechanism. This thermodynamically stable chelate was also found to be photochemically inactive, and a unique mechanism was discovered which allows for radical polymerization. Rate-determining opening of the chelate by an ancillary ligand followed by additional chain walking allows the metal to migrate to the α-carbon of the acrylate moiety. Ultimately, the molecular parameters necessary for blue-light-triggered Pd-C bond homolysis from this α-carbon to form a carbon-centered macroradical species were established. This intermediate is understood to initiate free radical polymerization of acrylic monomers, thereby facilitating block copolymer synthesis from a single Pd(II) complex. Key intermediates were isolated and comprehensively characterized through exhaustive analytical methods which detail the mechanism while confirming the structural integrity of the polyolefin-polar blocks. Chain walking combined with blue-light irradiation functions as the mechanistic switch from coordination insertion to radical polymerization. On the basis of these discoveries, robust di- and triblock copolymer syntheses have been demonstrated with olefins (ethylene and 1-hexene) which produce amorphous or crystalline blocks and acrylics (methyl acrylate, ethyl acrylate, n-butyl acrylate, and methyl methacrylate) in broad molecular weight ranges and compositions, yielding AB diblocks and BAB triblocks.

Full text of DOI:10.1021/jacs.0c10588

pubs.acs.org/JACS
Article
Dual Polymerization Pathway for Polyolefin-Polar Block Copolymer
Synthesis via MILRad: Mechanism and Scope
Huong Dau, Anthony Keyes, Hatice E. Basbug Alhan, Estela Ordonez, Enkhjargal Tsogtgerel,
Anthony P. Gies, Evelyn Auyeung, Zhe Zhou, Asim Maity, Anuvab Das, David C. Powers,
Dain B. Beezer,* and Eva Harth*
Cite This: https://dx.doi.org/10.1021/jacs.0c10588
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: This work explores the mechanism whereby a
cationic diimine Pd(II) complex combines coordination insertion
and radical polymerization to form polyolefin−polar block
copolymers. The initial requirement involves the insertion of a
single acrylate monomer into the Pd(II)−polyolefin intermediates,
which generate a stable polymeric chelate through a chain-walking
mechanism. This thermodynamically stable chelate was also found
to be photochemically inactive, and a unique mechanism was
discovered which allows for radical polymerization. Rate-
determining opening of the chelate by an ancillary ligand followed
by additional chain walking allows the metal to migrate to the α-
carbon of the acrylate moiety. Ultimately, the molecular
parameters necessary for blue-light-triggered Pd−C bond homolysis from this α-carbon to form a carbon-centered macroradical
species were established. This intermediate is understood to initiate free radical polymerization of acrylic monomers, thereby
facilitating block copolymer synthesis from a single Pd(II) complex. Key intermediates were isolated and comprehensively
characterized through exhaustive analytical methods which detail the mechanism while confirming the structural integrity of the
polyolefin−polar blocks. Chain walking combined with blue-light irradiation functions as the mechanistic switch from coordination
insertion to radical polymerization. On the basis of these discoveries, robust di- and triblock copolymer syntheses have been
demonstrated with olefins (ethylene and 1-hexene) which produce amorphous or crystalline blocks and acrylics (methyl acrylate,
ethyl acrylate, n-butyl acrylate, and methyl methacrylate) in broad molecular weight ranges and compositions, yielding AB diblocks
and BAB triblocks.
an established platform.2e,4 However, an alternative way of
INTRODUCTION
■
thinking abandons improving the tolerance of a single
Polyolefin−polar block copolymers are of interest due to their
polymerization technique and instead seeks to develop a
unique physical properties which arise from the covalent
methodology which merges multiple polymerization platforms
linkage of two or more distinct monomers in a block-type
using a single metal complex, accommodating polar and
architecture. The combination of polar and nonpolar proper-
nonpolar monomers.5
ties in a block copolymer can greatly tune important properties
An attractive approach that would be of broad interest is the
such as toughness, adhesion, surface properties, solvent
linking of coordination insertion and radical polymerization
resistance, etc.1 Specifically, the phase separation of block
techniques, which have been used separately to polymerize
copolymers allows for properties that arise from their
nonpolar and polar monomers with great control, respectively.
microstructure, whose application will boost the performance
An ideal candidate to achieve this duality is Brookhart’s
of already known plastics and offer avenues to unexplored
palladium(II) diimine complex, which has been well studied in
applications.2 However, the limited platforms available to
terms of coordination insertion polymerization,6 while also
generate these highly valued materials has led to a scarcity of
polyolefin−polar block copolymers and serve as the bottleneck
for future plastic development.3 In part, this can be attributed
Received: October 5, 2020
to the fundamental difference in reactivity between polar and
nonpolar monomers and has led to divergent synthetic
techniques optimized for each monomer class. These
approaches can now accommodate a few specific polar and
nonpolar monomers by tuning the tolerance and reactivity of
https://dx.doi.org/10.1021/jacs.0c10588
© XXXX American Chemical Society
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
A

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
having been reported to initiate radical polymerization.7
However, attempts to bring these two polymerization
techniques together have mostly been unsuccessful and
highlight the mechanistic obstacles associated with this
approach.7a−c In an attempt to copolymerize olefins and
acrylics with Brookhart catalysts, Espinet et al. sought to access
a dual-polymerization platform in which congruent coordina-
tion insertion and radical polymerization would generate
copolymers.7b In a similar approach, Ye et al. observed that a
mixture of homopolymers is obtained as a result of the
decoupled coordination insertion and radical mechanisms.7f
Monteil et al. have shown that a salicylaldiminato Ni(II)
catalyst performs both coordination insertion and radical
polymerization, yet the radical polymerization is only possible
following the irreversible reductive elimination of the Ni(II)
species to Ni(0).8 As such, radical polymerization is only
attainable from the deactivated catalysts which no longer
contain the polyolefin, preventing block copolymer formation.
Recently, a method to successfully combine coordination
insertion with radical polymerization, metal−organic insertion
light-initiated radical (MILRad) polymerization, has been
developed in our group, producing for the first time
polyolefin−polar block copolymers with a single Pd(II)
complex.7d However, this approach was limited to low-
molecular-weight olefin block segments, and the integrity of
the blocks remained a contentious facet of the work, as no
mechanistic understanding could explain the success of this
method.
We believe an understanding of the mechanism by which the
Brookhart-type complex can generate radicals will provide the
necessary insight needed for the generation of macroradical
initiators which are capable of block copolymer synthesis.
Herein, we report a comprehensive mechanism for block
copolymer formation based upon the isolation and character-
ization of key intermediates, combined with a detailed analysis
of the resultant polymers. Following our mechanistic findings,
we were able to produce new materials such as triblock
copolymers as well as higher molecular weight diblock
copolymers whose amorphous and crystalline nature can be
tuned.
synthesis is explored with various monomers such as 1-hexene,
ethylene, methyl (meth)acrylate, ethyl acrylate, and n-butyl
acrylate, using either a benzhydryl-derived ligand or a binuclear
palladium complex, which led to the generation of diblock
copolymers including a crystalline PE segment and amorphous
ABA triblock copolymers.
In part 1, the livingness of the coordination insertion
polymerization was investigated at room temperature to
determine the time frame in which the livingness of the
polymerization is maintained. In part 2, radical and spin
trapping experiments were performed to identify the palladium
complexes susceptible to Pd−C bond homolysis via EPR,
NMR, and MS analyses. In part 3, photolysis of a Pd−C bond
to generate a polyolefin macroradical was investigated as well
with radical and spin trapping experiments via EPR, NMR, and
MS analyses. Finally, a key intermediate is identified which
plays a critical role in linking coordination insertion to radical
polymerization. This mechanistic transition was successfully
applied to facilitate di- and triblock copolymer syntheses
including ethylene and α-olefins as well as acrylic and
meth(acrylic) monomers.
Part 1: Insertion Polymerization. Insertion polymer-
ization was used to polymerize α-olefins and ethylene and
operates through a chain-walking mechanism6a,b consisting of
three main processes beyond initiation: (1) chain propagation,
(2) metal migration along the polymer chain, and (3) chain
transfer.6d,9 For our approach, we hypothesized that the
palladium catalyst must be attached to the polymer chain for
successful block copolymer synthesis; thus, particular attention
was paid to the integrity of the catalyst during insertion
polymerization. The livingness of Brookhart catalysis has been
reported at low temperatures.9,10 However, radical polymer-
ization under these conditions is not ideal. As such, this work
explores conditions at room temperature which favor living
coordination insertion polymerization while being suitable for
radical polymerization. We envision the livingness of
coordination insertion polymerization depends on preventing
chain transfer, which leads to the decomposition of the
catalyst. To study the extent of chain transfer present in
coordination insertion polymerization at ambient temperature,
the evolution of molecular weight and conversion over time
1
RESULTS AND DISCUSSION
was monitored. While monitoring the reaction with H NMR
■
techniques, we observed that the polymerization continued to
occur in aliquots, and therefore calculated conversions would
be inaccurate. The conventional way to stop the polymer-
ization is to quench the reaction with Et3SiH. However, a
significant overlap of the ethyl groups of Et3SiH and the
The mechanism for block copolymer synthesis has been
divided into three sections (Figure 1). Part 1 focuses on the
insertion polymerization of 1-hexene and ethylene with a
“classic” palladium diimine at ambient temperatures (2a;
Figure 2). Part 2 explores the photoinitiated radical polymer-
ization of methyl (meth)acrylates. Part 3 investigates the
photoinitiated “switch” which allows for the synthesis block
copolymers. Subsequently, the scope of block copolymer
1
methyl groups of the polymers was observed in the H NMR
and led to inaccurate results. It is known that the addition of
methyl acrylate to Brookhart catalysts significantly retards the
rate of olefin insertion.6d,9 Excess methyl acrylate was then
adopted as an alternative additive to significantly slow the rate
of propagation, thereby avoiding experimental errors. As a
complementary technique to monitor conversion, we explored
the use of in situ FTIR as a more direct method (section 4.1 in
the Supporting Information).11 In situ FTIR was applied to
monitor real-time conversion with minimum invasion and
1
results are in agreement with H NMR of MA-doped aliquots
(Figures S9 and S11). To our knowledge, this is the first use of
this technique to monitor conversion for coordination
insertion polymerization.
Dichloromethane (DCM) and chlorobenzene (PhCl) have
been ubiquitously used in typical coordination insertion
Figure 1. Outline of the mechanistic study for polyolefin−polar block
copolymer synthesis in three separate parts.
B
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Figure 2. Coordination insertion polymerization of 1-hexene using catalyst 2a. Molecular weight (blue) and polydispersity (orange) vs conversion
graphs for polymerization reaction kinetics. (A) 1-Hexene polymerization in DCM. (B) 1-Hexene polymerization in PhCl. (C) SEC traces showing
successive chain extensions from poly(1-hexene).
polymerizations for polar and nonpolar monomers and hence
were chosen for these studies. For 1-hexene polymerization in
DCM, a linear increase in molecular weight vs conversion was
seen for the first 50 min of the polymerization (Figure 2a,
Table S2, and Figures S9−S12). After 50 min, deviation from
living behavior was observed, which is attributed to chain
transfer and catalyst decomposition. In addition to chain
transfer, isomerization was also detected over the course of the
polymerization (Figures S9 and S12), which had a negligible
effect on the chain growth of the polymer within the living
time frame.9 The behavior of the insertion polymerization in
PhCl shows trends dissimilar to those seen with DCM (Figure
2B, Table S3, and Figures S14−S17). For polymerization in
PhCl, a linear agreement between molecular weight and
conversion was observed over a longer time frame in
comparison to DCM. We attribute this observation to the
ability of PhCl to compete for the fourth coordination site of
palladium that is necessary for β-hydrogen elimination to occur
and, in turn, reduce the rate of chain transfer.12 Further
investigation of the livingness of the coordination insertion
polymerization at room temperature was established by chain
extension studies. 1-Hexene polymerization was started and
then stopped by the removal of the monomer under high
vacuum. Fresh monomer was subsequently added to reinitiate
the chain growth. The process was repeated twice and tracked
via in situ FTIR (Figure S18) and GPC analysis (Figure 2C
and Figure S19). The livingness was evidenced by the increase
in molecular weight of the pre-existing polymers while narrow
polydispersity was maintained (Đ < 1.1). Seeking to expand
the scope to more industrially relevant monomers such as
ethylene, we studied the livingness of ethylene polymerization
at room temperature. Employing a approach similar to that for
the 1-hexene study, the living time frame was determined to be
30 min in PhCl at room temperature (Table S10 and Figure
S35). Taken together, these results establish conditions which
are suitable for coordination insertion polymerization at room
temperature.
Part 2: Radical Polymerization. Conflicting reports exist
on the ability and inability of complex 2a to initiate radical
polymerization. Espinet et al. reported that 2a does not initiate
radical polymerization. However, Ye et al. reported that 2a
only initiates radical polymerization upon the addition of a
small amount of norbornene. Additional studies have shown
that light-promoted metal to ligand charge transfer is necessary
for radical formation in palladium(II) species which produce
alkyl radicals.7e,13 We sought to shed more light on the reactive
intermediates that play a role in the various steps of the radical
polymerization. Although Brookhart-type Pd(II) complexes are
reported to undergo radical polymerization, none of these
works have focused on the mechanism through which this
occurs.7,14 A better understanding of the mechanism by which
these complexes facilitate photoinitiated radical polymerization
is paramount in our investigation probing the transformation
from coordination insertion polymerization to radical polymer-
ization. A combination of radical trapping experiments with
TEMPO, spin trapping with EPR, and MS were used to
characterize intermediates responsible for radical generation.
These approaches together create a fundamental under-
standing of the photoinitiated radical polymerization involving
the Brookhart-type complexes.
Identity of Radical Species and Mechanism of Radical
Generation. In an effort to ascertain the identity of the
generated radical species, 2a was photolyzed in the presence of
TEMPO acting as a suitable radical trap (Figure 3A). The
resultant products from the reaction of 2a and TEMPO has
been fully characterized by NMR (Figure S37). As the reaction
progressed, NMR analysis showed the gradual disappearance
of the methyl peak attached to Pd(II) at 0.51 ppm and the
development of a singlet at 3.59 ppm which corresponds to
TEMPO-Me(2). Additionally, downfield shifts of isopropyl
methine resonances from 2.86−2.90 to 3.11−3.20 ppm
suggested the formation of a new Pd α-diimine species in
which asymmetry is introduced. MS data pointed toward a
three-membered-ring Pd−N−O species, which was later
C
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
of 8:1 (Figure S40). These adducts were isolated by column
chromatography and fully characterized by NMR (Figures S42
and S43). Surprisingly, adduct 2 was not observed in this
radical trapping experiment. On the basis of this outcome, one
conceivable pathway for the formation of 3 is through the
photoinduced Pd−Me bond homolysis, and the resultant Me
radical starts to propagate along one MA unit before being
quenched by TEMPO. However, previous mechanistic studies
revealed that migratory insertion of electron-deficient MA into
the Pd−C bond occurs rapidly in a 2,1-fashion to form a four-
coordinate intermediate which subsequently undergoes chain
walking and rearrangement to a stable six-membered
chelate.6c,d,15 Additionally, compound 4 rules out methyl
radicals having been the initiating radicals, as the dimeric
acrylate complex arises from insertion of two acrylate
monomers into the palladium complex, as observed by
Brookhart et al.6d This proposes a more plausible scenario
by which coordination insertion of MA into the complex
precedes Pd−C bond homolysis.6c The MA-inserted species
could cleave in the light to react with TEMPO immediately,
Figure 3. (A) Radical trapping of cation palladium diimine and
methyl radicals with TEMPO. (B) Radical trapping of cation
palladium diimine and methyl acrylate radicals with TEMPO.
confirmed by X-ray single-crystal diffraction analysis (Figure
4). The TEMPO-Me adduct 2 was isolated by column
1
yielding 3, which is the dominant product evidenced by H
NMR analysis. The minor product 4 reflects the dimerization
that occurs when a second MA inserts into the palladium
complex, which can then cleave in the light subsequently
1
followed by trapping with TEMPO (Scheme S1). H NMR
spectrum of the reaction prior to irradiation (t = 0 min) of the
reaction mixture with MA monomer showed quantitative
conversion of 2a to the 2a-MA-chelate complex, denoted by
the disappearance of the Pd−CH3 peak at 0.51 ppm and
formation of the Pd−CH2−CH2 peak as a pentet at 0.66 ppm
(Figure S40). This is in agreement with the mechanistic
studies reporting the fast insertion of MA into the cationic
palladium complex.6d This observation coupled with the
absence of adduct 2 speaks against CH3 radical generation in
the presence of MA and instead indicates that insertion of MA
precedes radical generation in the mechanism.
To confirm that insertion precedes radical generation, the
2a-MA-chelate complex was synthesized and then subjected to
photolysis in the presence of TEMPO (Figure 5). Surprisingly,
Figure 4. ORTEP diagram of Pd(II)-TEMPO adduct 1. Hydrogen
atoms and the BArF counterion are omitted for clarity. Selected
interatomic distances (Å) and angles (deg): Pd1−O1 1.978(1); Pd1−
N1 2.072(2); N1−O1 1.366 (2); Pd1−N2 2.043(1); Pd1−N3
2.081(2); N2−Pd1−O1 39.33(6); N1−Pd1−N2 154.03(6); N1−
Pd1−N3 128.75(6).
chromatography and characterized by 1H NMR and 13C
NMR (Figure S35). From these results, it was concluded that
Pd−C bond homolysis generates an intermediary diimine
Pd(I) radical which is then trapped by TEMPO, yielding
adduct 1. Simultaneously, the methyl radical is trapped by
TEMPO to form adduct 2.
Similar radical trapping experiments were carried out with
2a, MA, and TEMPO to determine the identity of the radical
initiator of polymerization (Figure 3B). The photolysis of 2a in
the presence of MA and TEMPO produced adduct 1 in a
similar yield. It is noteworthy that the rate of Pd−C bond
fragmentation in the presence of MA is faster in a more
coordinating solvent such as PhCl in comparison to DCM,
whereas the rate for the complex alone showed no significant
Figure 5. (A) Radical trapping of 2a-MA-chelate with TEMPO. (B)
Radical trapping of 2a-MA-chelate with TEMPO and MA.
minimal conversion (<1%) was observed in the light over 3 h,
in both the presence and absence of MA (Figure S44), which is
in conflict with the result obtained when 2a was used. This
suggests that the chelate complex alone is ineffective at
generating radicals to initiate a radical polymerization. As such,
we hypothesized that the chelate must be opened for the
effective homolysis of the Pd−C bond to take place. To assess
1
differences (Figures S38 and S41). ESI-MS and H NMR
analyses of the crude reaction mixture also suggested the
generation of a Me-MA-TEMPO adduct (3) and dimethyl
heptanedioate-TEMPO (DMH-TEMPO) adduct (4) in a ratio
D
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
this, the radical trapping experiment with the 2a-MA-chelate
was performed with 1 equiv of acetonitrile (ACN) as an
additive. The rate of Pd(II)-TEMPO formation in this reaction
was comparable to the rate of photolysis of the 2a complex
(Tables S12 and S13). To gain insight into anticipated
pathways for the radical generation, radical trapping experi-
ments were conducted with different amounts of ACN with
respect to the chelate complex. Kinetic studies showed that the
conversion rate of 2a-MA-chelate increased proportionately
with the amount of ACN added, indicating that the ancillary
ligand plays a crucial role in the photoinitiation step (Figures
S47 and S48). In terms of radical generation efficiency, the
quantum yield for the photolysis of complex 2a in the presence
of MA was determined to be much lower (Φ = 0.25) than that
of 2a in the absence of MA (Φ = 0.86) (section 5.8 in the
Supporting Information). Furthermore, the rate of photolysis
in the presence of MA (κ = 0.0039) is lower in comparison to
the photolysis without MA (κ = 0.0104) (Figures S38 and
S41). These observations taken together suggest that the lower
photolysis efficiency observed can be attributed to the
enhanced stability of the chelate complex compared to that
of just 2a.15 Special attention is drawn to these results, as they
confirm that a methyl radical is not the initiating species
responsible for radical polymerization under the typical
conditions; rather, an alkyl radical is generated through
photolysis of the open form of the chelate.
To further understand the role that ligand binding strength
has on Pd−C bond homolysis, the photolysis of the 2a-MA-
chelate complex under blue light was examined in the presence
of TEMPO and different coordinating ligands in the same time
frame of 18 h (Figure S42). Adding excess methyl propionate,
a nonpolymerizing analogue of MA, resulted in a 45−49%
conversion of the chelate complex to Pd(II)-TEMPO adduct
1. The lower conversion in comparison to MA (89%, Figure
S44) under the same conditions can be understood by the
difference of available coordinating sites in each analogous
compound. Additionally, when a stronger and sterically less
bulky donor such as acetonitrile was used, full conversion was
obtained within the same reaction conditions. These studies
highlight that the efficiency of homolysis is proportional to the
rate of chelate opening. Building on our hypothesis that chelate
opening is required for Pd−C bond homolysis to occur,
structures of adduct 3 and 4 from TEMPO trapping studies
would suggest that chain walking is also required so that radical
generation takes place at the position α to the carbonyl group.
To investigate this, the photolysis of complex 2a in the
presence of TEMPO and MMA was investigated (Figure 6). It
is known that the reaction of MMA with 2a proceeds slowly via
1,2-insertion to yield a five-membered-ring chelate.16 Due to
this 1,2-insertion of MMA, the chelate formed upon reacting
with 2a has no β-hydrogens and leads to a resting state which
is incapable of further chain walking. This unique chelation
process was utilized to evaluate the role of chain walking in
radical generation. The photolysis experiment’s crude 1H
NMR spectra showed the complete conversion of 2a into
adduct 1 and the MMA-inserted chelate complex 11 in a ratio
of 1.95:1 by the end of the reaction. The TEMPO-Me adduct 2
was also found and isolated from the reaction mixture. In
addition, the isolated MMA-inserted chelate complex 11 was
prepared and was subjected to TEMPO trapping experiments
in the presence and absence of MMA monomers. Interestingly,
no conversion was observed in either case (Figure S65). From
these studies, it is understood that the Pd−C bond of complex
Figure 6. Radical trapping of palladium and methyl radicals with
TEMPO in the presence of MMA.
2a, and not the 2a-MMA-chelate, breaks to form Pd(I) radical
species together with a methyl radical which initiates the
homopolymerization of MMA through free radical polymer-
ization. The importance of chain walking is solidified, as the
lack of β-hydrogens for the MMA chelate prevents access to
the carbon α to the carbonyl. The proposed mechanism
confirms the requisite of β-hydrogen in the chelate complex,
which facilitates chain walking to the labile intermediate
(Figure 7).
Electron paramagnetic resonance (EPR) spectroscopy in
combination with ESI-MS was used to directly observe and
confirm the presence and structure of the initiating free radicals
generated during photolysis of 2a with and without monomer
present. The advantage of this technique is the ability to detect
radicals at very low concentrations.17 Spin-trapping experi-
ments were performed with 2a and excess α-phenyl-N-tert-
butylnitrone (PBN) in PhCl (Figure S49). When the mixture
was irradiated with blue light, a distinct EPR resonance
corresponding to the nitroxyl radical was detected. Modeling
of the experimental spectrum suggests the reaction mixture
comprises a Me-PBN spin adduct with coupling constants of
aH = 3.43 G and aN = 14.92 G. FTMS was also performed for
the reaction, and the Me-PBN adduct was observed at [M +
H] = 192.1374 (Figure S52). Additionally, when this
experiment was conducted in the presence of MA, a similar
nitroxyl radical was the main EPR resonance observed and is
consistent with an alkyl-PBN spin adduct (Figure 8). Me-PBN
was not found though FTMS spectra, and this supports our
understanding that Me radicals are not generated in the
presence of MA. Spin trapping experiments were also
performed with the 2a-MA-chelate in the presence of PBN
and ACN. FTMS for this experiment showed the presence of
Me-MA-PBN at [M + H] = 278.1748 (Figure S54). The EPR
and ESI-MS data again confirm that the opening of the chelate
in blue light leads to the photolysis of the Pd−C bond, as
witnessed by the presence of the Me-MA-PBN species.
Throughout the EPR experiments, no spectral features
attributed to Pd(I) species were observed, which is similar to
previously reported EPR analyses of reactions that proceed
through Pd−C bond homolysis.18 This phenomenon may be
due to the poor nucleophilicity of the putative Pd center
E
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Figure 8. CW X-band (9.5 MHz, rt) EPR spectra of the trapped
radical from the irradiated reaction mixture of 2a in the presence of
MA and PBN. A nonirradiated reaction mixture is shown in the
control experiment.
radical polymerization was also investigated using NMR, MS,
and MALDI-Tof (section 5.10 in the Supporting Information).
Conventional free radical polymerization is usually conducted
at an elevated temperature that yields high-molecular-weight
polyacrylates. However, the chain growth can be significantly
hampered by increasing the amount of photoinitiator (which is
complex 2a herein) while lowering monomer concentration
and maintaining the low temperature (25 °C).19 As such, this
1
method produced a short PMA polymer (∼2000 g/mol). H
NMR and ESI-MS spectra of the reaction showed a mixture of
unreacted 2a-MA-chelate and PMA (Figures S61 and 62).
MALDI-ToF of the polymers reveals a series of peaks that
were separated by one MA unit (m/z 86.04). Each peak’s
isotopic distribution was found to be identical with the
simulated distribution of a 1:1 mixture of PMA with saturated
and unsaturated chain ends (Figure S63). These data, coupled
1
with the olefinic resonances observed in H NMR, suggests
that the termination pathway proceeds through a disproportio-
nation mechanism. This is in good agreement with previous
reports of Coote and Yamago for the radical polymerization of
MA.20 With an understanding of both the coordination
insertion polymerization along with our new insights into the
mechanism responsible for the radical polymerization, the
“switch” between theses platforms was then investigated.
Part 3: Photoinitiated “Switch”: Radical Generation
from the Polyolefin. Following insights gained from the
mechanistic studies of both polymerization pathways, the
mechanism by which an active radical can be generated at the
polyolefin chain end was investigated. It has been well
established that adding MA to the 1-hexene polymerization
results in the rapid formation of the six-membered-ring chelate
complex, which impedes further propagation.6c,d,21 On the
basis of previous studies discussed above, we hypothesized that
the opening of the chelate followed by photolysis of the Pd−C
bond produces a polyolefin macroradical, allowing for an
irreversible switch from insertion to radical polymerization. In
this section, we present experiments geared toward a better
understanding of radical generation from the polyolefin.
Synthesis and Characterization of Polyolefin Macro-
chelate, the Key Intermediate. In situ formation of the
macrochelate presents challenges for directly characterizing the
Figure 7. Proposed mechanism for the major pathway of radical
polymerization of MA using palladium diimine complexes.
radical in comparison to alkyl radicals and the relatively short
lifetime of the Pd(I) species.
These results together with the structures of isolated
compounds 3 strongly suggest the major mechanistic pathway
for light-initiated radical polymerization of MA, as shown in
Figure 7. Displacement of ACN in complex 2a followed by
rapid 2,1-insertion in an excess amount of MA yields 6.
Subsequent chain walking generates the stable chelate complex
5, which is not susceptible to fragmentation under blue light.
In the presence of ancillary ligands, 6 exhibits in equilibrium
with 5 via the classical β-hydride elimination/reinsertion
pathway. The intermediate 8 undergoes photodriven Pd−C
bond homolytic cleavage to generate the reactive radical 10,
which is able to facilitate the free radical polymerization of MA
(section 5.9 in the Supporting Information). The cationic
Pd(I) radical species is not stable and further decomposes to
form Pd black. The termination pathway of light-initiated
F
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
radical generation, and as such, developing novel synthetic
methods for these reaction intermediates was necessary. The
synthesis of the Pd-poly(1-hexene)-MA chelate is performed
by reacting catalyst 2a with 1-hexene and MA, followed by
careful removal of all volatiles to obtain a stable macrochelate
(section 6.1 in the Supporting Information). Characterization
of the macrochelate was performed using NMR (Figure S68−
suggested that a carbon-centered radical generation was
triggered by blue-light irradiation. The results from the EPR
experiments confirm the generation of alkyl radicals from the
macrochelates and strongly suggest that the alkyl radical is
formed in the light-induced “switch” process. Having
confirmed the generation of the alkyl radical, we next studied
the location of the bond homolysis in the macrochelate using
radical trapping methods.
1
S78) and ESI-MS analyses (Figure S67). H DOSY NMR
Radical Trapping Characterization of the “Switch”. As
it has already been shown that TEMPO is effective in capturing
the alkyl radical generated in the light cycle (Figure 4), similar
photolysis experiments were performed using the Pd-poly(1-
hexene)-MA and Pd-PE-MA macrochelates (sections 6.4 and
6.5 in the Supporting Information). ESI-MS shows a loss of the
palladium moiety (C28H40N2Pd, 510.22 Da), and the addition
of TEMPO (C9H18NO, 156.14 Da) accounts for an overall
loss in mass of 354.08 Da (Figures S69 and S93). This
observation suggests that the palladium chelate is formed and
attached to the polymer chain, and photolysis of the Pd−C
bond results in the formation of the polyolefin-TEMPO adduct
(Figure 10). From these results, the location of radical
confirmed the connectivity of the Pd species to the poly(1-
hexene) chains, indicating no unbound palladium catalysts
(Figure 9).
Figure 9. 1H DOSY NMR (600 MHz, TCE-d2, 298 K) spectra of the
Pd-poly(1-hexene)-MA chelate. The resonances corresponding to the
palladium complex and the polyolefin show similar diffusion behavior.
The diffusion behavior of the BArF is indicative of a weakly
coordinating counterion.
ESI-MS shows a narrow poly(1-hexene) distribution bearing
one MA unit and the palladium moiety, where each peak
exhibits a palladium isotopic pattern. Interestingly, 1-D and 2-
D NMR suggests a variety of conformations that exist for the
synthesized macrochelates, similar to the structure of the 2a-
MA-chelate. Pd-polyethylene(PE)-MA chelate samples were
prepared and characterized similarly to the Pd-poly(1-hexene)-
MA macrochelates (section 6.2 in the Supporting Informa-
tion). Pd-PE-MA chelate samples show a similar narrow
distribution bearing one MA unit and the palladium moiety
(Figure S81). Once we identified the structure of the
macrochelates, they were then used to probe the switch
mechanism that allows for the formation of block copolymers.
Spin Trapping Analysis of the “Switch”. First, we
investigated the identity of radical species generated from the
macrochelates using EPR studies established in the radical
section. As aforementioned, before the “switch”, a Pd-poly(1-
hexene)-MA chelate was formed and fully characterized. Next,
a macrochelate with a molecular weight of 2.8 kDa was
subjected to EPR experiments in the absence and presence of
MA monomers and ACN (Figure S90). A triplet of doublets
was observed in the recorded spectra, which is consistent with
a well-known PBN trapped alkyl radical with coupling
constants of aH = 2.79 G and aN = 14.65 G.22 In support of
the EPR results, the high mass region of the FTMS spectra
shows a distribution of poly(1-hexene)-PBN adducts differing
by one 1-hexene mass unit (Figure S91). In addition to these
observations, no signal was observed in the dark, which highly
Figure 10. Radical trapping experiments with a Pd-(1-hexene)-MA
chelate in the presence of TEMPO. Relative percentages of poly(1-
hexene)-TEMPO adducts were determined by quantitative 13C NMR.
generation along the polyolefin chains was probed via a series
1
of NMR experiments (1H, 13C, H−13C HSQC and HMBC,
1H−15N HMBC) (Figures S94−S99). 1H NMR analysis shows
resonances in the region from 4.15 to 4.30 ppm, which
corresponds to the methine proton α to the carbonyl where
the TEMPO is attached. The quantitative 13C NMR spectra of
the sample exhibit C−H resonances from 84.5 to 86.5 ppm,
which correspond to methine carbons attached to which are α
to the carbonyl (60%). Another C−H resonance is located at
82.0 ppm, which corresponds to methine carbons attached to
the oxygen of TEMPO which is β to the carbonyl (14%). C−
H2 resonances from 76.5 to 81.5 correspond to methylene
carbons where the TEMPO is attached at the end of the butyl
G
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
or methyl branch (26%). 1H−15N HMBC is also applicable to
these unique structures and was used to identify a three-bond
correlation between the polymeric hydrogen α to the nitrogen
through the oxygen of the TEMPO moiety, confirming their
connectivity (Figure 11). Additional 2-D NMR experiments
were employed to further confirm the identity of the TEMPO
adducts.
The TEMPO-trapped polymers are a result of the chain-
walking nature of the palladium, which allows the palladium to
chain walk to the α carbonyl when the chelate is opened. All
structures also contain only one TEMPO trapped unit per
chain, confirming that, for every polymer chain bearing the
catalyst, only one alkyl radical is produced. This explains why
diblock copolymers are selectively formed through this
pathway. The key mechanistic features of the polymerization
which allow for polyolefin macroradical generation are as
follows (Figure 12). First, the addition of methyl acrylate to a
growing polyolefin chain rapidly inserts and forms a Pd-
polyolefin-MA chelate. The opening of the chelate promotes
chain walking α to the carbonyl,14 where Pd−C bond
homolytic cleavage occurs under blue light, generating a
carbonyl-stabilized macroradical. To a lesser extent, chain
walking to the end of the nearest branch followed by Pd−C
bond homolysis generates other alkyl macroradicals. The
distribution of trapped alkyl radicals observed can be explained
by the resting state of the Pd(II)-alkyl complex during chain
walking, in which Pd-n-alkyl is more thermodynamically
favorable than Pd-iso-alkyl.23 The enhanced stability of the
chelate allows for the continuous generation of free radicals at
a low concentration, which is an essential requirement in free
radical polymerization techniques.24 The ability to initiate
radicals from a single site along a polyolefin chain was then
applied to the synthesis of block copolymers.
Scope of MILRad Polymerization: Polyolefin−Polar
Block Copolymer Synthesis and Characterization. The
main takeaway from the mechanistic investigations is
summarized as follows with regard to block copolymer
synthesis. Macroradical formation requires homolysis between
the Pd−C bond where the palladium is attached to the
polyolefin. Therefore, living conditions were used for the
insertion coordination polymerization, ensuring that each
polymer chain was bound to palladium. Conditions that
avoid decomposition of the Pd complex are necessary to
maintain access to the macroradicals. Furthermore, conditions
which prevent formation of Pd-hydride via chain transfer are
vital, as MA insertion and subsequent photolysis from the
hydride species can generate homopolymers. The implemen-
tation of these key mechanistic findings results in a robust
methodology that facilitates access to a variety of polyolefin−
1
Figure 11. (top) H−13C phase edited HSQC (600 MHz, TCE-d2,
298 K) of poly(1-hexene)-TEMPO adducts (methine shown in red,
methylene shown in blue). (bottom) H−15N gHMBC (600 MHz,
1
TCE-d2, 298 K) of poly(1-hexene)-TEMPO. Both spectra highlight
the connectivity between the polymer and the TEMPO moiety.
Figure 12. Proposed mechanism for polyolefin−polar block copolymer formation.
H
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
polar block copolymers. This section culminates in a broad
range of polyolefin−polar di- and triblock copolymers with
different compositions now available in light of the mechanistic
studies (Figure 13). As polyolefin-b-polyacrylate polymers
high conversion, and then the reaction mixture was quenched
with Et3SiH. The mass spectra revealed a main distribution
corresponding to PH-b-PMA block copolymers (Figure S113).
For example, the peak at m/z 2080 is assigned to block
copolymers with a total DP = 24 (Figure 14, left). Within this
Figure 14. Expansion of the MALDI-ToF spectra of (left) PH-b-MA
(Table 1, entry 1) and (right) PH-b-PEA (Table 1, entry 2), showing
the comonomer distribution for DPs 24 and 34.
cluster, PH11-b-MA13 and PH12-b-MA12 block copolymers with
a masses of 2082.5 and 2080.6 Da were found, respectively.
This small mass difference between block copolymer
compositions (Δ = 2 Da) within similar DPs results in a
significant signal overlap in the mass spectra (Figure S113b for
complete assignment).25 In order to obtain more resolved
spectra, methyl acrylate was replaced with ethyl acrylate to
create a higher difference in mass (Δ = 16 Da).26 In this
spectrum, a comonomer distribution of poly(1-hexene) and
poly(ethyl acrylate) is seen within the range for a given total
DP = 34 (Figure 14, right, and Figure S114).27 The larger Δ
value in this spectrum allows for better resolution that confirms
the block structure of the polymers formed. Additionally,
unsaturated and saturated chain ends were found in each peak
cluster (Figures S113 and S114). This is consistent with results
from MA homopolymerization in which termination under-
goes disproportionation. This method for characterizing
polyolefin−polar block copolymers is a powerful technique
that has not been previously explored for these polymers.
Size-Exclusion Chromatography (SEC) and Purification of
the Block Copolymers. Following short block copolymer
syntheses, larger block copolymers were synthesized in a one-
pot, sequential monomer addition manner with variations in
the size of the polyolefin block. On the basis of our results
from the insertion coordination polymerization, different
length poly(1-hexene) blocks were synthesized within the
living window for both PhCl and DCM by varying the time
while keeping the concentration of 2a constant. All polymer-
izations exhibited the same phenomenon throughout the
reaction, as the photolysis of the Pd−C bond of the opened-
form macrochelate generates macroradicals that initiate free
radical polymerization of MA generating block copolymers. As
evidenced from the GPC analysis of the crude reaction
mixture, we observed a mixture of block copolymers and
unreacted macrochelates (section 7 in the Supporting
Information). The generation of macroradicals during the
reaction produces palladium species that decompose to Pd
black. The buildup of Pd black, witnessed by the darkening of
the reaction mixture, prevents effective Pd−C bond photolysis
for other macrochelates, as light penetration is drastically
Figure 13. Types of di- and triblock copolymers via MILRad
polymerization described in this work (Table 1).
largely remain a rare and underexplored material platform, a
complete characterization of the new materials was accom-
plished by employing a combination of characterization
techniques such as MALDI-ToF, DOSY, SEC/GPC, small-
angle X-ray scattering (SAXS), dynamic light scattering (DLS),
transmission electron microscopy (TEM), and thermal
analysis.
Diblock Copolymers Containing Amorphous Poly(1-
hexene) (PH) Segments. Matrix-Assisted Laser Desorp-
tion/Ionization-Time of Flight (MALDI-ToF). Initially, short
polyolefin−polar block copolymers were synthesized so that
they could be characterized by MALDI-ToF, whose distribu-
tion would be indicative of block copolymer formation and
also provide evidence as to the possibility of chain transfer.
The Pd-poly(1-hexene)-MA chelate was employed for the
synthesis of low-molecular-weight PH-b-PMA block copoly-
mers. Photoinitiated radical polymerization of MA proceeded
for 3 h and was then quenched with Et3SiH. This time frame
was chosen so that the composition of the reaction mixture
would allow for characterization by MALDI-ToF. The spectra
of the resultant polymer show two distributions in which the
lower molecular weight distribution is the dominant species
and was identified as a saturated poly(1-hexene) bearing a
single MA unit (Figure S112). The origin of this low-
molecular-weight polymer is understood to be the quenched
product of the macrochelate. Unsaturated polyolefins, as a
result of the chain transfer pathway, were not observed. This
indicates that these conditions do not lead to the formation of
small molecule palladium chelates, and thus the generation of
PMA homopolymers is not possible under these conditions. In
a second example, MA was allowed to react for 24 h, reaching
I
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
h
Table 1. Selected Polyolefin-Polar Block Copolymers
a
b
c
Complex used for the syntheses: 2a. Complex used for the syntheses:2b activated in situ with 1.2 equiv of NaBArF. Complex used for the
d
syntheses: bis-2a-DHFC. Molecular weight and polydispersity index (Đ) were determined by GPC analysis with samples run in THF at 40 °C
calibrated to poly(methyl methacrylate) standards. Molecular weight and polydispersity index (Đ) were determined by GPC analysis with samples
run in CDCl3 at 40 °C calibrated to poly(methyl methacrylate) standards. Molecular weight and polydispersity index (Đ) were determined by
GPC analysis with samples run in TCB at 150 °C calibrated to polystyrene standards. Sunlight was used instead of blue light. Samples
correspond to polyolefin−polar block copolymers synthesized as described in the text with a variety of block compositions. Abbreviations: PH =
poly(1-hexene), PE = poly(ethylene), PMA = poly(methyl acrylate), PEA = poly(ethyl acrylate), PMMA = poly(methyl methacrylate), P(nBA) =
poly(n-butyl acrylate).
e
f
g
h
reduced throughout the reaction. To further confirm the
nature of the unreacted polyolefin, block copolymer
purification techniques were employed. Soxhlet extraction is
a common technique for separating polymers on the basis of
the variance in solubility exhibited between different polymers.
It has been reported that this method was successful in
separating a homopolymer mixture of polyethylene and
poly(methyl methacrylate).8b,28 Soxhlet extraction was in-
effective at separating the block copolymer from the unreacted
polyolefin, which is attributed to the aggregation properties of
the block copolymer in a selective solvent (Figure S102).
Dynamic light scattering (DLS) analysis reveals that
micellization of the block copolymer produces sub-micrometer
aggregates in both polar and nonpolar solvents which can
travel through the pores of the extraction thimble (6 μm)
(section 7.1.7 in the Supporting Information). If a mixture of
pure homopolymers were generated, this method would have
been successful. In light of this observation, alternative
methods were developed to separate the polymer mixture. A
centrifugation method was effective at removing unreacted
polyolefins (detailed and validated in section 7.1.1 in the
Supporting Information). This method takes advantage of the
partial solubility of the block copolymer in comparison to the
complete solubility of the polyolefin in nonpolar solvents at
room temperature. The block copolymers can be recovered at
high yield (∼80%) as pellets, whereas the supernatant contains
the totality of unreacted polyolefins and a minority of block
copolymers (Table S18 and Figure S103). 1H NMR analysis of
the separated polymers revealed that both spectra were absent
of olefinic resonances (5.0−6.0 ppm), highlighting that chain
transfer is absent under these synthetic conditions (Figure
S116). The absence of chain transfer rules out the generation
of Pd-hydride, which subsequently excludes a pathway for the
formation of homopolymers. As a result, PHs with different
molecular weights (11−32 kDa) were prepared, and
subsequent radical polymerization of MA yielded block
copolymers with a molecular weight range of 60−101 kDa
(Table 1, entries 3 and 4, and Table S18). As mentioned
above, the insertion of MMA into the Pd−alkyl bond gives a
complex inert toward light-induced radical polymerization.
Consequently, light-induced block copolymerization with
sequential addition of 1-hexene and MMA was not successful.
However, by using a Pd-poly(1-hexene)-MA chelate as an
intermediate, poly(1-hexene)-b-poly(methyl methacrylate)
(PH-b-PMMA) block copolymers were accessible (Table 1,
entry 5; detailed in section 7.1.3 in the Supporting
Information).
Small-Angle X-ray Scattering (SAXS). The purified block
copolymers were submitted for SAXS analysis, and all
polymers exhibited principal scattering peaks, confirming the
existence of block copolymer structures (section 7.1.6 in the
Supporting Information). The interplanar spacing (calculated
as d spacing = 2π/q*) changes are most likely attributed to
different molecular weights of each segment within the diblock
copolymers.
Thermal Properties. The purified block copolymers and
isolated polyolefins were subjected to differential scanning
calorimetry (DSC) measurements. The DSC traces of the
block copolymers exhibit two glass transition temperatures
(Tg), which are in the range of poly(1-hexene) (−60 °C,
broad) and poly(methyl acrylate) (11 °C), respectively (Figure
S119a, left). This is indicative of block copolymers containing
long block segments which are in heterogeneous phases.29 The
isolated polyolefin exhibits a single Tg value at −60 °C
(broad), indicative of a homogeneous phase (Figure S119a,
right).
Diblock Copolymers Containing Amorphous-PE or
Crystalline-PE Segment. To further confirm the scope of
J
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
block copolymer polymerization with other monomers, blocks
consisting of polyethylene-b-polyacrylate (PE-b-PMA and PE-
b-PnBA) were synthesized. PE-b-PMA was synthesized by first
conducting coordination insertion polymerization of an olefin
and then generating the chelate by addition of MA. Following
the synthesis of the macrochelate, MA monomers and 1.2
equiv of ACN were added in the presence of light to facilitate
Pd−C bond photolysis, which produced the final block
copolymers. The efficiency of light-induced generation of the
polyolefin macroradical was also demonstrated by utilizing
sunlight as opposed to blue LEDs (Table 1, entry 7). After 12
h irradiation by sunlight, a PE-b-PMA block copolymer with
Mn = 85000 g/mol was successfully obtained from its Pd-PE-
MA precursor. It should be noted that the conversion of MA
monomer was found to be slower than that when blue light
was used (Table S28). It is known that the classical Brookhart
catalyst 2a generates amorphous polyolefins as a result of the
chain-walking mechanism. Thus, in order to explore the
versatility of the Pd(II) diimine platform, catalyst 2b was used
to achieve semicrystalline PE blocks. Block copolymerization
of ethylene and n-butyl acrylate (nBA) were performed in a
one-pot, sequential addition fashion to generate semicrystalline
polyolefin−polar block copolymers (section 7.2.2 in the
Supporting Information). By variation of the amount of nBA
monomer, different P(nBA) lengths were achieved (Table 1,
entries 9 and 10). A comprehensive analysis by different
techniques such as SAXS, DSC, NMR, and GPC again
confirms the existence of block copolymer architectures for
these materials (section 7.2.4−7.2.6 in the Supporting
Information).
Figure 15. 1H DOSY NMR (600 MHz, TCE-d2, 298 K): (top)
mixture of unreacted PE precursor and PE-b-PMA; (bottom) purified
PE-b-PMA. The resonances corresponding to the PE block and the
PMA block show similar diffusion behavior.
1H Diffusion-Ordered NMR Spectroscopy (DOSY). For this
1
Aggregation Behavior. Amphiphilic block copolymers are
known for their ability to self-assemble into micelles in a
selective solvent (Figure 16). PE-b-P(nBA) (Table 1, entry 9)
study, PE-b-PMA (Table 1, entry 8) was selected and H
DOSY experiments were performed on both the crude sample
and purified sample in which unreacted PE was removed. The
2D spectra of the crude polymer mixture shows two distinct
resonances corresponding to the PE-b-PMA diblock copolymer
and unreacted Pd-PE-MA chelate, which exhibited two
diffusion coefficients (Figure 15, top). Following purification
of the sample, a single resonance was obtained where the
signals of PE (1.26−0.9 ppm) and PMA (3.65 ppm) were
aligned at the same diffusion coefficients, confirming a block
copolymer architecture (Figure 15, bottom).
Thermal Analysis. Similar to the PH-b-PMA materials,
amorphous PE-b-PMA diblocks synthesized using 2a exhibit
two glass transition temperatures (Tg), corresponding to its
homo polyethylene and homo polyMA segments, at −66 and
12 °C respectively (Figure S132). In contrast, due to its slow
chain-walking properties, 2b can generate PE with dramatically
lower degree of branching (20 B/1000 C) as opposed to its
classic version 2a (90 B/1000 C) (Figures S129−S130).30 As a
result, the PE-b-P(nBA) block copolymers synthesized using
2b exhibit melting points of more than 100 °C (Table 1,
entries 9 and 10). In comparison to the homo polyethylene
obtained from the same catalyst, a small reduction (1−3 °C) in
melting temperatures of these materials was observed.
However, the addition of the P(nBA) segment led to a
significant drop in crystallization temperature in the cooling
cycles (Figure S133). This phenomenon is usually observed in
crystalline−amorphous (C-A) diblock copolymers, which is
caused by the perturbation of amorphous blocks in the
nucleation, chain folding, and orientation of the crystallizable
segment.31
Figure 16. Micellization of PE-b-P(nBA) (Table 1, entry 9) block
copolymer in THF confirmed by DLS analysis and TEM imaging.
The average diameter was estimated to be ∼42 nm.
was found to create a clear solution at a concentration of 1
mg/mL in THF, and this sample was analyzed by both
dynamic light scattering (DLS) and transmission electron
microscopy (TEM) (Figure 16). Correlogram and size
distribution histograms can be found in section 7.2.7 in the
Supporting Information.
Triblock Copolymer PMA-b-PE-b-PMA. In comparison
to diblock copolymers, triblock copolymers exhibit a superior
property as compatibilizers for polymer blends and many other
applications.3a However, the preparation of triblock copoly-
mers combining PE and polar blocks remains a great challenge.
The common strategy to access these types of materials is
through telechelic polyolefins bearing suitable functional
groups for the subsequent polymerization.32 Mechanistic
K
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
insights into this study revealed that the Pd−C bond of the 2a-
MA-chelate complex can be homolyzed under blue light when
this is assisted by opening of the chelate by an ancillary ligand.
This chelate is also capable of polymerizing ethylene in a living
fashion under suitable conditions.10 With this in mind, we
developed a binuclear Pd(II) complex (bis-2a-DHFC) bearing
a 1,6-bis(acryloyloxy)-2,2,3,3,4,4,5,5-octafluorohexane linkage
(detailed synthesis and characterization can be found in
section 3.5 in the Supporting Information). This strong
electron-withdrawing linker was carefully selected to ensure
fast initiation and, as a result, a uniform chain growth from
both metal centers.6c This complex is stable for weeks at −30
°C under inert conditions even though its mono analogue, Pd-
fluorinated octyl acrylate (FOA) chelate complex, was found to
be unstable and decomposed with time and handling.6c The
structure of the complex was verified by single-crystal X-ray
diffraction analysis (Figure 17). The livingness of ethylene
Figure 18. (A) Synthesis of PMA-b-PE-b-PMA triblock copolymers
(Table 1, entry 11) and hydrolysis experiment, GPC traces of (B) the
triblocks after the light reaction and their PE precursor and (C)
before and after hydrolysis.
remove unreacted PE and exhibited a mono distribution over
triple-detection SEC/GPC analysis (Table 1, entry 11, and
Figure S139). The identity of the PMA-b-PE-b-PMA triblocks
was also evidenced through a cleavage experiment with DBU,
yielding a single trace for the resultant PE-b-PMA diblock
copolymers. It is noticeable that the molar ratio of ethylene
and MA repeating units remained unchanged before and after
the hydrolysis, which also points toward the uniform activity of
both palladium centers (Figure S140). Further GPC and NMR
data can be found in section 8.2 in the Supporting Information.
DSC, SAXS, and DLS data again confirm the block structure of
the materials (Figure S141).
Figure 17. ORTEP diagram of the bis-2a-DHFC complex. Hydrogen
atoms and BArF counterions are omitted for clarity.
polymerization catalyzed by bis-2a-DHFC was investigated
through kinetic studies (section 8.1 in the Supporting
Information). During the course of the reaction, the polymer-
ization exhibited a linear increase in molecular weight over
time while maintaining narrow polydispersity (Đ = 1.03−
1.11), which indicates a high level of controlled polymerization
with negligible chain transfer (Table S30 and Figure S137a,b).
To the best of our knowledge, this is one of the rare examples
of binuclear palladium catalysts that can achieve this narrow
molecular weight distribution (Đ > 1.46 for other catalysts).33
1H NMR analysis of the obtained PE in chloroform-d showed
the distinctive peak of COOCH2CF2 at 4.76 ppm, suggesting
the attachment of the fluorinated ester linkage. As evidence of
simultaneous chain growth, the obtained PE was subjected to a
hydrolysis experiment. The hydrolyzed products exhibit a
single peak whose molecular weight is half that of the parent
CONCLUSION
■
The development of next-generation materials will arise from
orthogonal platforms that can seamlessly generate polyolefin−
polar block copolymers, which stem from approaches that seek
to combine multiple polymerization techniques into a single
platform. Herein, we report a mechanism in which
coordination insertion and radical polymerization techniques
are bridged through the use of cationic diimine Pd(II)
complexes. The experimental results illustrate the interplay
among chain walking, chelation, and radical initiation for block
copolymer synthesis. Because chelates derived from methyl
acrylate insertion serve as a thermodynamic sink in
copolymerization with olefins, stable macroinitiators can be
prepared with tunable molecular weights in a facile manner.
The photochemically inactive chelate requires reversible
opening while also avoiding chain transfer, and it is this subtle
interplay that has been overlooked and has prevented
comparable systems from achieving block copolymers in a
similar manner. The mechanism of radical generation is now
understood to follow a series of steps in which opening of the
chelate is facilitated by a coordinating ligand first, and
subsequent chain walking allows access to the carbon α to
the carbonyl moiety. This bond is suitable for homolysis in
1
species, and a H NMR spectrum of the species showed the
disappearance of the COOCH2CF2 peak (Table S30 and
Figure S136). Having successfully identified the living
condition for ethylene polymerization with the bis-2a-DHFC
complex, we subsequently exploredtriblock formation. Using
the bis-2a-DHFC complex, ethylene polymerization was
performed for 2 h, and MA monomer was added to the
reaction mixture while the temperature was kept low (section
8.2 in the Supporting Information). The reaction was used for
block copolymerization and analysis of the generated macro-
1
chelate. H NMR analysis of the macrochelate shows a 1:2
ratio of the ester linkage to the diimine ligand, which indicates
the insertion of MA into Pd-PE chains on both sides (Figure
S138). Free radical polymerization of MA under blue light with
the addition of an ancillary ligand was performed, affording
triblock copolymers (Figure 18). The product obtained after
the light reaction was purified by the centrifugation method to
L
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
blue light and generates a polyolefin macroradical capable of
free radical polymerization of methyl acrylate, ethyl acrylate,
and methyl methacrylate. Radical trapping studies combined
with NMR, EPR, and MS have identified these intermediates
within the mechanism and have led to improvements in the
synthesis. Access to a broad range in molecular weights and
compositions for block copolymers has been demonstrated.
We have shown for the first time that this mechanism is
suitable for a one-pot synthesis of polyethylene-b-polyacrylate
block copolymers. Additionally, a binuclear palladium(II)
catalyst has been synthesized with living olefin polymerization
kinetics, and subsequent BAB triblock copolymers have been
prepared in a similar one-pot fashion with acrylate-olefin-
acrylate structures. With the expansion of the scope and
identification of key requirements for a successful reaction, a
new perspective toward triggering homolysis in chain-walking
catalysts with external stimuli can finally bridge the gap in
metal−organic catalysts to access amphiphilic block copoly-
mers.
Enkhjargal Tsogtgerel − Department of Chemistry, Center of
Excellence in Polymer Chemistry (CEPC), University of
Houston, Houston, Texas 77004, United States
Anthony P. Gies − The Dow Chemical Company, Lake
Jackson, Texas 77566, United States; orcid.org/0000-
0002-3558-593X
Evelyn Auyeung − The Dow Chemical Company, Lake
Jackson, Texas 77566, United States
Zhe Zhou − The Dow Chemical Company, Lake Jackson,
Texas 77566, United States; orcid.org/0000-0003-0869-
2112
Asim Maity − Department of Chemistry, Texas A&M
University, College Station, Texas 77843, United States;
orcid.org/0000-0002-5923-8596
Anuvab Das − Department of Chemistry, Texas A&M
University, College Station, Texas 77843, United States;
orcid.org/0000-0002-9344-4414
David C. Powers − Department of Chemistry, Texas A&M
University, College Station, Texas 77843, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c10588
ABBREVIATIONS
■
Abbreviations of related structures can be found in section 2 of
the Supporting Information.
Funding
Welch #H-E-0041, A-1907, CAREER-1848135.
Notes
ASSOCIATED CONTENT
■
sı
* Supporting Information
The authors declare no competing financial interest.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c10588.
ACKNOWLEDGMENTS
■
The authors thank the Robert A. Welch Foundation for
generous support of this research (#H-E-0041) through the
Center of Excellence for Polymer Research. A.K. acknowledges
the Center of Excellence in Polymer Chemistry Fellowship. We
thank Yu-Sheng Liu for his help with GPC samples. We are
grateful to Dr. Xiqu Wang for collecting diffraction data and
solving the X-ray structures. We thank Dr. Laurence J. Dangott
at the Protein Chemistry Lab of TAMU for MALDI-ToF
measurements. We thank Dr. Steve Swinnea for SAXS
measurements at UT Austin. We thank Dr. David Truong
for SAXS and DSC measurements at TAMU. We thank Rufeng
Li from the Cai Lab for FTMS at UH. We thank Professor
Olafs Daugulis and Professor Thomas Teets for helpful
discussions. We thank Dr. Scott Smith for his support with
NMR. We also thank the Dr. Randall Lee group for their
assistance and allowing us to use the DLS instrument for
particle analysis. D.C.P., TAMU, thanks the Welch foundation
(A-1907) and the National Science Foundation for support
(CAREER-1848135).
Experimental procedures, characterization of key inter-
mediates, and additional spectroscopic data (PDF)
Details of the crystal structure data and refinements
(PDF)
Crystallographic data (CIF)
Crystallographic data (CIF)
Crystallographic data (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
Dain B. Beezer − Department of Chemistry, Center of
Excellence in Polymer Chemistry (CEPC), University of
Houston, Houston, Texas 77004, United States;
Email: dbeezer@fisk.edu
Eva Harth − Department of Chemistry, Center of Excellence in
Polymer Chemistry (CEPC), University of Houston, Houston,
Texas 77004, United States; orcid.org/0000-0001-5553-
0365; Email: harth@uh.edu
REFERENCES
■
Authors
(1) Boffa, L. S.; Novak, B. M. Copolymerization of Polar Monomers
with Olefins Using Transition-Metal Complexes. Chem. Rev. 2000,
100 (4), 1479−1494.
Huong Dau − Department of Chemistry, Center of Excellence
in Polymer Chemistry (CEPC), University of Houston,
Houston, Texas 77004, United States
(2) (a) Mayes, A. M.; Olvera de la Cruz, M. Cylindrical versus
spherical micelle formation in block copolymer/homopolymer blends.
Macromolecules 1988, 21 (8), 2543−2547. (b) Lyatskaya, Y.;
Gersappe, D.; Gross, N. A.; Balazs, A. C. Designing Compatibilizers
To Reduce Interfacial Tension in Polymer Blends. J. Phys. Chem.
1996, 100 (5), 1449−1458. (c) Janert, P. K.; Schick, M. Phase
Behavior of Binary Homopolymer/Diblock Blends: Temperature and
Chain Length Dependence. Macromolecules 1998, 31 (4), 1109−
1113. (d) Rebizant, V.; Venet, A.-S.; Tournilhac, F.; Girard-Reydet,
E.; Navarro, C.; Pascault, J.-P.; Leibler, L. Chemistry and Mechanical
Properties of Epoxy-Based Thermosets Reinforced by Reactive and
Anthony Keyes − Department of Chemistry, Center of
Excellence in Polymer Chemistry (CEPC), University of
Houston, Houston, Texas 77004, United States
Hatice E. Basbug Alhan − Department of Chemistry, Center
of Excellence in Polymer Chemistry (CEPC), University of
Houston, Houston, Texas 77004, United States
Estela Ordonez − Department of Chemistry, Center of
Excellence in Polymer Chemistry (CEPC), University of
Houston, Houston, Texas 77004, United States
M
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Nonreactive SBMX Block Copolymers. Macromolecules 2004, 37 (21),
8017−8027. (e) Ruokolainen, J.; Mezzenga, R.; Fredrickson, G. H.;
Kramer, E. J.; Hustad, P. D.; Coates, G. W. Morphology and
Thermodynamic Behavior of Syndiotactic Polypropylene-Poly-
(ethylene-co-propylene) Block Polymers Prepared by Living Olefin
Polymerization. Macromolecules 2005, 38 (3), 851−860. (f) Robinson,
J. W.; Zhou, Y.; Qu, J.; Bays, J. T.; Cosimbescu, L. Highly branched
polyethylenes as lubricant viscosity and friction modifiers. React.
Funct. Polym. 2016, 109, 52−55. (g) Wang, F.; Lei, S.; Li, C.; Ou, J.;
Xue, M.; Li, W. Superhydrophobic Cu Mesh Combined with a
Superoleophilic Polyurethane Sponge for Oil Spill Adsorption and
Collection. Ind. Eng. Chem. Res. 2014, 53 (17), 7141−7148.
(3) (a) Ruzette, A. V.; Leibler, L. Block copolymers in tomorrow’s
plastics. Nat. Mater. 2005, 4 (1), 19−31. (b) Leone, A. K.; Dewyer, A.
L.; Kubo, T.; Zimmerman, P. M.; McNeil, A. J. Toward one-pot
olefin/thiophene block copolymers using an in situ ligand exchange. J.
Polym. Sci., Part A: Polym. Chem. 2019, 57 (14), 1601−1605.
(c) Souther, K. D.; Leone, A. K.; Vitek, A. K.; Palermo, E. F.;
LaPointe, A. M.; Coates, G. W.; Zimmerman, P. M.; McNeil, A. J.
Trials and tribulations of designing multitasking catalysts for olefin/
thiophene block copolymerizations. J. Polym. Sci., Part A: Polym.
Chem. 2018, 56 (1), 132−137.
(4) (a) Dommanget, C.; D’Agosto, F.; Monteil, V. Polymerization of
Ethylene through Reversible Addition-Fragmentation Chain Transfer
(RAFT). Angew. Chem., Int. Ed. 2014, 53 (26), 6683−6686.
(b) Wolpers, A.; Bergerbit, C.; Ebeling, B.; D’Agosto, F.; Monteil,
V. Aromatic Xanthates and Dithiocarbamates for the Polymerization
of Ethylene through Reversible Addition-Fragmentation Chain
Transfer (RAFT). Angew. Chem., Int. Ed. 2019, 58 (40), 14295−
14302. (c) Dai, S.; Chen, C. Palladium-Catalyzed Direct Synthesis of
Various Branched, Carboxylic Acid-Functionalized Polyolefins:
Characterization, Derivatization, and Properties. Macromolecules
2018, 51 (17), 6818−6824. (d) Chen, M.; Yang, B.; Chen, C.
Redox-Controlled Olefin (Co)Polymerization Catalyzed by Ferro-
cene-Bridged Phosphine-Sulfonate Palladium Complexes. Angew.
Chem., Int. Ed. 2015, 54 (51), 15520−15524. (e) Carrow, B. P.;
Nozaki, K. Synthesis of Functional Polyolefins Using Cationic
Bisphosphine Monoxide-Palladium Complexes. J. Am. Chem. Soc.
2012, 134 (21), 8802−8805. (f) Chai, Y.; Wang, L.; Liu, D.; Wang,
Z.; Run, M.; Cui, D. Polar-Group Activated Isospecific Coordination
Polymerization of ortho-Methoxystyrene: Effects of Central Metals
and Ligands. Chem. - Eur. J. 2019, 25 (8), 2043−2050.
(5) Keyes, A.; Basbug Alhan, H. E.; Ordonez, E.; Ha, U.; Beezer, D.
B.; Dau, H.; Liu, Y.-S.; Tsogtgerel, E.; Jones, G. R.; Harth, E. Olefins
and Vinyl Polar Monomers: Bridging the Gap for Next Generation
Materials. Angew. Chem., Int. Ed. 2019, 58 (36), 12370−12391.
(6) (a) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.;
Brookhart, M. Mechanistic Studies of Pd(II)-α-Diimine-Catalyzed
Olefin Polymerizations1. J. Am. Chem. Soc. 2000, 122 (28), 6686−
6700. (b) O’Connor, K. S.; Lamb, J. R.; Vaidya, T.; Keresztes, I.;
Klimovica, K.; LaPointe, A. M.; Daugulis, O.; Coates, G. W.
Understanding the Insertion Pathways and Chain Walking Mecha-
nisms of α-Diimine Nickel Catalysts for α-Olefin Polymerization: A
13C NMR Spectroscopic Investigation. Macromolecules 2017, 50
(18), 7010−7027. (c) Johnson, L. K.; Mecking, S.; Brookhart, M.
Copolymerization of Ethylene and Propylene with Functionalized
Vinyl Monomers by Palladium(II) Catalysts. J. Am. Chem. Soc. 1996,
118 (1), 267−268. (d) Mecking, S.; Johnson, L. K.; Wang, L.;
Brookhart, M. Mechanistic Studies of the Palladium-Catalyzed
Copolymerization of Ethylene and α-Olefins with Methyl Acrylate.
J. Am. Chem. Soc. 1998, 120 (5), 888−899.
Palladium(II) Complexes and Their Use in Ethene Homopolymeriza-
tion and Norbornene Copolymerizations. Organometallics 2010, 29
(14), 3160−3168. (d) Keyes, A.; Basbug Alhan, H. E.; Ha, U.; Liu, Y.-
S.; Smith, S. K.; Teets, T. S.; Beezer, D. B.; Harth, E. Light as a
Catalytic Switch for Block Copolymer Architectures: Metal-Organic
Insertion/Light Initiated Radical (MILRad) Polymerization. Macro-
molecules 2018, 51 (18), 7224−7232. (e) Keyes, A.; Dau, H.; Basbug
Alhan, H. E.; Ha, U.; Ordonez, E.; Jones, G. R.; Liu, Y.-S.; Tsogtgerel,
E.; Loftin, B.; Wen, Z.; Wu, J. I.; Beezer, D. B.; Harth, E. Metal-
organic insertion light initiated radical (MILRad) polymerization:
photo-initiated radical polymerization of vinyl polar monomers with
various palladium diimine catalysts. Polym. Chem. 2019, 10 (23),
3040−3047. (f) Xiang, P.; Ye, Z. Homo- and Co-polymerization of
norbornene and methyl acrylate with Pd-diimine catalysts. J.
Organomet. Chem. 2015, 798, 429−436.
(8) (a) Leblanc, A.; Grau, E.; Broyer, J.-P.; Boisson, C.; Spitz, R.;
Monteil, V. Homo- and Copolymerizations of (Meth)Acrylates with
Olefins (Styrene, Ethylene) Using Neutral Nickel Complexes: A Dual
Radical/Catalytic Pathway. Macromolecules 2011, 44 (9), 3293−3301.
̈
̈
(b) Olscher, F.; Gottker-Schnetmann, I.; Monteil, V.; Mecking, S.
Role of Radical Species in Salicylaldiminato Ni(II) Mediated Polymer
Chain Growth: A Case Study for the Migratory Insertion Polymer-
ization of Ethylene in the Presence of Methyl Methacrylate. J. Am.
Chem. Soc. 2015, 137 (46), 14819−14828.
(9) Gottfried, A. C.; Brookhart, M. Living and Block Copolymeriza-
tion of Ethylene and α-Olefins Using Palladium(II)-α-Diimine
Catalysts. Macromolecules 2003, 36 (9), 3085−3100.
(10) Gottfried, A. C.; Brookhart, M. Living Polymerization of
Ethylene Using Pd(II) α-Diimine Catalysts. Macromolecules 2001, 34
(5), 1140−1142.
(11) Haven, J. J.; Junkers, T. Online Monitoring of Polymerizations:
Current Status. Eur. J. Org. Chem. 2017, 2017 (44), 6474−6482.
(12) Dall’Anese, A.; Rosar, V.; Cusin, L.; Montini, T.; Balducci, G.;
D’Auria, I.; Pellecchia, C.; Fornasiero, P.; Felluga, F.; Milani, B.
Palladium-Catalyzed Ethylene/Methyl Acrylate Copolymerization:
Moving from the Acenaphthene to the Phenanthrene Skeleton of α-
Diimine Ligands. Organometallics 2019, 38 (19), 3498−3511.
(13) (a) Boisvert, L.; Denney, M. C.; Hanson, S. K.; Goldberg, K. I.
Insertion of Molecular Oxygen into a Palladium(II) Methyl Bond: A
Radical Chain Mechanism Involving Palladium(III) Intermediates. J.
Am. Chem. Soc. 2009, 131 (43), 15802−15814. (b) Smoll, K. A.;
Kaminsky, W.; Goldberg, K. I. Photolysis of Pincer-Ligated PdII-Me
Complexes in the Presence of Molecular Oxygen. Organometallics
2017, 36 (7), 1213−1216.
(14) Nagel, M.; Sen, A. Intermediacy of Radicals in Rearrangement
and Decomposition of Metal-Alkyl Species: Relevance to Metal-
Mediated Polymerization of Polar Vinyl Monomers. Organometallics
2006, 25 (20), 4722−4724.
(15) Philipp, D. M.; Muller, R. P.; Goddard, W. A.; Storer, J.;
McAdon, M.; Mullins, M. Computational Insights on the Challenges
for Polymerizing Polar Monomers. J. Am. Chem. Soc. 2002, 124 (34),
10198−10210.
(16) Borkar, S.; Yennawar, H.; Sen, A. Methacrylate Insertion into
Cationic Diimine Palladium(II)-Alkyl Complexes and the Synthesis of
Poly(alkene-block-alkene/carbon monoxide) Copolymers. Organo-
metallics 2007, 26 (19), 4711−4714.
(17) Dikanov, S. A.; Crofts, A. R., Electron Paramegnetic Resonance
Spectroscopy. In Handbook of Applied Solid State Spectroscopy; Vij, D.
R., Ed.; Springer US: Boston, MA, 2006; pp 97−149.
(18) (a) Ratushnyy, M.; Kvasovs, N.; Sarkar, S.; Gevorgyan, V.
Visible-Light-Induced Palladium-Catalyzed Generation of Aryl
Radicals from Aryl Triflates. Angew. Chem., Int. Ed. 2020, 59 (26),
10316−10320. (b) Huang, H.-M.; Koy, M.; Serrano, E.; Pfluger, P.
̈
M.; Schwarz, J. L.; Glorius, F. Catalytic radical generation of π-
allylpalladium complexes. Nat. Catal. 2020, 3 (4), 393−400.
(19) (a) Chen, Y.; Hu, Z.; Xu, D.; Yu, Y.; Tang, X.; Guo, H. Studies
of Free Radical Polymerization Initiated by Visible Light Photoredox
Catalysis. Macromol. Chem. Phys. 2015, 216 (10), 1055−1060. (b) Su,
W.-F., Radical Chain Polymerization. In Principles of Polymer Design
́
́
́
(7) (a) Albeniz, A. C.; Espinet, P.; Lopez-Fernandez, R. Polymer-
ization of Acrylates by Neutral Palladium Complexes. Isolation of
Complexes at the Initial Steps. Organometallics 2003, 22 (21), 4206−
́
́
́
4212. (b) Lopez-Fernandez, R.; Carrera, N.; Albeniz, A. C.; Espinet,
P. Dual Behavior of Cationic Palladium Pentafluorophenyl Complexes
as Catalysts for the Homopolymerization of Acrylates and of
Nonpolar Olefins. Organometallics 2009, 28 (17), 4996−5001.
(c) Chen, Y.; Mandal, S.; Sen, A. Synthesis of (N-O)-Ligated
N
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
and Synthesis; Springer Berlin Heidelberg: Berlin, Heidelberg, 2013;
pp 137−183.
(20) Nakamura, Y.; Lee, R.; Coote, M. L.; Yamago, S. Termination
Mechanism of the Radical Polymerization of Acrylates. Macromol.
Rapid Commun. 2016, 37 (6), 506−513.
(21) Chen, Z.; Brookhart, M. Exploring Ethylene/Polar Vinyl
Monomer Copolymerizations Using Ni and Pd α-Diimine Catalysts.
Acc. Chem. Res. 2018, 51 (8), 1831−1839.
́
̂
(22) Kermagoret, A.; Debuigne, A.; Jerome, C.; Detrembleur, C.
Precision design of ethylene- and polar-monomer-based copolymers
by organometallic-mediated radical polymerization. Nat. Chem. 2014,
6 (3), 179−187.
(23) Shultz, L. H.; Tempel, D. J.; Brookhart, M. Palladium(II) β-
Agostic Alkyl Cations and Alkyl Ethylene Complexes: Investigation of
Polymer Chain Isomerization Mechanisms. J. Am. Chem. Soc. 2001,
123 (47), 11539−11555.
(24) Meira, G. R.; Silveston, P. L. Polymerization Under
Modulation. In Periodic Operation of Chemical Reactors; Silveston, P.
L., Hudgins, R. R., Eds.; Butterworth-Heinemann: Oxford, 2013;
Chapter 7, pp 171−203.
(25) Wilczek-Vera, G.; Yu, Y.; Waddell, K.; Danis, P. O.; Eisenberg,
A. Detailed structural analysis of diblock copolymers by matrix-
assisted laser desorption/ionization time-of-flight mass spectrometry.
Rapid Commun. Mass Spectrom. 1999, 13 (9), 764−777.
(26) Crecelius, A. C.; Becer, C. R.; Knop, K.; Schubert, U. S. Block
length determination of the block copolymer mPEG-b-PS using
MALDI-TOF MS/MS. J. Polym. Sci., Part A: Polym. Chem. 2010, 48
(20), 4375−4384.
(27) Town, J. S.; Jones, G. R.; Haddleton, D. M. MALDI-LID-ToF/
ToF analysis of statistical and diblock polyacrylate copolymers. Polym.
Chem. 2018, 9 (37), 4631−4641.
(28) Desurmont, G.; Tokimitsu, T.; Yasuda, H. First Controlled
Block Copolymerizations of Higher 1-Olefins with Polar Monomers
Using Metallocene Type Single Component Lanthanide Initiators.
Macromolecules 2000, 33 (21), 7679−7681.
(29) Daimon, H.; Okitsu, H.; Kumanotani, J. Glass Transition
Behaviors of Random and Block Copolymers and Polymer Blends of
Styrene and Cyclododecyl Acrylate. I. Glass Transition Temperatures.
Polym. J. 1975, 7 (4), 460−466.
(30) Dai, S.; Sui, X.; Chen, C. Highly Robust Palladium(II) α-
Diimine Catalysts for Slow-Chain-Walking Polymerization of Ethyl-
ene and Copolymerization with Methyl Acrylate. Angew. Chem., Int.
Ed. 2015, 54 (34), 9948−9953.
(31) (a) Van Horn, R. M.; Steffen, M. R.; O’Connor, D. Recent
progress in block copolymer crystallization. Polym. Cryst. 2018, 1 (4),
No. e10039. (b) Nandan, B.; Hsu, J. Y.; Chen, H. L. Crystallization
Behavior of Crystalline-Amorphous Diblock Copolymers Consisting
of a Rubbery Amorphous Block. J. Macromol. Sci., Polym. Rev. 2006,
46 (2), 143−172. (c) Shiomi, T.; Takeshita, H.; Kawaguchi, H.;
Nagai, M.; Takenaka, K.; Miya, M. Crystallization and Structure
Formation of Block Copolymers Containing a Rubbery Amorphous
Component. Macromolecules 2002, 35 (21), 8056−8065.
(32) (a) Yan, T.; Guironnet, D. Amphiphilic triblock copolymers
containing polypropylene as the middle block. Angew. Chem., Int. Ed.
2020, DOI: 10.1002/anie.202009165. (b) Makio, H.; Fujita, T.
Synthesis of Chain-End Functionalized Polyolefins with a Bis-
(phenoxy imine) Titanium Catalyst. Macromol. Rapid Commun.
2007, 28 (6), 698−703.
(33) (a) Wang, F.; Chen, C. A continuing legend: the Brookhart-
type α-diimine nickel and palladium catalysts. Polym. Chem. 2019, 10
(19), 2354−2369. (b) Takano, S.; Takeuchi, D.; Osakada, K.;
Akamatsu, N.; Shishido, A. Dipalladium Catalyst for Olefin Polymer-
ization: Introduction of Acrylate Units into the Main Chain of
Branched Polyethylene. Angew. Chem., Int. Ed. 2014, 53 (35), 9246−
9250. (c) Wang, R.; Sui, X.; Pang, W.; Chen, C. Ethylene
Polymerization by Xanthene-Bridged Dinuclear α-Diimine NiII
Complexes. ChemCatChem 2016, 8 (2), 434−440. (d) Ye, J.; Ye, Z.
Living” Polymerization of Ethylene and 1-Hexene Using Novel
Binuclear Pd-Diimine Catalysts. Polymers 2017, 9 (12), 282−303.
O
https://dx.doi.org/10.1021/jacs.0c10588
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX