Isospecific Polymerization of Methyl Methacrylate by Intramolecular Rare-Earth Metal Based Lewis Pairs?

Article abstract of DOI:10.1002/cjoc.202000441

A series of cationic rare-earth aryloxide complexes, i.e., [LREOAr']⁺[B(C₆F₅)₄]^– (L = CH₃C(NAr)CHC(CH₃)(NCH(R)CH₂PPh₂); RE = Y, Lu; Ar' =2,6-tBu₂

Full text of DOI:10.1002/cjoc.202000441

Chinese Journal
of Chemistry
CJC
中 国 化 学 - An International Journal
Accepted Article
Title: Isospecific polymerization of methyl methacrylate by intramolecular rare-
earth metal based Lewis pairs
Authors: Yiqun Zhou, Shengjie Jiang, Xin Xu*
This manuscript has been accepted and appears as an Accepted Article online.
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Chinese Journal
of Chemistry
rare-earth, Lewis pair, tridentate ligand, polymerization, tacticity
Report
CJC
中
国 化 学 - An International Journal
Isospecific polymerization of methyl methacrylate by intramolecular
rare-earth metal based Lewis pairs
Yiqun Zhou, Shengjie Jiang, Xin Xu*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou
215123, P. R. China
Cite this paper: Chin. J. Chem. 2019, 37, XXX—XXX. DOI: 10.1002/cjoc.201900XXX
Summary of main observation and conclusion
A =
series of cationic rare-earth aryloxide complexes, i.e., [LREOAr']+[B(C6F5)4]- (L
CH3C(NAr)CHC(CH3)(NCH(R)CH2PPh2); RE = Y, Lu; Ar' = 2,6-tBu2-C6H3, 2,6-(PhCMe2)2-4-Me-C6H2; Ar = 2,6-iPr2-C6H3, 2,6-(Ph2CH)2-4-iPr-C6H2; R = H, CH3, iPr,
Ph), were prepared and applied to the Lewis pair polymerization of methyl methacrylate (MMA). The stereoregularity of the resulting PMMA was
significantly affected by the R substituent on the pendant arm of the tridentate NNP ligand, and was found to increase with increases in the steric
hindrance of R. When using a Ph group as R, the Y complex produced a highly isotactic polymer with an mm value of 95% and a Tg of 54.6 oC. In contrast,
the steric hindrance of the Ar and Ar' groups had no effect on the tacticity of the resulting polymer, presumably because these two substituents were
situated such that they pointed outward from the cyclic intermediates. Kinetics studies demonstrated that the polymerization was a first-order process
with regard to the monomer concentration prior to catalyst deactivation. End group analysis indicated that the polymerization was accompanied by two
possibly competing chain-termination side reactions that proceeded via intramolecular backbiting cyclization.
value of 91% using an Al/P Lewis pair at -78 oC.15 Several chiral
phosphines were also examined for use as Lewis bases in LPP as a
means of increasing stereoselectivity under mild conditions.
Background and Originality Content
Since the discovery of frustrated Lewis pairs (FLPs) in 2006 by
Stephan and coworkers,1 FLP chemistry has become one of the
However, these compounds only gave syndio-rich PMMA with rr
most promising research fields in main group chemistry.2,3 FLPs
values of 74-75%, with no enhancement compared with PMMA
typically comprise sterically bulky main group Lewis acids and
derived from achiral LPs. These results can likely be attributed to
bases and can react with a large number of small molecules via
binding of the Lewis base to the initiating chain end, which has a
synergistic acid/base interactions.4 This has led to the applications
negligible effect on the polymer tacticity when the polymer chain
of FLPs in various organic syntheses, such as hydrogenation,5
becomes sufficiently long. It is also noteworthy that there have
hydroamination,6 and C-H functionalization.7 Notably, FLPs can
been no reports concerning the synthesis of isotactic or iso-rich
also be used in the field of polymer synthesis. In a seminal work,
PMMA using LPP.
Chen and coworkers found that FLPs based on the main group
Recently, the replacement of the main group Lewis acid in FLP
alane, Al(C6F5)3, effectively promoted the polymerization of
chemistry by a transition-metal Lewis acid has emerged and
methyl methacrylate (MMA) and its cyclic analogues.8 A novel
shown some unique and promising results, because the Lewis
acidity of transition-metals is diverse and can be finely-tuned.16
Specifically, employing rare-earth (RE) metals as Lewis acids is
expected to provide new opportunities in both FLP and RE
chemistry.17 On this basis, our group previously designed a new
type of tridentate NNP ligand and prepared the corresponding
cationic RE aryloxide complexes (Scheme 1),18 which were found
to be active for the polymerization of MMA. The isolation of
active intermediates (Scheme 1) and end group analysis indicated
that this polymerization was initiated by an internal RE/P FLP-type
1,4-addition rather than the traditional and ubiquitous initiation
pathway in RE metal-catalyzed MMA polymerization, i.e., covalent
RE-E (E = H, C, N) bond insertion or single-electron transfer.19
conjugate-addition mechanism was proposed based on a FLP-type
1,4-addition as the initiating step. Stimulated by this research,
there has been significant development in the study of Lewis pairs
polymerization (LPP) over the past decade.9 As an example, the
scope of monomers has been expanded to include a variety of
polar alkenes, including Michael-type compounds and their
derivatives10 as well as heterocyclic esters.11 The
living/controlled12
and
complete
chemoselective
(co)polymerization13 have been successfully achieved through the
use of LPP. Furthermore, application of LPP as a platform for the
preparation of advanced materials have also realized.14 However,
despite these advances, there are still various challenges in the
field of LPP. One issue is that, although the stereoregularity of the
polymer backbone plays a crucial role in defining the properties of
the material, there have been few reports concerning the
synthesis of polymers with a high degree of stereospecificity via
LPP. Chen et al. demonstrated the highly stereospecific
polymerization of MMA to obtain syndiotactic PMMA with a rr
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Scheme 1. Cationic RE aryloxide complexes and corresponding cyclic
intermediates in LPP of MMA.
N
N
O
OMe
DIPP
N
P
N
RE
DIPP
RE
(MMA)
Ph
Ph
O
O
P
Ar'
O
Ar'
Ph
OMe
Ph
=
2-C
iPr 6H3
DIPP
2,6-
-
=
Ar'
tBu
H3
C6
2,6-
2
R
N
P
N
Modified site
Ar
RE
O
- counterions
[B(C6 5)4]
Ph
F
Ar'
Cations
with
Ph
However, the resulting polymers were found to be atactic. In
single-site metal-catalyzed MMA polymerization, the steric
hindrance of ancillary ligands usually exerts considerable
influence on the stereoregularity of the resulting polymer.20 Thus,
with the aim of achieving a high degree of stereospecificity, we
modified NNP ligand precursor and prepared new RE aryloxide
complexes (Scheme 1). A systematic study of polymerization
promoted by various RE complexes with different NNP ligands
indicated that isotactic PMMA could be obtained with an mm
value of up to 95%. The study reported herein assessed the
effects of the supporting ligands, the RE Lewis acids, and the
reaction conditions on the catalytic activity and on the
stereoregularity of the polymeric product.
Scheme 2. Preparation of the NNP ligand precursors 3.
Our previous work indicated that complexes containing Sc,
the smallest RE metal, showed significantly lower activity in the
LPP of MMA compared with the performance obtained from the
Y and Lu analogues.18 Thus, Sc complexes with new NNP ligands
were not prepared in this study. The target RE complexes were
synthesized using a three-step process based on our prior
research, as depicted in Scheme 3. The alkane elimination
reactions of the ligand precursors 3 with RE(CH2SiMe3)3(THF)2 (RE
= Y, Lu) afforded the RE dialkyl complexes 4b - 4d in moderate
yields under mild conditions. However, attempts to prepare
complex 4e-Y using this same method failed, likely due to the
presence of the bulky aryl group. Thus, the salt elimination
reaction was applied and gave the desired dialkyl complex 4e-Y in
a 70% isolated yield (for detail, see Supporting Information).
Treatment of the RE dialkyls 4 with one equivalent of
alkyl-substituted phenol in toluene generated the mixed
alkyl/aryloxide species 5. Notably, complexes 5b - 5d were
isolated as mixtures of two diastereomers, for which the
respective molar ratios were determined by 1H and 31P NMR
spectroscopy (see Supporting Information for details). This
outcome is attributed to the coexistence of the unsymmetrical RE
center and the chiral carbon at the ligand backbone.18,23 These
results suggest that partial racemization occurred during the
preparation of the ligand precursors. Finally, the target cationic
RE aryloxide complexes 6 were obtained by the protonolysis of 5
Results and Discussion
Synthesis and characterization of new tridentate NNP ligand
precursors and corresponding cationic RE complexes. Ligand
precursors were prepared by the condensation reactions of
enamines 1 with β-aminophosphines 2 in the presence of a
catalytic amount of p-toluenesulfonic acid (Scheme 2). The
starting materials 2b and 2c were accessed through
well-established protocols from naturally-occurring chiral amino
acids,21 while 2d was obtained from the chiral sulfinamide.22 To
achieve satisfactory yields, these condensation reactions
required relatively harsh conditions (>110 oC, several days).
Consequently, partial racemization of compounds 3 occurred, as
confirmed by further transformations (vide infra). In all cases, no
isomer from tautomerization of the resulting products was
observed in solution. Compound 3c was also characterized by
single crystal X-ray diffraction and the molecular structure is
provided in Supporting Information.
with
equimolar
amounts
of
the
Brønsted
acid
[PhNMe2H][B(C6F5)4]. Selected NMR spectroscopic data for
complexes 6 are presented in Table 1 for comparisons. Based on
the present 31P NMR spectroscopic observations together with
the results of our previous work, we propose that internal RE...P
interactions were present in complexes 6 in the solution state.24
RE complexes 4c-Lu and 5c-Y were also characterized by single
crystal X-ray diffraction. The molecular structure of complex 5c-Y
is depicted in Figure 1, while structure of 4c-Lu is provided in
Supporting Information.
R
NH2
R
NH
N
p-TsOH
Ar
+
HN
O
or
toluene
xylene
PPh2
Ar
PPh2
reflux
2
1
,
=
2-C
iPr 6H3
=
Ar
3a
3b
3c
),
R
H
iPr
Ph
3d
( )
CH
2,6-
(
),
,
(
),
(
3
=
=
R
H 3e
(
Ar
-4-iPr-C6H2
CH)2
2,6-(Ph2
)
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temperature (Table 2, entry 1). However, the resulting polymer
showed an isotacticity of 70% mm, in sharp contrast to our prior
research. Upon lowering the temperature to -30 oC, the
polymerization became much more rapid such that 90% of the
MMA was consumed after 5 h, and PMMA with an 77% mm was
obtained [molecular weight (Mn) = 8400 g/mol, polydispersity
(PDI) = 1.62; Table 2, entry 2]. However, the stereoregularity of
the resulting polymer was significantly reduced (mm = 57%)
when polymerization was performed at -40 oC, and the activity
was also lower such that only 65% MMA conversion was
observed after 12 h (Table 2, entry 3). PhF, PhBr, and toluene
were also investigated as solvents for the polymerization at -30
oC under the same conditions. These polymerizations achieved
83-91% monomer conversions after 12 h but only produced
isotactic-rich polymers (59% mm in PhF, 58% mm in PhBr, and
60% mm in toluene; Table 2, entries 4-6). Thus, complex 6c-Y
showed superior performance when polymerization was
conducted in PhCl at -30 oC, likely because the corresponding
cyclic, zwitterionic intermediate involved in the polymerization
was relatively stable under these conditions.
R
N
P
N
R
N
P
N
Ar
Ar
SiMe
RE
RE(CH2
-
3)3(THF)2
Ar'OH
SiMe4
RE
O
3
-
SiMe4
SiMe3
Ar'
SiMe3
SiMe3
Ph
Ph
Ph
Ph
4
5
N
N
R
Ar
F5
B(C6
F
[PhNMe2H][B(C6 5)4]
)
RE
4
-
-
PhNMe2
SiMe4
O
P
=
RE
Lu
Ph
Y,
=
Ar'
Ph
,
=
2-C
iPr 6H3 Ar'
2-C
tBu 6H3
2,6-
Ar
,
2,6-
=
R
H
6a
6b iPr 6c Ph 6d
CH
(
),
(
),
(
),
(
);
2-C
3
=
=
=
=
R H
Ar
Ar
CH)2-4-iPr-C6H2 Ar'
tBu 6H3
6e
);
,
,
2,6-(Ph2
2-C
2,6-
(
=
=
H
iPr 6H3 Ar'
-4-Me-C6H2
R
6f
( ).
,
,
2,6-
2,6-(PhCMe2)2
Scheme 3. The synthetic pathway to the cationic RE aryloxide complexes
6.
Table 2. Polymerization of MMA by using complex 6c-Y.a
(Insertion here)
A series of cationic RE aryloxide complexes based on
tridentate NNP ligands were subsequently examined in LPP of
MMA under the optimized conditions (2 mol% of catalyst
o
loading, -30 C, 0.4 mL of PhCl as a solvent, Table 3). Both 6a-Y
and 6a-Lu generated atactic PMMA while exhibiting moderate
activity (Table 3, entries 1 and 2). In contrast, the RE complexes
6b-Y and 6b-Lu, in which a methyl group was present on the
pendant arm, polymerized MMA to isotactic-rich PMMA (6b-Y:
67% mm, 6b-Lu: 50% mm; Table 3, entries 3 and 4). Complex
6c-Lu achieved a 60% monomer conversion after 12 h and
yielded a polymer with an isotacticity of 63% mm (Mn = 9300
g/mol, PDI = 1.31; Table 3, entry 5). The incorporation of a more
sterically hindered phenyl group on the pendant arm resulted in
the formation of highly isotactic PMMA (95% mm) but of a low
monomer conversion of 20% (Table 3, entry 6). The resulting
polymer had a glass transition temperature (Tg) of 54.6 oC (Figure
S88 in Supporting Information), which is consistent with the
reported values for it-PMMA.25 These observations indicate that
the substituent on the pendant arm of the NNP ligand had a
significant effect on the stereoregularity of the resulting polymer,
with stereoregularity decreasing in the order of Ph > iPr > Me >
H. In addition, the polymerization results were affected by the RE
metal ion and the Y complexes showed higher activity and gave
more stereoregular products than the Lu analogues. Increasing
the steric hindrance of the NAr (in the case of the 6e-Y) and OAr
groups (in the case of the 6f-Lu) did not improve the
stereoregularity of the polymeric product, but rather afforded
atactic PMMA (Table 3, entries 7 and 8). This most probably
occurred because these two Ar substituents extended outward
from the cyclic intermediate, and so had a negligible effect on
the stereoregularity during polymerization. The effects of the RE
catalyst and monomer concentrations on the polymerization
were also examined, and lower concentrations were found to
greatly improve the ability to regulate the stereospecificity of the
Figure 1. Molecular structure of complex 5c-Y. Hydrogen atoms are
omitted for clarity; displacement ellipsoids are drawn at the 30%
probability level. Selected bond lengths (Å) and angles (o): Y1-O1 2.107(3);
Y1-N1 2.346(4); Y1-N2 2.324(4); Y1-C49 2.396(6); Y1-P1 3.0650(12);
N1-C2 1.319(6); C2-C3 1.416(7); C3-C4 1.395(7); N2-C4 1.338(6); N2-C6
1.490(6); C6-C7 1.555(8); C6-C10 1.490(6); N1-Y1-N2 81.54(14);
O1-Y1-C49 109.86(17); N2-Y1-P1 64.77(10).
Table 1. Selected NMR spectroscopic data for cationic RE aryloxide
complexes 6.a
(Insertion here)
Polymerization of MMA by intramolecular rare-earth metal
based Lewis pairs. We first examined the polymerization of MMA
using 2 mol% of complex 6c-Y by varying both the reaction
temperature and solvent, and the results are summarized in
Table 2. The polymerization led to a monomer conversion of 43%
after 12 h when the reaction was conducted in PhCl at room
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polymer. As an example, complex 6c-Y produced highly isotactic
o
PMMA (mm = 90%, Mn = 7800 g/mol, PDI = 1.33, Tg = 56.2 C),
although with a sacrifice of the catalytic activity (12 h, 58% MMA
conversion; Table 3, entry 9).
Table 3. Polymerization of MMA by various cationic RE aryloxide
complexes.a
(Insertion here)
Kinetic and mechanistic studies of polymerization. To obtain
additional information concerning the RE-based LPP of MMA, we
examined the polymerization kinetics using the RE catalysts 6b-Y,
6c-Y, 6c-Lu, and 6d-Y with an initial MMA concentration of 2.5 M
in PhCl at -30 oC. As shown in Figure 2, all of four systems rapidly
initiated the polymerization but suffered from catalyst
deactivation after approximately 2 h, after which there were no
further
increases
in
MMA
conversion.
Plots
of
ln([MMA]0/[MMA]t]) vs. time (prior to catalyst deactivation) for
all trials generated good straight lines (Figure 3), indicating a
first-order dependence on the MMA concentration. Notably, this
type of polymerization kinetics is different with those observed
during the LPP of MMA with main-group Lewis acids, for which
zero-order kinetics are characteristic.9
Figure 3. Plots of the first-order kinetics of ln([MMA]0/[MMA]t]) vs. time
(min) for the polymerization of MMA by RE catalysts 6b-Y (●), 6c-Y (■),
6c-Lu (▲) and 6d-Y (▼) in PhCl at -30 oC, [MMA]0 = 2.5 M.
Figure 2. Plots of conversion (%) vs. time (min) for the polymerization of
MMA by RE catalysts 6b-Y (●), 6c-Y (■), 6c-Lu (▲) and 6d-Y (▼) in PhCl
at -30 oC, [MMA]0 = 2.5 M.
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Running title
Chin. J. Chem.
Figure 4. MALDI-TOF MS spectrum of the isolated PMMA produced by 5 mol% of 6c-Y at -30 oC in PhCl.
It is well-established that chain initiation in LPP is caused by
the cooperative activation of the monomer substrate by the
Lewis acid and base. In our previous work, we established that
the cationic RE aryloxide complex based on NNP ligand 3a
underwent an FLP-type RE/P 1,4-addition to conjugated carbonyl
compounds such as MMA, affording product with
a
nine-membered metallocyclic framework (Scheme 1, vide supra),
that was found to serve as the initiating species for the MMA
polymerization.18 This result is in contrast to the traditional
initiating process during polymerization catalyzed by RE
complexes, which involves σ-bond insertion or single-electron
transfer. To provide further evidence for this initiation
mechanism and to better understand the chain termination or
catalyst deactivation processes that occur during the
polymerization, we subsequently investigated the chain end
structure of the polymer. An oligomerization of 20 equivalents of
o
MMA using the 6c-Y at -30 C was performed and the resulting
PMMA was analyzed by MALDI-TOF MS spectrum. As can be seen
from Figure 4, a major series of ions were observed at m/z
intervals of 100.2, which corresponds to the mass of the MMA
Figure 5. Plot of m/z values from the MALDI-TOF MS spectrum vs. the
number of MMA repeating units (n).
monomer.26 A plot of the m/z values associated with this series
against the number of MMA repeating units gave a straight line
with a slope of 100.17 and an intercept of 781.9 (Figure 5). This
intercept value is not equal to the sum of the masses (513.3) of
H+ and the ligand precursor 3c, as was observed following
polymerization with 6a in our previous work, indicating a distinct
chain termination pathway. We propose that side reactions
occurred during the RE-based LPP of MMA and competed with
chain propagation, and that this phenomenon also resulted in
low activity in the present system during polymerization. Given
that the chain initiation end is believed to comprise the ligand
precursor 3c (with a mass of 512.3), the value of 269 for the
Chin. J. Chem. 2018, template
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residual segment obtained by subtracting this value from the
intercept would be expected to equal the mass of the chain
termination end. We assume that intramolecular nucleophilic
backbiting of the growing polymer at the adjacent ester group
occurred to generate a six-membered lactone chain end, as has
been reported to occur during the LPP of polar alkenes by
stereocontrol compared with those achieved using its Lu
analogue. Kinetics data confirmed that the current
polymerization exhibits
a first-order dependence on the
monomer concentration prior to catalyst deactivation, which is
different from the behavior previously reported for the Al-based
LPP of MMA. End group analysis indicated that the
polymerization was terminated by an unexpected backbiting
cyclization reaction, resulting in the low activity of this RE-based
LP system during MMA polymerization. This study provides a
comprehensive understanding of cationic RE aryloxide complexes
during the LPP of MMA and also represents the first example of
the isospecific polymerization of acrylate monomers by this
process.
Al-based systems.27 The sum of the masses of such
a
six-membered lactone moiety (169) and an MMA repeating unit
corresponds to the segment mass of 269. It should be noted here
that there is another possible pathway for intramolecular
backbiting cyclization that generates a β-ketoester chain end with
a mass of 269,27b but we cannot differentiate between these two
possible chain ends at present. Even so, based on the above
observations, we can assume that growth of the polymer chains
was hampered by the occurrence of the backbiting side reaction
during the late stage of polymerization. This process formed
either six-membered lactone or β-ketoester chain ends, and led
to complete catalyst deactivation. Overall, these data show that
the polymerization of MMA by cationic RE aryloxide complexes
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
based on tridentate NNP ligands is initiated by
a RE/P
Acknowledgement
1,4-addition and terminated by a backbiting cyclization reaction.
The stereospecific polymerization of MMA to give a highly
isotactic polymer is a remarkable feature of our RE-based Lewis
pair catalyst system. The triad analysis of the resulting polymer
stereomicrostructure by 1H NMR indicated that the stereocontrol
mechanism cannot be simply associated with either chain end
control (4[rr][mm]/[mr]2 = 1)25a-c,28 or enantiomorphic site control
(2[rr]/[mr] = 1)25a-d,29. This was further confirmed by the pentad
analysis of the polymers using quantitative 13C NMR (for more
details, see Supporting Information).25a,29a,30 On the basis of
polymerization results, it appears that the stereospecificity of the
product is greatly affected by the substituent on the ligand
pendant arm. In addition, the isotacticity of the resulting PMMA
is increased with increasing steric hindrance of the corresponding
substituent. Therefore, we tentatively propose that increasing
the steric effect of the cyclic chain propagation intermediate may
result in a slower rate of the subsequent conjugated addition
reaction compared with that of enolate isomerization, thus
render MMA coordination at the same site. This stereocontrol
mechanism has been previously suggested to occur during the
isospecific polymerization of MMA promoted by a chiral RE
metallocene amide.31
This work was supported by the National Natural Science
Foundation of China (21871204), 1000-Youth Talents Plan, and
PAPD.
References
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Conclusions
In summary, we synthesized a series of tridentate NNP ligand
frameworks and prepared the corresponding cationic RE
aryloxide complexes, which were systematically investigated in
LPP of MMA. The results clearly demonstrated that the
stereoregularity of the resulting PMMA was strongly affected by
the steric hindrance of the ligand motif, and that isospecific
polymerization could be realized by introducing substituents on
the pendant arm of the ligand. Remarkably, the isotacticity of the
PMMA was increased with increasing steric hindrance of the
substituent on the ligand backbone, achieving an mm value of up
to 95% when a phenyl group was introduced. The outcome of the
polymerization was also dependent on the nature of RE metal,
and an
Y complex showed higher activity and greater
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(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2019
Manuscript revised: XXXX, 2019
Manuscript accepted: XXXX, 2019
Accepted manuscript online: XXXX, 2019
Version of record online: XXXX, 2019
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Table 1. Selected NMR spectroscopic data for cationic RE aryloxide complexes 6.a
Complex
6ab
6b
6c
6d-Y
6e-Y
6f-Lu
1H
NCHR
3.22 (Y)
3.38 (Lu)
2.24 (Y)
2.26 (Lu)
4.98 (Y)
4.94 (Lu)
46.8 (Y)
45.7 (Lu)
28.8 (Y)
27.5 (Lu)
94.5 (Y)
101.5 (Lu)
-6.8 (Y)
6.8 (Lu)
-21.5
3.45 (Y)
3.60 (Lu)
2.32 (Y)
2.38 (Lu)
4.69 (Y)
4.84 (Lu)
53.9 (Y)
53.8 (Lu)
34.5 (Y)
34.0 (Lu)
89.2 (Y)
93.0 (Lu)
-10.9 (Y)
-1.8 (Lu)
-22.0
3.25 (Y)
3.34 (Lu)
2.68 (Y)
2.62 (Lu)
4.66 (Y)
4.81 (Lu)
64.5 (Y)
64.4 (Lu)
32.2 (Y)
31.2 (Lu)
88.0 (Y)
89.4 (Lu)
-13.2 (Y)
-4.4 (Lu)
-22.0
4.36
3.53
2.43
PCH2
γ-CH
NCHR
PCH2
γ-CH
2.65
4.95
62.1
36.8
94.7
-9.0
2.56
5.08
48.5
34.0
97.6
-10.3
-21.4
2.43
4.79
44.6
28.7
98.3
-1.1
13C{1H}
31P{1H}
31P{1H}c
-22.0
-21.5
aNMR in PPM. bRef. 18. c 31P{1H} of corresponding ligand precursor.
Table 2. Polymerization of MMA by using complex 6c-Y.a
c
Temp.
(oC)
Time
(h)
Conv.b
Mn
PDIc
mmd
(%)
mrd
(%)
rrd
Entry
Solvent
(%)
43
90
65
83
91
87
(g/mol)
4100
8400
5500
5700
6300
4600
(Mw/Mn)
(%)
1
2
3
4
5
6
25
PhCl
PhCl
12
5
1.93
1.62
1.47
1.30
1.29
70
77
57
59
58
60
18
13
21
18
17
20
12
10
22
23
25
20
-30
-40
-30
-30
-30
PhCl
12
12
12
12
PhF
PhBr
toluene
1.44
b
aConditions: Polymerizations were conducted in 0.4 mL of solvent, where n[6c-Y] = 20 μmol, [M]/[C] = 50/1. Monomer conversions were determined by 1H
NMR spectroscopy. cMn and PDI were determined by GPC in THF relative to PS standards. drr, mr, mm = polymer methyl triads were measured by 1H NMR
(PMMA).
Table 3. Polymerization of MMA by various cationic RE aryloxide complexes.a
c
Time
(h)
Conv.b
(%)
Mn
PDIc
mmd
(%)
mrd
(%)
rrd
Entry
Cat.
(g/mol)
(Mw/Mn)
(%)
1
2
3
4
5
6
7
8
9e
6a-Y
6a-Lu
6b-Y
6b-Lu
6c-Lu
6d-Y
6e-Y
12
12
12
12
12
24
12
8
71
54
68
43
60
20
83
93
58
7900
6900
6400
6100
9300
2800
5300
7300
7800
1.48
1.67
1.48
1.25
1.31
1.20
1.19
1.15
1.33
28
20
61
50
63
95
28
24
90
22
22
18
19
19
3
50
58
21
31
18
2
21
39
7
51
37
3
6f-Lu
6c-Y
24
aConditions: Polymerizations were conducted at -30 oC in 0.4 mL of PhCl, where n[cat.] = 20 μmol, [M]/[C] = 50/1. bMonomer conversions were determined
by 1H NMR spectroscopy. cMn and PDI were determined by GPC in THF relative to PS standards. drr, mr, mm = polymer methyl triads were measured by 1H
NMR (PMMA). ePolymerization was conducted in 0.8 mL of PhCl.
Chin. J. Chem. 2018, template
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Running title
Chin. J. Chem.
Entry for the Table of Contents
Page No.
Isospecific
polymerization
of
methyl
methacrylate by intramolecular rare-earth
metal based Lewis pairs
RE P
intramolecular
/
Lewis
Ar
pairs
O
N
N
R
O
O
RE
OMe
OMe
to 95%
O
OMe
P
Ph
Ar'
mm
Ph
up
=
R
Me, iPr, Ph
Cationic rare-earth aryloxide complexes based on tridentate NNP ligands are prepared and applied
for RE/P Lewis pair polymerization of methyl methacrylate, which afford isotactic polymers when
using ligands with substituent on the pendant arm.
Yiqun Zhou, Shengjie Jiang, Xin Xu*
Chin. J. Chem. 2018, template
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