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Development of Group 3 Catalysts for Alternating Copolymerization of Ethylene and Styrene Derivatives Shihui Li, Dongtao Liu, Zichuan Wang, and Dongmei Cui ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00885 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018
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ACS Catalysis
Development of Group 3 Catalysts for Alternating Copolymerization of Ethylene and Styrene Derivatives Shihui Li,† Dongtao Liu, † Zichuan Wang†,‡ and Dongmei Cui*†. †
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People′s Republic of China; ‡
University of Chinese Academy of Sciences, Changchun Branch, Changchun 130022, People′s
Republic of China.
ABSTRACT: Alternating copolymers have the most clear and defined microstructures among man-made polymers, having been promising building blocks to access synthetic polymers able to mimic biomaterials. The most successful approaches are employing donor-accepter monomer couples, enantiomers with different substituents as well as the specially designed cyclic monomers containing various units through ionic and living radical polymerizations. Herein we report the catalytic behaviors of rare-earth metal based catalyst systems towards the direct copolymerizations of ethylene with a series of unmasked polar styrenes and non-polar styrenes. For the copolymerization of ethylene with para-methoxystyrene, the pyridyl side-armed fluorenyl supported yttrium catalyst was inert, while its scandium analogue displayed a moderate activity to give a random copolymer; and the half-Sandwich fluorenyl scandium catalyst provided a gel product. In contrast, the methyl substituted N-heterocyclic-carbene (NHC) sidearmed fluorenyl scandium catalyst showed the highest activity, 3.19 × 105 g molSc-1 h-1, which was ten times higher than its analogue bearing the steric bulky trimethylphenyl substituted NHC fluorenyl ligand, although it couldn’t initiate any polar styrene homopolymerization. The ACS Paragon Plus Environment
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catalytic performance was extended to the other polar styrenes, such as meta-methoxystyrenes, 6-methoxy-2-vinylnaphthalene, para-methylthiostyrene, diphenyl(4-vinylphenyl)phosphine and para-(N,N-diethylamino)styrene. All the resultant copolymers are constituted by pseudoalternating microstructure despite of polymerization conditions. In particular, when para-(N,Ndimethylamino)styrene was used as the comonomer, perfect alternating product was generated with an as high as 83% comonomer conversion. The relationships among the structural factors and electronics of the precursors and their catalytic performances and the resultant copolymer compositions and the sequence distributions, were established.
KEYWORDS: ethylene, polar styrene, rare-earth metal, Alternating copolymerization, sequence-regulated polymer. Introduction Natural materials exert complex and diverse functionalities, which are attributed to their precisely regulated composition and sequences of the primary structures that determine further assembly and final properties.1 Comparatively, synthetic catalysts can’t tune the microstructures as precisely as enzymes do, thus, to synthesize sequence-controlled polymers has been a chased research target of chemists. In the past decades, by means of modern polymer synthetic methods of anionic living polymerizations,2 controlled radical polymerizations3 and ring-opening metathesis polymerization,4 in combination with “click reaction”, Suzuki coupling etc. organic reactions,5-8 many acrylates and cyclic esters based polar polymers with random, gradient, block as well as topological 1D or 2D primary structures have been prepared.9 Whilst the discoveries of the oscillating catalysts,10 living coordinative chain transfer polymerization (LCCTP),11,12 the degenerative transfer13,14 as well as the “chain-shuttling” polymerizations,15-18 allow to access 2 ACS Paragon Plus Environment
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ACS Catalysis
many olefins based copolymers bearing regio- and stereo multi-block sequences through the coordination-insertion
mechanism.
Among
the
man-made
macromolecules
alternating
copolymers are special, because they bear the most clear and defined microstructures19. Moreover, they are usually prepared from known monomer pairs bearing different functionalities, thus have totally different properties from the corresponding homopolymers, which open interesting perspectives for mimicking natural materials20. However, the alternating copolymerization has been far from success, since it relies on much higher cross-propagation rates than self-growth rates of both monomers. Therefore, monomer pairs able to perform alternating copolymerization are very limited to those bearing rather different Q-e values such as the electron-donating styrenic derivatives and the electron-withdrawing maleic anhydrides through the classic radical mechanism. By means of control radical polymerization (CRP), polymers bearing polystyrenic backbones regularly inserted by maleic imides have been achieved (Chart 1, I)21. These donor-acceptor AB repetitive sequenced or pre-programmed polymers may carry biology functionality and intelligent information to mimic natural materials22. Choosing a monomer pair with one being low active or inert, is another commonly accepted strategy to reach alternating copolymerization between olefinic monomers. The nonpolymerizable
1,1-diphenylethylene
is
selected
to
perform
the
anionic
alternating
copolymerization with other monomers (Chart 1, II)23. Similarly, the cationic copolymerization of para-methoxystyrene and 4-hydoxy-3-methoxy-β-methylstyrene affords an alternating copolymer (Chart 1, III)24. An ethylene-propene copolymer with the alternating content of 83% has been synthesized via coordination mechanism by using a metallocene catalyst25. Ethylenecycloolefin copolymers with high alternating sequences (comonomer content > 47 mol%)26,27 have also been successfully obtained at the presence of excess amount of the low active
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cycloolefins (Chart 1, IV and V). Ring-opening polymerization of enantiomeric lactones (Chart 1, VI),28 metal template monomers or initiators containing a built-in recognition site (Chart 1, VII),29,30 and macrocyclic ring-opening metathesis polymerization,31,32 are also employed to produce polymers with the alternating arrangement of the side functional groups. On contrary, the alternating copolymerization of ethylene with polar monomers has still been a bare research area, although their copolymerizations have been extensively investigated.33 Because the employed early transition metal catalysts are strong oxophilic and easily poisoned by polar groups, thus, coordination/insertion of a polar monomer facilely arouses termination, resulting in low polar monomer incorporations even though they are masked34.
Chart 1. Representative comonomer pairs for alternating copolymerization. Polyethylene and polystyrene are two highly produced and widely applied plastics, and their chemical and physical properties are mutual supplementary. The significance of ethylene (E) and styrene (St) copolymerization has never been overlooked,35 neither to say their alternating copolymerization that gives a completely new material. However, most of the catalysts employed in the copolymerization of E and α-olefins can hardly catalyze E-St copolymerization, owing to the rather different nature of the active species (Ti4+ for E and Ti3+ for St) and reactivity ratios36. The most effective catalysts for E-St copolymerization are limited to the Ti-CGC catalysts and the half-Sandwich scandium catalysts, which can give pseudo-random and multi-block E-St 4 ACS Paragon Plus Environment
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ACS Catalysis
copolymers35,37-42. To date, no catalysts have been reported able to catalyze the copolymerization of E and unprotected polar styrenes35a. Recently, we found that polar groups didn’t always poison the active species, and the homoand co-polymerization of methoxystyrenes has been realized by using the properly designed rareearth metal catalysts43-45. Herein, we report a strategy to achieve alternating copolymerization of E with a series of non-polar and polar styrenes by using rare-earth metal precursors 1-5 (Chart 2). For the first time, the pseudo-, and perfect alternating copolymers of E and polar styrenes have been synthesized with distinguished activities, high molecular weights and high comonomer conversions. And the nature behind is revealed, which is the concert effects of the electronics and sterics of the ligands as well as the interactions of the penultimate chain end and/or the polar group with the active central metal. These results are proved by fully characterizing the microstructures of the isolated copolymers and investigating the reactivity ratios of the monomer couples. Results and Discussion Ethylene and para-methoxystyrene copolymerization. The copolymerization of E and paramethoxystyrene (pMOS) was first tested by using the catalyst system consisted of the pyridyl side-armed fluorenyl yttrium complex 1 and coactivators [PhMe2NH][B(C6F5)4] and AliBu3, which is a highly active and syndioselective catalytic system for pMOS homopolymerization44. However, only a trace amount of homopolymer Poly(pMOS) was isolated (Table 1, entry 1), probably owing to the extremely low activity of 1 for E homopolymerization (Table S1, entry 1). A strong Lewis acidic scandium analogue 2 was employed, as expected, which showed moderate activity for E polymerization (1.7×105 g molSc-1 h-1, Table S1, entry 2) but a slightly lower 5 ACS Paragon Plus Environment
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activity for pMOS homopolymerization as compared to 144. To our delight, system 2 catalyzed E-pMOS copolymerization with a moderate activity of 0.86 ×105 g molSc-1 h-1 to give a copolymer containing 41.3 mol% of pMOS units (Table 1, entry 2). The 1H and
13
C NMR
spectrum analyses revealed that the obtained E-pMOS copolymer (soluble in CHCl3) possesses random microstructure containing Poly(pMOS) segments (Figures S4-S6). This suggests that precursor 2 has more opening coordination sphere to allow continuous insertion of the steric pMOS. Switching to the half-Sandwich complex 3, a highly active catalyst system for E polymerization (5.50×105 g molSc-1 h-1, Table S1, entry 3), only trace amount of gel-like product (insoluble in both CHCl3 and THF) was isolated (Table 1, entry 3). Complex 3, without electron donating side-arm in the ligand, is more electron deficient as compared to complex 2, thus, the interaction of pMOS with the active metal center becomes much stronger, which means pMOS steadily occupies the coordination site to stop E coordination/propagation. Moreover, 3 might facilely abstract the ortho phenyl proton of methoxy substituent in pMOS, which leads to step polymerization along with the addition polymerization as the previously reported pMOS homopolymerization with the analogous scandium precursor46,47. As a result, a copolymer with complicated (topological or cross-linked) structures was obtained. Then, complex 4 bearing the fluorenyl ligand with a bulky carbene side-arm, that is less electron-donating than the pyridyl side-arm in 2,48 was employed firstly to catalyze E homopolymerization. The activity of 3.35×105 g molSc-1 h-1 is reasonably higher than system 2 but lower than system 3 (Table S1, entries 2-4). Unfortunately, system 4 was inert towards pMOS homopolymerization (Table 1, entry 4). To our surprise, the copolymerization indeed took place to afford a soluble copolymer albeit in a low activity (0.23 × 105 g molSc-1 h-1) (Table 1, entry 5).
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ACS Catalysis
Chart 2. Rare-earth metal dialkyl precursors 1-5. Table 1. Copolymerization of E with pMOS by rare-earth metal precursors 1-5 a.
Entry
Cat.
pMOS
E
(mmol) (atm)
Temp Time
Yield
(oC)
(min)
(g)
Act.b
Conv.
Incorp.c
Mnd
(%)
(mol%)
(104)
Mw/Mnd
T ge (oC)
1
1
3
4
70
15
0.017
0.07
-
100
-
-
-
2
2
3
4
70
10
0.14
0.86
26
41.3
0.52
1.64
24.5
3
3
3
4
70
10
0.012
0.07
-
-
-
-
-
4
4
3
0
70
10
trace
-
-
-
-
-
-
5
4
3
4
70
10
0.04
0.23
5.4
20.7
0.69
1.62
-
6
5
3
4
70
10
0.41
2.48
85
49.1
2.10
1.90
34.3
7
5
4
4
70
10
0.49
2.93
75
49.2
2.65
1.94
37.8
8
5
5
4
70
10
0.49
2.95
60
49.3
2.98
1.91
38.8
9
5
6
4
70
10
0.53
3.19
54
49.9
3.55
1.84
39.6
10
5
3
1
70
20
0.18
0.52
34
49.8
0.81
1.49
34.6
11
5
3
2
70
10
0.23
1.37
47
49.5
0.57
2.11
32.2
12
5
3
6
70
6
0.36
3.62
73
46.8
1.07
1.68
34.2
13
5
3
4
50
10
0.37
2.21
74
47.8
1.05
1.64
32.4
14
5
3
4
60
10
0.41
2.43
82
47.9
1.14
1.84
34.6
15
5
3
4
80
6
0.30
2.98
60
48.0
0.78
1.95
30.9
16
5
3
4
90
6
0.19
1.89
39
49.2
0.47
2.55
31.3
[a] Conditions: toluene, 3 mL; Ln, 10 µmol; [Ln]:[PhMe2NH][B(C6F5)4)]:[AliBu3] = 1:1:20 (mol/mol/mol). [b] Given in 105 g molLn-1 h-1. [c] Styrene derivative incorporation
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was determined by 1H NMR in CDCl3 at 25 oC. [d] Determined by GPC in THF at 40 oC against polystyrene standard. [e] Determined by DSC.
Figure 1. X-ray crystal structures and bite angles of complexes 4 and 5. The 1H and
13
C NMR spectra of the resultant copolymer show the content of pMOS units is
low (20.7 mol%) and no consecutive pMOS sequences are found. This gave us a hint that the low activity of complex 4 probably stemmed from the crowded coordination sphere around the active metal center, which impeded the continuous coordination-insertion of the bulky pMOS. Thus, a precursor bearing a ligand with the similar electron-donating carbene side-arm as that in 4 but less hindered is anticipated to facilitate the copolymerization. Then, we designed complex 5 containing the smaller methyl substituent on the carbene moiety (1H and 13C NMR spectra of 5 are shown in Figures S1 and S2). X-ray diffraction analysis reveals that complex 5 (Figure S3) is a structural analogue of complex 4 but possesses a more opening coordination sphere around the metal center as shown in Figure 1. This is consistent with the smaller bite angle of Flucent-Sc-C being 100.1° in 5 than 105.5° in 4, which might facilitate the coordination/insertion of the bulky styrenic momomers. Strikingly, the system based on precursor 5 successfully catalyzed the copolymerization of E and pMOS with an activity over 10 times higher than that of analogous 4. Moreover, the incorporation ratio of pMOS reached up to 49.1 mol% together with a very high 8 ACS Paragon Plus Environment
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ACS Catalysis
pMOS conversion (85%, Table 1, entry 6). If keeping E pressure constantly while boosting pMOS concentration from 1.0 mol/L to 1.7 mol/L, the activity was not deteriorated but slightly increased from 2.48 × 105 to 3.19 × 105 g molSc-1 h-1 (Table 1, entries 6-9). These results are in sharp contrast to the copolymerizations of E with other polar monomers catalyzed by groups 4 and 10 transition metal catalysts, in which the activity is often depressed by increasing the polar monomer concentration49-56. Meanwhile, increasing E pressure brought about more obvious increment of the activity (0.5 × 105 g molSc-1 h-1 at 1 bar vs 3.62 × 105 g molSc-1 h-1 at 6 bar) (Table 1, entries 6 and 10-12). Performing the polymerization at elevated temperatures also slightly accelerated the copolymerization process (2.21 ×105 g molSc-1 h-1 at 50 °C vs 2.98 ×105 g molSc-1 h-1 at 80 °C), whereas too high temperature might arouse decomposition of the active species and lowered the yield (Table 1, entries 13-16).
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Figure 2. 1H NMR (a) and 13C NMR (b) spectra of E-pMOS copolymer (CDCl3, 25 °C) (Table 1, entry 9) The 1H and 13C NMR spectra of a representative E-pMOS copolymer (Table 1, entry 9) were shown in Figure 2. The methine proton Ha and the methylene protons Hb from pMOS units give broad signals at δ = 2.17 ppm and 1.34 ppm, respectively. The integration intensity ratio Ha:Hb of approximate 1:4 excludes the presence of consecutive pMOS units in the copolymer backbone (Figure 2a). The methylene protons Hc at δ = 0.83 ppm are arising from E units. The sequence distribution was defined by 13C NMR spectrum data (Figure 2b). The resonance at δ = 44.7 ppm is attributed to the tertiary carbons (Tδδ) arising from SoESo and SoEESo (So = pMOS) sequences. The corresponding secondary carbons Sαδ and Sαγ from these sequences and those having longer E units SoExSo (x ≥ 3) are observed around δ = 37.2 ppm35. The signal at δ = 39.8 ppm attributed to Tββ of the continuous SoSo sequence as observed in poly(pMOS) is absent,44 suggesting that pMOS inserts into the macromolecular backbone of polyethylene as a discrete unit. Moreover, the signal at δ = 25.5 ppm assignable to the secondary Sββ carbons from the alternating SoESo sequences is overwhelming stronger than those at δ = 29.9 ppm and 27.6 ppm attributed to the secondary carbons Sγγ, Sδδ, Sγδ and Sβδ from the EE sequences, indicative of the mainly anisoleethylene alternating microstructure (assignments are further confirmed by DEPT135 characterization, see Figure S15). Noted that all the E-pMOS copolymers produced by system 5 under various conditions have over 46.8 mol% contents of pMOS and incorporate only discrete pMOS units (Figures S7-S28). To explain the monomer distribution of these E-pMOS copolymers, the competitive reactivity ratios calculated according to the Fineman-Ross plot are rpMOS = 0.096 and rE = 0.13. Both values are rather close and much smaller than 1 (Figure S60).
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ACS Catalysis
Therefore, either pMOS or E prefers cross-propagation to self-propagation, tending to form the alternating E-pMOS sequence. Table 2. Copolymerizations of E with polar styrenes and styrene by using precursor 5a
Comonomer
Tol.
Time Yield Act.b
(mmol)
(mL)
(min)
(g)
1
A(3)
3
10
0.426
2
B(3)
3
10
3
C(3)
3
10
Entry
a
TOFc
Conv. Incorp.d
Mne
Mw/Mne
(mol %) (104)
T gf (oC)
E
Comonomer
(%)
2.13
1618
1567
87
49.2
1.01
1.84
20.7
0.623
3.74
1822
1751
97
49.0
2.77
1.74
76.6
0.436
2.62
1493
1493
83
50.0
0.97
1.63
59.2
4
C(6)
3
15
0.528
2.11
1205
1205
50
50.0
1.99
1.63
61.3
5
D(3)
3
6
0.399
3.99
1996
1957
65
49.5
0.75
1.64
39.4
6
E(6)
6
6
0.949
9.49
6539
5096
90
43.8
0.22
2.89
19.6
7
F(3)
3
6
0.740
7.40
2784
2296
77
46.4
0.64
1.24
61.4
8
G(3)
3
6
0.376
3.76
3142
2764
92
46.8
1.11
1.58
18.4
9
G(12)
12
5
0.794
9.5
7672
7082
49
48.0
5.00
1.70
29.5
10
G(18)
12
5
0.839
10.1
8007
7511
35
48.4
6.87
2.04
29.0
11
G(24)
12
5
0.854
10.2
8024
7679
27
49.4
9.66
2.12
31.6
12
G(42)
12
5
0.977
11.7
8981
8838
18
49.7
10.45
2.56
33.2
13
H(12)
12
5
1.161
13.9
9589
9513
66
49.8
2.18
2.27
38.1
Conditions: toluene, 3 mL; Polymerization temperature, 70 °C; Ethylene pressure, 4 bar;
Sc, 10 µmol; [Sc]:[PhMe2NH][B(C6F5)4]:[AliBu3] = 1:1:20 (mol/mol/mol). bGiven in 105 gmolSc-1h-1. cmol•molSc-1h-1. 1
d
Styrene and its derivative incorporation was determined by
H NMR in CDCl3 at 25 °C. eDetermined by GPC in THF at 40 °C against polystyrene
standard. fDetermined by DSC. Copolymerization of ethylene with other polar/non-polar styrenes. Stimulated by the above results, the copolymerizations of E with other polar styrene derivatives were investigated by using system 5. The copolymerization of E with meta-methoxystyrene (mMOS), an analogue of pMOS, showed the similar results with respect of catalytic activity and comonomer 11 ACS Paragon Plus Environment
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incorporation (49.2 mol%) as well as the pseudo-alternating composition (Table 2, entry 1; Figures S29-S31). This is in sharp contrast to the copolymerizations of St with pMOS and mMOS to provide multi-block and gradient microstructures45. System 5 also exhibited excellent catalytic performance towards the copolymerization of E with the steric bulky polar comonomer 6-methoxy-2-vinylnaphthalene (MVN) in a high catalytic activity (3.74×105 g molSc-1 h-1) and distinguished comonomer incorporation ability (49.0 mol%). A pseudo-alternating copolymer with high molecular weight (Mn = 2.77×104 gmol-1) and narrow molecular weight distribution (PDI = 1.74, Table 2, entry 2) was afforded. The most striking result belonged to the copolymerization of E with para-(N,N-dimethylamino)styrene (DMAS) by giving the perfect alternating poly(E-alt-DMAS) product with a 50:50 molar ratio of the two monomer units, which was proved by NMR analyses. The resonance at δ = 0.89 ppm is assigned to the methylene protons Hc between two discrete DMAS units; the secondary carbons Sββ, Sαδ and Sδδof E-alt-DMAS units appear at δ = 25.6, 37.3 and 44.6 ppm; the consecutive EE carbons showing around δ = 29.9 and 27.6 ppm in 13C NMR are not observed (Figure 3 and Figure S36). In addition, the copolymerization performed in a much controllable manner. If doubling DMAS loading from 3 mmol to 6 mmol while keeping E loading unchanged, the molecular weight of the resultant alternating copolymer was almost doubled (0.97 × 104 gmol-1 vs 1.99 × 104 gmol-1, Table 2, entries 3 and 4). Extending to more bulky para-(N,N-diethylamino)styrene (DEAS) and (para-vinylphenyl)diphenylphosphine (DPVP) and less polar para-methylthiostyrene (MTS) comonomers, the copolymerizations exhibited higher activities but provided copolymers with flawed alternating arrangement (Table 2, entries 5-7).
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ACS Catalysis
Figure 3. 1H NMR (a), and
13
C NMR (b) spectra of E-DMAS copolymer (CDCl3, 25 °C)
(Table 2, entry 3) Taking methoxystyrene (MOS) as the comonomer and complex 5 as the catalyst precursor, the reasons behind the above results might be explained as depicted in Scheme 1. First and foremost, the fluorenyl ligand with the carbene side-arm in precursor 5 provides a proper steric bulkiness around the active Sc3+ center to permit one MOS coordination-insertion but prohibit its consecutive propagation. Only the smaller E monomer can be incorporated into the polymer chain after a MOS unit. Secondly, the penultimate MOS unit of the active Sc-polymer chain tends to η6-coordinate with the Sc3+ active species, which prohibits E from contacting with the 13 ACS Paragon Plus Environment
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latter38,39d. Meanwhile, the Lewis basic MOS readily coordinates to the strong Lewis acidic Sc3+ cation in the Sc-σ-O coordination mode (dormant), alternatively, polar monomers have coordination priority than E.45 Then, MOS swings from the Sc-σ-O mode to the active Sc-π-C=C mode followed by insertion to finish one MOS propagation after E growth. Thirdly, the electrondonating NHC side-arm in the fluorenyl ligand partially compensates the electron deficiency of the cationic metal center Sc3+ to reduce its interaction strength with the polar group. This makes the switch from the dormant Sc-σ-O mode to the active Sc-π-C=C mode much easily, and increases chance for MOS coordination/insertion. Therefore, as only as E presence in the polymerization procedure, MOS keeps propagating from E chain-end, which should be the reason for high conversions of polar styrene monomers in all cases. This work represents the first example of alternating copolymerization of ethylene with unmasked polar monomers, particularly, in high activity and high polar comonomer conversion.
Scheme 1. Ethylene and MOS insertion pathways Intrigued by this finding, we turned back to investigate the copolymerization of E and St. Fortunately, it performed fluently ([St]:[Sc] = 300) at 4 bar E pressure and 70 °C for 6 min. An 14 ACS Paragon Plus Environment
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activity of 3.21 × 105 g molSc-1 h-1 and a 46.8 mol% St incorporation under a high level conversion (92%) were achieved (Table 2, entry 8). When the molar ratio of St-to-Sc stepwise increased from 1200:1 to 4200:1 (under a constant toluene volume), the catalytic activity increased obviously from 9.5 ×105 to 11.7 ×105 g molSc-1 h-1 accompanied by steadily high St incorporation (48.0-49.7 mol%) (Table 2, entries 9-12). These results are in sharp contrast to those observed by using the group 4 metal based catalyst35-39, Me2Si(Me4Cp)(N-tBu)TiCl2/MAO, where styrene is loaded in an excess amount (91 mol% St vs 9 mol% ethylene) but only one third is transferred into the copolymer37. Switching to the electron-donating para-methylstyrene (pMS) as the comonomer, striking consequences were observed like distinguished catalytic activity (13.9×105 gmolSc-1h-1), high pMS content (49.8 mol%) and conversion (66%) (Table 2, entry 13). The σ-donating effect of the methyl substituent increases the electron density of the reaction double bond which facilitates pMS coordination/insertion and subsequently elevates the activity and incorporation ratio57,58. As far as we are aware, the catalyst system based on precursor 5, represents the first example that realizes the alternating copolymerization of ethylene with both polar and nonpolar styrenes. Conclusion We have demonstrated that the copolymerizations of ethylene with polar styrene derivatives can be achieved by using the fluorenyl-carbene scandium catalyst to give pseudo- or ideal alternating copolymers with narrow molecular weight distributions and high comonomer conversions under mild conditions. This is mainly attributed to the fluorenyl ligand bearing the carbene side-arm, which constitutes a constrain-geometry-configuration (CGC-like) around the active metal center. Thus, the coordination sphere of the CGC-ligand is steric bulky, which blocks the consecutive propagation of any styrene monomers. Consequently, the scandium active 15 ACS Paragon Plus Environment
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species attaching to a polar styrene chain-end prefers to propagate the smaller ethylene monomer to form a pS-E unit (pS: polar styrenes). Meanwhile, the effect of the penultimate styrene unit and the interaction between the Lewis acidic active metal center and the Lewis basic polar monomers result in coordination priority for the polar styrene derivatives, therefore the scandium active species attaching to ethylene chain end tends to propagate polar styrene monomers. On the other hand, the electron-donating ability of the carbene fluorenyl CGC ligand is weaker than the pyridyl fluorenyl CGC ligand but stronger than the half-Sandwich fluorenyl ligand, which results in proper interaction strength between the polar styrene and the active metal center, thus to construct a “buffer” system, allowing cross propagation of ethylene after polar and non-polar styrenes. This study sheds a new light on how to promote copolymerization of unprotected polar styrenes with ethylene in an alternating manner via designing appropriate catalyst’s ligands to regulate the interaction strength between the polar monomers and the active metal center. ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Detailed experimental procedures, characterization data for complexes 5, the ethylene homopolymerization data, NMR spectra, and DSC data of copolymers. AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] ORCID: Shihui Li: 0000-0002-4121-9119 16 ACS Paragon Plus Environment
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Dongmei Cui: 0000-0001-8372-5987 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is partially supported by the NSFC (project Nos. 21634007, 51773193, 21674108), the “973” project (2015CB654702) and department of science and technology of Jilin province for project (20180101171JC). REFERENCES (1) Church, G. M.; Gao, Y.; Kosuri, S., Next-Generation Digital Information Storage in DNA. Science 2012, 337, 1628. (2) Szwarc, M., Living Polymers. Nature 1956, 178, 1168−1169. (3) Patten, T. E.; Xia, J. H.; Abernathy, T.; Matyjaszewski, K., Polymers with very low polydispersities from atom transfer radical polymerization. Science 1996, 272, 866−868. (4) Bielawski, C. W.; Grubbs, R. H., Living ring-opening metathesis polymerization. Prog. Polym. Sci. 2007, 32, 1−29. (5) Kade, M. J.; Burke, D. J.; Hawker, C. J., The Power of Thiol-ene Chemistry. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 743−750. (6) Solleder, S. C.; Meier, M. A. R., Sequence Control in Polymer Chemistry through the Passerini Three-Component Reaction. Angew. Chem., Int. Ed. 2014, 53, 711−714. (7) Gacal, B.; Durmaz, H.; Tasdelen, M. A.; Hizal, G.; Tunca, U.; Yagci, Y.; Demirel, A. L., Anthracene-maleimide-based Diels-Alder “click chemistry” as a novel route to graft copolymers. Macromolecules 2006, 39, 5330−5336. (8) Bates, F. S.; Hillmyer, M. A.; Lodge, T. P.; Bates, C. M.; Delaney, K. T.; Fredrickson, G. H., Multiblock Polymers: Panacea or Pandora’s Box? Science 2012, 336, 434−440. (9) Lutz, J-F.; Meyer, T. Y.; Ouchi, M.; Sawamoto, M., Sequence-Controlled Polymers: Synthesis, Self-Assembly, and Properties. Acs Symposium Series 1170, American Chemical Society, Washington, DC (2014). (10) Coates, G. W.; Waymouth, R. M., Oscillating Stereocontrol-a Strategy for the Synthesis of Thermoplastic Elastomeric Polypropylene. Science 1995, 267, 217−219. (11) Jian, Z. B.; Cui D. M.; Hou Z. M.; Li, X. F., Living catalyzed-chain-growth polymerization and block copolymerization of isoprene by rare-earth metal allyl precursors bearing a constrained-geometry-conformation ligand. Chem. Commun. 2010, 46, 3022−3024. (12) Wei, J.; Zhang, W.; Wickham, R.; Sita, L. R., Programmable Modulation of Co-monomer Relative Reactivities for Living Coordination Polymerization through Reversible Chain Transfer between “Tight” and “Loose” Ion Pairs. Angew. Chem., Int. Ed. 2010, 49, 9140−9144. 17 ACS Paragon Plus Environment
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Table of Contents
N
Sc SiMe3 SiMe3
N
[PhMe2NH][B(C6F5)4]/AliBu3
X
X
n
X
O N
O
N
P
S
O
Ph
Ph
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