Living Polymerization of Conjugated Polar Alkenes Catalyzed by N

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Living Polymerization of Conjugated Polar Alkenes Catalyzed by N-heterocyclic Olefin-based Frustrated Lewis Pairs Qianyi Wang, Wuchao Zhao, Sutao Zhang, Jianghua He, Yuetao Zhang, and Eugene Y.-X. Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00333 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Catalysis

Living Polymerization of Conjugated Polar Alkenes Catalyzed by N-Heterocyclic Olefin-Based Frustrated Lewis Pairs Qianyi Wang,† Wuchao Zhao,† Sutao Zhang,† Jianghua He,† Yuetao Zhang,†,* and Eugene Y.-X. Chen‡ †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun, Jilin, 130012, China. ‡

Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United

States

ABSTRACT: The living polymerization of conjugated polar alkenes such as methacrylates by a noninteracting, authentic frustrated Lewis pair (FLP) has remained elusive ever since the report on the FLPpromoted polymerization in 2010. Here we report that the polymerization of alkyl methacrylates by a FLP system based on strongly nucleophilic N-heterocyclic olefin (NHO) Lewis base and sterically encumbered, but modestly strong Lewis acid MeAl(4-Me-2,6-tBu2-C6H2O)2 is not only rapid but also living. This living polymerization was indicated by the formation of a linear, living chain, capped with NHO/H chain-ends, without backbiting-derived cyclic chain-ends. The true livingness of this FLPpromoted polymerization has been unequivocally verified by five lines of evidence, including predicted polymer number-average molecular weight (Mn, up to 351 kg·mol-1) coupled with low dispersity (Ð = 1.05) values; obtained high to quantitative initiation efficiencies; observed linear increase of polymer Mn vs. monomer conversion and monomer-to-initiator ratio; found precision in multiple chain extensions;

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and formed well-defined diblock and ABA triblock copolymers with narrow molecular weight distributions (Đ = 1.09–1.13), regardless of the comonomer addition order.

KEYWORDS: block copolymer, frustrated Lewis pairs, living polymerization, N-heterocyclic olefin, polar alkenes.

INTRODUCTION The field of the “frustrated Lewis pair” (FLP) chemistry has continued to receive sustained intense interest ever since the seminal work reported by Stephan and Erker.1 The application of FLPs has now been well established in the small molecule chemistry, such as activation of small molecules,2 catalytic hydrogenation reactions,3 and new reactivity/reaction developments.4 In the area of macromolecular synthesis promoted by a Lewis pair (LP), since the first report on the polymerization by FLPs based on a bulky N-hetereocyclic carbene (NHC) or phosphine Lewis base (LB) and a bulky, strong Lewis acid (LA), such as Al(C6F5)3,5 the polymerization promoted by LPs, either a FLP or a classical Lewis adduct (CLA), has also attracted increasing attention.6 Such LPs can initiate rapid polymerization of polar vinyl monomers, including linear and cyclic acrylic monomers,6a,6h-6l,6n,6s,6t,7 monomers bearing the C=C-C=N functionality,8 and chemoselective polymerization of dissymetric divinyl polar monomers.9 LPs based on an N-heterocyclic olefin (NHO), a potent nucleophile and strong donor,10 also exhibited high activity for polymerization of acrylates and acrylamides.11 Although FLPs or CLAs exhibited high activity for polymerization of conjugated polar alkenes, the application of such polymerization is hampered by both low initiation efficiencies and chain-termination side reactions,11,12 evidenced by the much higher observed number-average molecular weight (Mn) than the calculated Mn and broad molecular weight distribution (MWD, or large Ð values) of the resulting polymers, thus giving rise to low initiation efficiencies (I*) and rendering the inability to produce well-defined block copolymers. In such LP polymerization, synergistic effects of the LA and LB sites of LPs is essential to achieve an effective and controlled polymerization system. This important point was further nicely demonstrated by the ACS Paragon Plus Environment

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ACS Catalysis

polymerization of bulky methacrylates such as t-butyl methacrylate using a CLA of AlMe3/PMe3, which was shown to be controlled (predictable Mn, narrow MWD, and high I* values), but the polymerization was not interrogated using the living polymerization protocol.13 We showed earlier that a CLA such as Ph3P·B(C6F5)3 is highly active for polymerization of a renewable cyclic acrylic monomer, while the non-interacting, true FLP such as PMes3/B(C6F5)3 or its linked intramolecular non-interacting FLP analogue is completely inactive for such polymerization.14 As revealed by the above overview, so far there is no report to date on the living polymerization of polar vinyl monomers by a non-interacting, true FLP, or LP-promoted living polymerization of less bulky methacrylates, particularly methyl methacrylate (MMA), a very important fundamental monomer in the polymer industry, which would enable the synthesis of well-defined block copolymers or polymers with controlled architectures. More recently, we found that NHO/Al(C6F5)3 LPs (which form CLAs) promoted highly effective, living ring-opening (co)polymerization of lactones.15 However, this type CLA still failed to promote the living polymerization of MMA. Here we report a significant development in this continuing effort: a true (non-interacting) FLP consisting of a strongly nucleophilic but bulky NHO and less acidic (relative to Al(C6F5)3) but bulky MeAl(4-Me-2,6-tBu2-C6H2O)2 (MeAl(BHT)2) cooperatively promotes high-speed living polymerization of conjugated polar alkenes, including MMA and benzyl methacrylate (BnMA) (Scheme 1).


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monomers:

O

O

OMe

OBn

polymers:

n O

MeO

Page 4 of 27

n O

BnO

n MeO

n

m

O BnO O MeO

O

Lewis acids (LA): Al(C6F5)3, Ph3Al OEt2, MeAl(BHT)2, AlMe3, AlEt3 Ph

Ph Lewis bases (LB):

N

N

NHO1

N

N

NHO2

N

N

NHO3

N

Ph N

NHO4

Scheme 1. Structures of the Lewis acids, Lewis bases, monomers, and the resulting polymers investigated in this study. RESULTS AND DISCUSSION MMA Polymerization by NHO-Based FLPs and CLAs. Four sterically hindered NHOs bearing the same exocyclic methyl groups but varied substituents on the ring carbons (4-Me, NHO1; 4,5-Me2, NHO2; 4-Me-5-Ph, NHO3; 4,5-Ph2, NHO4, Scheme 1) were employed as the nucleophilic LB for the MMA polymerization performed in toluene at room temperature (RT). Control experiments using NHOs alone showed that incomplete monomer conversions were observed up to 24 h, affording polymers with broad MWDs (Ð = 1.25 – 2.86, runs 1 – 4, Table 1). When employed as LPs in combination with LA Al(C6F5)3, which was premixed with MMA, followed by addition of an NHO, the polymerization activity was drastically enhanced for all the NHO/Al(C6F5)3 LPs, achieving quantitative monomer conversion in 30 s with 0.125% LP loading (runs 5 – 8, Table 1). For all the [MMA]/[LP] (LP = NHO1/Al(C6F5)3) ratios varied from 200 to 3200, this LP initiated rapid polymerization of MMA and achieved quantitative monomer conversion within 15 min, producing PMMA with predicted Mn, coupled with low Ð values (1.03 – 1.08), thus giving rise to high initiation efficiencies of 68-107% ACS Paragon Plus Environment

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ACS Catalysis

(Table S2). The three other NHO/Al(C6F5)3 LPs all exhibited comparable polymerization activity with that for NHO1/Al(C6F5)3 (Table S2). However, because NHO1 and Al(C6F5)3 form a CLA, both the polymerization activity and initiation efficiency were drastically decreased when the LA and the LB were premixed before addition of monomer (vide infra), indicating the CLA formation significantly inhibited the chain initiation process. Furthermore, chain-extension by NHO/Al(C6F5)3 LPs was partially successful if the second batch of MMA was added immediately after full conversion of the first batch of monomer has been achieved (run 2, Table S8). As revealed by GPC analysis (Figure S23), the coexistence of a major high MW peak along with a minor small MW peak indicated the partial deactivation of the active species; even if the second batch of MMA was added just 10 minutes after the first batch of monomer reached completion, no conversion of the added MMA was detected in up to 5 h, suggesting the complete deactivation of the active species. To probe the initiation and termination process, we analyzed low MW PMMA samples produced by NHO1/Al(C6F5)3 with matrix-assisted laser desorption/ionization time-of-flight mass spectrum (MALDI-TOF MS). The spectrum (Figure 1a) consisted of only one series of molecular ion peaks corresponding to the linear PMMA with NHO1 and the cyclic β-ketoester or δ-valerolactone chain-ends (Figure 2a), where NHO1 was derived from the chain initiation by NHO1 (152) and the cyclic chain-end [100 (MMA)-31 (loss of MeO) = 69] was derived from the backbiting chain-termination process during the polymeization.11,12 Overall, these results showed that, although the polymerization by the CLA-forming LP, NHO/Al(C6F5)3, is rapid and controlled (over MW and MWD), it is not a living process. Table 1. Results of the polymerization of MMA by NHO based FLPs a Entry

LB

LA

M:LB:L A

Time

Conv.b

Mnc

Mn(calcd)

Ð

I*d

(%)

(kg·mol-1)

(kg·mol-1)

(Mw/Mn)

(%)

1

NHO1

-

800:1

24 h

48.3

63.5

38.8

1.25

61

2

NHO2

-

800:1

24 h

88.8

86.1

71.3

1.36

83

3

NHO3

-

800:1

24 h

85.3

87.3

68.6

2.82

79

4

NHO4

-

800:1

24 h

72.8

68.4

58.6

2.86

86


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5

NHO1

Al(C6F5)3

800:1:2

30 s

>99

88.6

80.2

1.03

91

6

NHO2

Al(C6F5)3

800:1:2

30 s

>99

94.6

80.3

1.04

85

7

NHO3

Al(C6F5)3

800:1:2

30 s

>99

80.6

80.3

1.05

100

8

NHO4

Al(C6F5)3

800:1:2

30 s

>99

85.9

80.4

1.07

94

9

NHO1

MeAl(BHT)2

800:1:2

40 min

>99

77.3

80.2

1.08

104

10

NHO1

Ph3Al·Et2O

800:1:2

5 min

96.6

103

77.5

1.10

75

11

NHO1

AlMe3

800:1:2

12 h

38.3

n.d.

n.d.

n.d.

n.d.

12

NHO1

AlEt3

800:1:2

12 h

81.9

n.d.

n.d.

n.d.

n.d.

13

NHO1

MeAl(BHT)2

200:1:2

2 min

>99

30.4

20.2

1.09

66

14

NHO1

MeAl(BHT)2

400:1:2

10 min

>99

45.6

40.2

1.09

88

15

NHO1

MeAl(BHT)2

1600:1:2

120 min

>99

146

160

1.07

110

16

NHO1

MeAl(BHT)2

3200:1:2

480 min

100

347

321

1.05

93

17

NHO2

MeAl(BHT)2

200:1:2

2 min

>99

27.9

20.2

1.14

72

18

NHO2

MeAl(BHT)2

400:1:2

10 min

>99

46.0

40.2

1.10

87

19

NHO2

MeAl(BHT)2

800:1:2

40 min

>99

78.6

80.3

1.08

102

20

NHO2

MeAl(BHT)2

1600:1:2

120 min

>99

152

160

1.08

105

21

NHO2

MeAl(BHT)2

3200:1:2

480 min

100

351

321

1.05

91

22

NHO3

MeAl(BHT)2

200:1:2

2 min

>99

27.2

20.3

1.09

75

23

NHO3

MeAl(BHT)2

400:1:2

10 min

>99

44.4

40.3

1.10

91

24

NHO3

MeAl(BHT)2

800:1:2

40 min

>99

75.4

80.3

1.08

106

25

NHO3

MeAl(BHT)2

1600:1:2

120 min

>99

137

160

1.09

117

26

NHO3

MeAl(BHT)2

3200:1:2

360 min

100

318

321

1.05

101

27

NHO4

MeAl(BHT)2

200:1:2

2 min

>99

29.6

20.3

1.10

69

28

NHO4

MeAl(BHT)2

400:1:2

10 min

>99

47.7

40.3

1.09

84

29

NHO4

MeAl(BHT)2

800:1:2

40 min

>99

77.7

80.3

1.07

103

30

NHO4

MeAl(BHT)2

1600:1:2

120 min

>99

143

160

1.06

112

31

NHO4

MeAl(BHT)2

3200:1:2

480 min

100

286

321

1.10

112


6

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a

Condition: carried out at RT in 4.5 mL toluene using procedure A; for a 200MMA/1LB/2LA ratio, [MMA]0 = 0.936 M, [LA]0 = 2[LB]0 = 9.36 mM. n.d.: not determined. b Monomer conversions measured by 1H NMR. c Mn and Ð determined by GPC relative to PMMA standards in DMF. d Initiation efficiency (I*)% = Mn(calcd)/Mn(exptl) × 100, where Mn(calcd) = [MW(MMA)]([MMA]0/[I]0)(conversion)+MW of chain-end groups.

Figure 1. MALDI-TOF MS spectrum of the low-MW PMMA sample produced by (a) NHO1/Al(C6F5)3 and (b) NHO1/MeAl(BHT)2 in toluene at RT. 4500

a

b

4000

y = 100.03x + 221.23 3500

Molar Mass (m/z)

Molar Mass (m/z)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

2500

1500

y = 100x + 153.16 3000 2000 1000

500

0

5

10

15

20

25

30

35

5

number of MMA unit (n)

10

15 20 25 number of MMA unit (n)

30

35

Figure 2. Plot of m/z values taken from Figure 1 vs the number of MMA repeat units (n) and the deduced corresponding polymer chain structure produced by (a) NHO1/Al(C6F5)3 or (b) NHO1/MeAl(BHT)2. To develop a living polymerization by LPs, side reactions such as irreversible chaintermination processes must be avoided. In the above MMA polymerization, the chain termination occurs via backbiting cyclization that proceeds through intramolecular nucleophilic attack of the LA-activated ester group of the growing polymer chain by the enolate active chain end.11,12


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Therefore, the acidity of the LA is essential for the activation of the ester carbonyl group. On one hand, activation of monomer is greater for higher acidity of the LA, but on the other hand, suppressing the side reaction calls for the reduced acidity of the LA. Furthermore, the steric and electronic interplay between the LA and the LB counterpart in the LP must also be finely balanced. In this context, a series of organoaluminum LAs with descending acidity in a relative scale16 (Table S1) were examined for MMA polymerization in an 800:1:2 MMA/NHO1/LA ratio: Al(C6F5)3 (100%) > (Ph)3Al·OEt2 (88%) ≈ MeAl(BHT)2 (86%) > AlMe3 (71%) ≈ AlEt3 (70%) (Table S3). This investigation led us to the bulky MeAl(BHT)2 with lower acidity relative to Al(C6F5)3, which, when combined with NHO1 (0.125% relative to MMA) achieved full monomer conversion within 40 min, producing PMMA with Mn = 77.3 kg·mol-1 and Ð = 1.08, thus yielding a high I* of 104% (run 9, Table 1). As revealed by MALDI-TOF MS spectrum of a low MW PMMA sample, only one series of molecular ion peaks was detected (Figure 1b), the corresponding analysis indicated that the produced polymer chain is a linear, living chain, capped with NHO1/H chain-ends and showed no evidence for a cyclic backbiting chain-end (Figure 2b). Interestingly, although (Ph)3Al·OEt2 possesses similar acidity to that of MeAl(BHT)2 and exhibited enhanced polymerization activity towards the polymerization of MMA and achieved quantitative monomer conversion in 5 min, producing PMMA with Mn = 103 kg·mol-1 and Ð = 1.10, thus obtaining a relative lower I* of 75% (run 10, Table 1), the produced polymer still contained a cyclic chain-end derived from backbiting cyclization side reactions as shown by MALDI-TOF MS spectrum (Figures S18 and S19). On the other hand, NHO-based LPs using a LA with further reduced acidity, such as AlMe3 or AlEt3, rendered incomplete MMA conversion up to 12 h (runs 11 and 12, Table 1). The MALDI-TOF MS spectrum (Figure S20) of a low MW PMMA sample produced by NHO1/AlEt3 indicated that


8

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there are mainly two sets of peaks with different intensities, corresponding to a mixture of a polymer chain capped with NHO1 and the cyclic β-ketoester or δ-valerolactone chain-ends derived from backbiting cyclization side reactions (Figure S21), and the other polymer chain capped with NHO1/H chain-ends (Figure S22). These results further highlight the importance of considering both electronic and steric factors when matching the LA with LB for generating a suitable LP that can promote a living polymerization. Mechanistic Aspects of Polymerization. To gain further insight into the above-described polymerization behavior, we investigated stoichiometric reactions of NHO1 with Al(C6F5)3 and MeAl(BHT)2, respectively. We observed the formation of the corresponding CLA, accompanied by some minor unidentifiable or decomposed species (Figures S7 and S8), for the NHO1/Al(C6F5)3, and exclusively generation of a non-interacting, true FLP for the NHO1/MeAl(BHT)2, since there were no detectable spectral changes before and after mixing of NHO1 and MeAl(BHT)2 (Figure S9). Likewise, the formation of FLPs was also observed for the LPs consisting NHO2-NHO4 and MeAl(BHT)2. Accordingly, we also performed a comparative polymerization study with two different activation procedures. For procedure A, a predetermined amount of a LA (2 equiv. relative to LB) was first dissolved in MMA and solvent, followed by rapid addition of a solution of NHO1 to initiate the polymerization; in contrast, for procedure B, NHO1 was premixed with the LA in solvent for 5 min, followed by rapid addition of MMA to start the polymerization. As anticipated, the activation procedure significantly affected the polymerization outcome for the interacting LPs. For the NHO1/Al(C6F5)3 that forms a CLA, both the polymerization activity and I* were drastically decreased from 30 s (the time for achieving >99% conversion) to 20 min and 90% to 15% (run 5 for procedure A, Table 1 vs run 1 for procedure B, Table S4). In sharp contrast, there exhibited essentially no effects of the


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activation procedure on the polymerization outcome by the NHO1/MeAl(BHT)2 FLP system (run 9 for procedure A, Table 1 vs run 2 for procedure B, Table S4). These results prompted us to further investigate the stoichiometric reaction of NHO1-NHO4 with MeAl(BHT)2·MMA in a 1:1 molar ratio, which cleanly generates zwitterionic enolaluminate intermediates as two isomers (Z/E = 6-9:1) (Figures S10-S13). We successfully isolated the corresponding zwitterionic intermediates (INT1 and INT1’) from the stoichiometric reaction of MeAl(BHT)2·MMA with NHO3 or NHO4, respectively, and characterize both INT1 (Figures S14 and S15) and INT1’ (Figures S16 and S17) through 1H and 13C NMR spectroscopy. Many attempts to get the crystal structures of INT1 and INT1’ were unsuccessful. Control experiments (Table S5) indicated that MeAl(BHT)2 alone is completely ineffective for the MMA polymerization while only 5.6% monomer conversion was obtained by the NHO1/MeAl(BHT)2 LP generated from the reaction of MeAl(BHT)2 with NHO1 in 1:1 ratio in up to 24 h. These results, coupled with the kinetics results (vide infra), indicate that the reaction proceeds through a bimolecular, activated monomer propagation mechanism (Scheme 2). With identification of the superior NHO1/MeAl(BHT)2 FLPs catalyst system, we employed such a FLP system to examine its efficacy for the control polymerization of BnMA and achieved quantitative monomer conversion in 60 min for the 800:1:2 ratio (Mn = 111 kg·mol-1 and Ð = 1.09 (run 1, Table S6). The three other NHO/MeAl(BHT)2 LPs all exhibited comparable polymerization activity with that for NHO1/MeAl(BHT)2 (runs 2–4, Table S6). To the investigation of the mechanistic aspects of the polymerization, we examined the kinetics of the MMA polymerization by NHO1/MeAl(BHT)2 employing a fixed [MMA]0/[NHO1]0 ratio of 800:1 but varied [MeAl(BHT)2]0 concentration: [MMA]0/[NHO1]0/[MeAl(BHT)2]0 = 800:1:2, 800:1:3, 800:1:4, 800:1:5. As can be seen from the representative kinetic plots of


10

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ACS Catalysis

[MMA]t/[MMA]0 vs time (Figure 3), the polymerization clearly showed no induction period and a strict zero-order dependence on [MMA] concentration for all the ratios investigated herein. A double-logarithm plot (insert) of the apparent rate constants (kapp), obtained from the slopes of the best-fit lines to the plots of [MMA]t/[MMA]0 vs time, as a function of ln[MeAl(BHT)2] was fit to a straight line (R2 = 0.995) with a slope of 1.006, revealing that the propagation is first order in [MeAl(BHT)2] concentration. In the second set of kinetic experiments, with a fixed amount of [MMA]0, the [MeAl(BHT)2]0/[NHO1]0 ratio was varied at 5:4, 3:2, 2:1, and 1.5:0.5 such that at each ratio there was 1 equiv of MeAl(BHT)2 left to activate the monomer upon formation of the active intermediate that consumes equimolar NHO1 and MeAl(BHT)2. The same zero-order dependence was observed for all the ratios investigated in this study (Figure 4). A double-logarithm plot (insert) of the apparent rate constants (kapp), obtained from the slopes of the best-fit lines to the plots of [MMA]t/[MMA]0 vs time, as a function of ln[NHO1] was fit to a straight line (R2 = 0.990) with a slope of 0.963, revealing that the propagation is first-order in [NHO1] concentration. Overall, the MMA polymerization by the NHO1/MeAl(BHT)2 pair follows a bimolecular, activated monomer propagation mechanism (Scheme 2), as previously predicted by a computational study for the MMA polymerization by the LB/Al(C6F5)3.6t Such kinetics are consistent with the propagation mechanism in that the C−C bond forming step via intermolecular Michael addition of the propagating species to the LA-activated monomer is the rate-limiting step and the release of the LA catalyst from its coordinated last inserted monomer unit in the growing polymer chain to the incoming monomer is relatively fast.17


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Figure 3. Zero-order kinetic plots for the MMA polymerization by the NHO1 /MeAl(BHT)2 system in toluene at RT : [MMA]0 = 0.936 M; [NHO1]0 = 1.17 mM; [MeAl(BHT)2]0 = 5.85 mM(△), 4.68 mM (▲), 3.51 mM (◇), 2.34 mM (◆). Inset: plot of ln(kapp) vs ln[ MeAl(BHT)2].


12

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ACS Catalysis

Figure 4. Zero-order kinetic plots for the MMA polymerization by the NHO1/MeAl(BHT)2 system in toluene at RT: [MMA]0 = 0.936 M; [NHO1]0 = 4.68 mM (△), 2.34 mM (▲), 1.17 mM (◇), 0.585 mM (◆); [MeAl(BHT)2]0 = 5.85 mM (△), 3.51 mM (▲), 2.34 mM (◇), 1.755 mM ( ◆). Inset: plot of ln(kapp) vs ln[NHO1]. O

O O

N

N

N LA

N

LA N

O

O O

LA

O

N

n O

O

LA

O

Scheme 2. Proposed mechanism for the polymerization of conjugated polar alkenes (represented by MMA here) by NHO-based LPs. Establishing Living Polymerization. The above results revealed a high degree control possessed by the NHO1/MeAl(BHT)2 FLP system. Therefore, we further interrogated the livingness features of the MMA polymerization. First, varying the [MMA]0/[FLP]0 ratios from


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200 to 3200 achieved quantitative monomer conversion for all the ratios and produced welldefined polymers with predicted Mn and low Ð values (1.05–1.09) and high initiation efficiencies (66–110%, runs 9, 13-16 Table 1). The Mn value of PMMA produced by NHO1/MeAl(BHT)2 increased linearly (R2 = 0.989) with an increase in the [MMA]0/[NHO1]0/[MeAl(BHT)2]0 ratio from 200:1:2 to 3200:1:2 (Figure 5), while the Ð value remained narrow. Second, GPC traces of PMMA samples produced by NHO1/MeAl(BHT)2 showed the gradual shift to the higher-molarmass region with an increase in the [M]0/[LP]0 ratio from 200 to 3200 while maintaining a narrow and unimodal MWD (Figure 6). Third, a plot of the PMMA Mn value vs monomer conversion at a fixed 800[MMA]0/[NHO1]0 ratio also gave a straight line (R2 = 0.999), which was coupled with low Ð values in the range of 1.04–1.16 (Figure 7). It is worth noting that MeAl(BHT)2 based FLPs with other three NHOs (NHO2–NHO4, runs 17-31, Table 1) also exhibited comparable polymerization activity and high degree of polymerization control with those observed for the NHO1/MeAl(BHT)2 FLP. The yielded PMMA materials all possessed syndio-biased tacticities (~67% rr for NHO/MeAl(BHT)2 and 75% rr for NHO/Al(C6F5)3, Table S7) due to the chain-end control nature of this polymerization system..


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400

3.2 R² = 0.9891

■Mn □Ð

2.8

300 2.4

200

2

Ð

Mn 103 (g/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1.6 100 1.2

0 0

500

1000

1500

2000

2500

0.8 3500

3000

[MMA]0/[NHO1]

Figure 5. Plots of Mn and Đ for PMMA vs [MMA]0/[NHO1]0 ratio.

200 MMA 400 MMA 800 MMA 1600 MMA 3200 MMA

18

20

22

24 26 28 Retention Time (min)

30

32

Figure 6. GPC traces of PMMA samples obtained with NHO1/[MeAl(BHT)2 at various [MMA]0/[NHO1]0 ratios at RT. Conditions: [MMA]0/[NHO1]0/[MeAl(BHT)2]0 = 200:1:2, 400:1:2, 800:1:2, 1600:1:2, 3200:1:2, [MMA]0 = 0.936 M.


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ACS Catalysis

10 2.8 R² = 0.9995

■Mn □Ð

2.4

6

2

4

1.6

2 0

Ð

8 Mn (104 g/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.2 0.8 0

20

40

60

80

100

120

Conv. (%)

Figure 7. Plots of Mn and PDI of PMMA vs MMA conversion catalyzed by NHO1/MeAl(BHT)2 at RT. Conditions: [MMA]0/[NHO1]0/[MeAl(BHT)2]0 = 800:1:2. Fourth, chain extension experiments (Table S9) were also carried out to provide more direct evidence for the living characteristics of the NHO1/MeAl(BHT)2 FLP system. Specifically, the PMMA with Mn = 30.4 kg·mol-1 and Ð = 1.09 was first prepared by polymerizing the first batch (200 equiv) of MMA to completion without quenching. Next, a second batch of MMA was added to the above mixture and afforded the resulting PMMA with Mn = 48.6 kg·mol-1 and Ð = 1.09. Full monomer conversion was still achieved with addition of the third and even fourth batch of MMA, producing well-controlled PMMA with Mn = 67.5 kg·mol-1 and Ð = 1.07 and PMMA with Mn = 93.5 kg·mol-1 and Ð = 1.08, respectively; both the third and fourth chain extensions gave a near quantitative re-initiation efficiency (Table S9). These multiple chain extensions with precision manifested the livingness and robustness of the current NHO1/MeAl(BHT)2 FLP system. Fifth, we examined the livingness of the NHO1/MeAl(BHT)2 FLP system via the synthesis of well-defined diblock and triblock copolymers. First, when MMA and BnMA were


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ACS Catalysis

polymerized by NHO1/MeAl(BHT)2 simultaneously, the copolymerization produced a random copolymer with Mn = 53.2 kg·mol-1 and Ð = 1.15 (run 1, Table S10). Second, sequential block copolymerization by polymerizing 200 equiv MMA first without quenching, followed by addition of 200 equiv of BnMA, afforded successfully diblock copolymer PMMA-b-PBnMA (run 2, Table S10). GPC traces (Figure 8a，red) provided further evidence for the well-defined block copolymer PMMA-b-PBnMA formation. Specifically, the PMMA (Mn = 30.4 kg·mol-1 and Ð = 1.09) produced during the initial MMA polymerization shifted to a higher molecular weight region (Mn = 49.6 kg·mol-1) while maintaining a low Ð value of 1.13, for the well-defined diblock copolymer formation. Third, through this sequential block copolymerization protocol, a well-defined ABA triblock copolymer, PMMA-b-PBnMA-b-PMMA (Figure 8a，blue, Mn = 65.9 kg·mol-1, Ð = 1.10, run 4, Table S10), was also successfully synthesized. In addition, we employed BnMA as the primary monomer A and MMA as the subsequent monomer B for the copolymerization and also obtained well-defined AB diblock copolymer PBnMA-b-PMMA (run 3, Table S10; Figure 8b, red) and ABA triblock copolymer PBnMA-b-PMMA-b-PBnMA (run 5, Table S10; Figure 8b, blue).


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PMMA PMMA-b-PBnMA PMMA-b-PBnMA-b-PMMA

a

18

20

22

24 26 28 30 Retention Time (min)

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PBnMA PBnMA-b-PMMA PBnMA-b-PMMA-b-PBnMA

b

32

18

20

22 24 26 28 30 Retention Time (min)

32

Figure 8. GPC traces of homopolymer (black), diblock copolymer (red), and ABA triblock copolymer (blue) produced from the sequential block copolymerization of MMA and BnMA by NHO1/MeAl(BHT)2 in toluene at RT: (a), polymerizing MMA first; (b), polymerizing BnMA first. CONCLUSIONS In summary, the first living polymerization of sterically less hindered methacrylates such as MMA and BnMA was achieved by non-interacting, true FLPs, NHO/MeAl(BHT)2, while the closely related interacting LPs, NHO/Al(C6F5)3, which can form CLAs, suffered from irreversible chain-termination side reactions due to LA-activated back-biting cyclization. The livingness and robustness of this FLP-promoted polymerization system has been unequivocally verified by five lines of evidence, including predictable polymer Mn and low Ð values; high to quantitative initiation efficiencies; a linear increase of polymer Mn vs. monomer conversion and


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ACS Catalysis

monomer-to-initiator ratio; precision in multiple polymer chain extensions; and synthesis of well-defined diblock and triblock copolymers, regardless of the comonomer addition order. The success of developing this living polymerization was achieved by striking a fine balance between the sufficient LA acidity needed for monomer activation and the lowest possible LA acidity to suppress the LA-activated side reaction, as well as a fine balance between the sufficient LP sterics to minimize the LA-LB interaction and the reduced LB sterics to ensure effective initiation of the reaction, all accomplished through understanding the steric and electronic interplay between the LA and the LB counterpart and site cooperativity in the LP-promoted polymerization. These results should stimulate future efforts in developing highly active and living LP polymerization systems for the synthesis of other well-defined polymers, thereby further expanding the utility of the FLP chemistry. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Full experimental details, NMR spectra and additional polymerization data (PDF, 28 pages). AUTHOR INFORMATION Corresponding Author * Email for Y. T. Zhang: [email protected] Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENTs This work was supported by the National Natural Science Foundation of China (Grant no. 21422401, 21374040, 21774042), 1000 Young Talent Plan of China funds, the startup funds from Jilin University, Talents Fund of Jilin Province to YZ, and the US National Science Foundation (NSF-1507702) to EYC.

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vs.

Polymerization.

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for Table of Contents use only


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Living Polymerization of Conjugated Polar Alkenes Catalyzed by N

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