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Fast Living Polymerization of Challenging Aryl Isocyanides Using an Air-Stable Bisphosphine-Chelated Nickel(II) Initiator Jaeho Lee, Suyong Shin, and Tae-Lim Choi* Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea

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S Supporting Information *

ABSTRACT: Here we report a highly efficient living polymerization of challenging electron-rich or sterically hindered aryl isocyanides using an air-stable, but highly active, bisphosphinechelated nickel(II) complex. Initially, the living character was examined by screening various Ni(II) complexes, and we identified o-Tol(dppe)NiCl as an excellent initiator for the living polymerization of aryl isocyanides. On the basis of chain extension experiments and in situ 31P NMR spectroscopy, we concluded that the high stability of the propagating species due to the tightly bound chelating ligand was crucial for successful living polymerizations. Not only reactive electron-poor aryl isocyanides but also more challenging electron-rich or sterically hindered aryl isocyanides underwent fast living polymerizations to give polymers having controlled Mn with narrow dispersity. In addition, we confirmed that the electronic character of the monomer significantly affected the polymerization efficiency by comparing the polymerization of 4-octyloxyphenyl isocyanide and 3octyloxyphenyl isocyanide, which have the same substituents at different positions on the phenyl ring. Furthermore, ABCDE pentablock copolymer containing various substituents was efficiently synthesized in only 1 min.

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INTRODUCTION

sterically congested arylisocyanides, but it required an excess of triphenylphosphine (PPh3) to stabilize the chain-ends.19,20 Meanwhile, Ni complexes have been widely used for the polymerization of isocyanides because of their low cost, easy handling, and high activity. Deming and Novak succeeded in the living polymerization of alkyl isocyanides for the first time by using the π-allyl Ni complex in noncoordinating solvents.21,22 A few decades later, Suginome and co-workers demonstrated that aryl Ni(II) complexes are suitable initiators for synthesizing helically chiral poly(aryl isocyanide)s23 and poly(quinoxaline2,3-diyl)s24−30 in a living manner. Recently, a nucleophile adduct of the tetra(tert-butylisocyano)Ni(II) complex was reported as an excellent initiator for the living polymerization of electron-poor aryl isocyanides.31 Furthermore, the Bielawski and Wu groups achieved a controlled poly(3-hexylthiophene)-bpolyisocyanide (P3HT-b-PI) block copolymerization of various isocyanides and diisocyanobenzenes by using Ni-terminated P3HT as a macroinitiator.32−36 Although many examples of Ni(II) initiators for the living polymerization of aryl isocyanides are available, monomers having electron-withdrawing groups have been primarily used. This narrow monomer scope is because the electron-rich aryl isocyanides are hard to insert into the M−C bond during propagation.37,38 Herein, we report the fast living polymerization of a wide range of aryl isocyanides, including the electron-rich and sterically hindered aryl isocyanides, which are challenging to polymerize, using air-

The polymerization of isocyanides using transition-metal complexes is a powerful tool to produce polyisocyanides, which are attractive polymers due to their CN conjugated backbone and their intrinsic helical conformation in solution.1−7 Isocyanides insert into the metal−carbon (M−C) bond of the transition-metal complex, forming the imino−metal propagating species, and the successive insertion of isocyanides drives the polymerization in a chain-growth manner.4−9 In the early days, zerovalent nickel, cobalt−carbonyl complexes (Ni(CO)4 and Co2(CO)8), or nickel(II) salts (NiCl2 and Ni(acac)2) were used for the polymerization of isocyanides.10 However, those simple transition-metal catalysts could not promote living polymerization. Since then, there have been many attempts to solve this issue; as a result, the transition-metal complexes including palladium (Pd), rhodium (Rh), and nickel (Ni) have been reported as suitable initiators for the living polymerization of isocyanides. Among them, the palladium−platinum (Pd−Pt) μethynediyl complex is a well-known catalyst for the living polymerization of isocyanides despite its limited stability in air.11,12 To overcome this issue, Wu and co-workers recently developed a series of air-stable ethynyl Pd complexes and succeeded in the living polymerizations of various isocyanides and diisocyanobenzene derivatives.13−15 They also expanded the utility to prepare other complex polymer architectures such as brush copolymers and star polymers.15−18 The only drawback was that the Pd-catalyzed polymerizations required high temperatures and long reaction times. In addition, aryl Rh complexes were developed for the living polymerization of © XXXX American Chemical Society

Received: May 24, 2018 Revised: September 3, 2018

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DOI: 10.1021/acs.macromol.8b01090 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Optimization of Ni(II) Initiator

entry

catalyst

m

n

Mna (kDa)

Đa

yieldb (%)

c

A B C D B C D D

50 50 50 50 50 50 50 50

0 0 0 0 50 50 50 50

30.3 9.7 9.7 9.2 17.2 16.8

4.45 1.04 1.04 1.03 1.04 1.04

95 90 85 76 90 78 46 73

1 2 3 4 5 6 7 8d

bimodal trace bimodal trace

a Determined by THF SEC calibrated using polystyrene (PS) standards. bIsolated yield after purification. c60 min reaction. d2.5 equiv of PPh3 was added.

Figure 1. THF SEC traces for a homopolymer and a chain-extended polymer (a) initiated by C and (b) D. 31P NMR spectra of the (i) free phosphine ligands, (ii) initiators, and (iii) reactions with 1 and (c) C and (d) D.

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stable o-Tol(dppe)NiCl. To demonstrate the versatility of this initiator, ABCDE pentablock copolymer containing all different substituents was successfully prepared. The key to these successful polymerizations was the stabilization of the propagating Ni complexes by the bidentate chelating ligand.

RESULTS AND DISCUSSION

First, we attempted to polymerize aryl isocyanide 1 with the Ni(dppp)Cl2 (A) complex that Bielawski’s group had used for the synthesis of the P3HT-b-PI block copolymers derived from B

DOI: 10.1021/acs.macromol.8b01090 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Controlled Polymerization of Various Aryl Isocyanides 1−9 with Initiator C

entry

monomer

[M]/[I]

time (min)

Mna (kDa)

Đa

yieldb (%)

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

1 1 1 1 1 1 2 2 2 2 2 3 3 3 4 4 4 5 5 5 6 6 7 7 7 7 8 8 8 8 8 8 8 9 9 9 9

50 100 250 500 750 1000 50 100 150 200 250 50 100 150 50 100 150 50 100 150 25 50 50 100 250 500 50 100 250 500 750 1000 1500 50 100 250 500

1 3 10 15 20 20 1 1 2 2 2 2 5 5 3 2 2 5 10 3 2 5 1 1 2 3 3 5 10 30 50 65 100 1 1 2 3

9.7 18.4 50.2 113.3 221.4 328.4 9.1 17.2 23.6 36.7 47.1 5.0 10.7 20.3 7.2 15.2 21.0 6.6 14.9 20.0 5.7 11.9 7.6 14.1 40.7 102.6 9.1 16.8 43.1 107.8 187.1 297.5 541.1 6.8 13.7 38.6 105.1

1.04 1.03 1.05 1.04 1.09 1.14 1.03 1.06 1.10 1.17 1.40 1.11 1.22 1.34 1.10 1.22 1.52 1.09 1.22 1.36 1.08 1.25 1.03 1.04 1.06 1.15 1.03 1.02 1.03 1.03 1.04 1.06 1.10 1.04 1.04 1.03 1.10

85 87 80 96 98 95 74 87 95 94 86 99 99 94 82 99 78 86 89 88 99 80 94 87 94 98 94 99 99 99 99 98 98 81 95 94 97

a

Determined by THF SEC calibrated using PS standards. bIsolated yield after purification.

various isocyanides.32 However, when 50 mol equiv (equiv) of 1 was added to the initiator A, the controlled polymerization failed and produced polyisocyanide with Mn of 30.3 kDa, a much higher value than the expected value, and with a broad Đ of 4.45 (Table 1, entry 1). During the synthesis of P3HT-b-PI, the initially formed propagating living end of P3HT acted as a well-

defined macroinitiator,33 while A reacting with 1 might not form well-defined propagating species. The importance of the initiating group on the catalysts, such as π-allyl,21,22 aryl,23 or carbene-like groups,31 is well-documented for the controlled polymerization of isocyanides, so we introduced the o-tolyl group as an actual initiating group by preparing o-Tol(dppp)C

DOI: 10.1021/acs.macromol.8b01090 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 2. Plots of Mn vs M/I ratios and their corresponding Đ values for (a) P1, (b) P2, (c) P7, and (d) P8. The actual M/I values were calculated from the initial feeding ratios and the final conversions.

NiCl (B), o-Tol(dppe)NiCl (C), and o-Tol(PPh3)2NiCl (D). When polymerizations of monomer 1 were performed using these three Ni(II) initiators at room temperature, gratifyingly, all polymerizations were completed rapidly in 1 min to produce P1 with Mn of 9.2−9.7 kDa and narrow dispersities of 1.03−1.04, implying well-controlled polymerizations (Table 1, entries 2− 4). We subsequently examined the living character of each initiator by chain extension experiments. When another 50 equiv of 1 was added to the reaction pot of the preformed P150 using the initiator B and C containing chelating ligands, SEC traces of the final polymers were completely shifted to a higher molecular weight region, and narrow dispersities were retained (Mn = 17.2 kDa, Đ = 1.04 and Mn = 16.8 kDa, Đ = 1.04, respectively) (Table 1, entries 5 and 6, Figure 1a, and Figure S1). However, the same experiment with the initiator D having monodentate PPh3 ligands showed a bimodal SEC trace, implying that the chain termination occurred. Although the addition of 2.5 equiv of PPh3 reduced the intensity of the lower molecular weight region peak, the bimodal trace was still observed. (Table 1, entries 7 and 8, and Figure 1b). It seemed that the stability of the propagating species from D was lower than that from B and C containing chelating phosphines; consequently, the additional PPh3 ligand slightly improved the stability of the propagating species. This assumption was further supported by monitoring the Ni complexes using 31P NMR. After the reaction of the initiator C with 10 equiv of 1, no free dppe (−12.82 ppm) was detected, implying its tight binding to the Ni center (Figure 1c). However, the same reaction using the initiator D showed that 70% of PPh3 (−5.49 ppm) readily dissociated from the Ni center, indicating significant decomposition of the catalyst (Figure 1d). In both cases, we did not observe the actual propagating species by 31P NMR, but these results indicate that the higher stability of C containing a chelating ligand promoted the living polymerization. Although both initiators B and C showed the living

polymerization, the user-friendly C was used as the optimal catalyst due to its easier synthesis and storage in air. To further demonstrate the living polymerization of 1, we varied the M/I ratios from 50 to 1000. The polymerization was completed efficiently in 1−20 min to produce P1 with controlled molecular weights (Mn of 9.7−328.4 kDa) and narrow dispersities (1.03−1.14) (Table 2, entries 1−6, and Figure 2a). The high activity of the initiator C led us to attempt more challenging polymerizations of electron-rich aryl isocyanides 2−7 because they were either inactive monomers or did not promote the living polymerizations with other Ni initiators.23,31 The polymerization of 2 containing an electrondonating linear alkyl chain at the para position was conducted with M/I ratios from 50 to 250, and Mn of P2 increased linearly from 9.1 to 47.1 kDa. Their resulting Đ values stayed narrow up to M/I = 200 (1.03−1.17), but broadened at a higher M/I ratio (Table 2, entries 7−11, and Figure 2b). Monomer 3 containing an electron-donating and bulky tert-butyl group, was also successfully polymerized in a controlled manner with M/I ratios from 50 to 150 to produce P3 with Mn of 5.0−20.3 kDa and narrow Đ values of 1.11−1.34 in only 2−5 min (Table 2, entries 12−14, and Figure S3a). N-Propylaminocarbonyl-substituted monomer 4 was reported to be unreactive with the Ni initiator used by Suginome’s group,23 but with the initiator C, we obtained P4 having Mn in the range of 7.2−21.0 kDa with M/I = 50−150, and narrow Đ values (1.10−1.22) were maintained up to M/I = 100 (Table 2, entries 15−17, and Figure S3b). Furthermore, another electron-rich monomer 5, substituted with the bulky (triisopropylsilyl)oxy group, was also rapidly polymerized to produce P5 with a linear increase in Mn from 6.6 to 20.0 kDa and narrow Đ values of 1.09−1.36 (Table 2, entries 18−20, and Figure S3c). It is remarkable that the high activity and stability of the initiator C enabled the controlled polymerization of challenging monomers, which were not possible with the previous Ni initiators. D

DOI: 10.1021/acs.macromol.8b01090 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. (a) Pentablock copolymerization of 1, 9, 2, 4, and 5. (b) SEC traces for corresponding block copolymers. (c) Their Mn and Đ values determined by THF SEC calibrated using PS standards.

34−37, and Figure S3d). As expected from its chiral side chain, a positive Cotton effect was observed from the circular dichroic (CD) spectra of P9100 measured at 367 nm (Δε367 = +7.4), confirming that P9 formed a single-handed helical conformation in solution (Figure S5). Having optimized the living polymerization, we attempted the synthesis of ABCDE pentablock copolymer composed of five different monomers because multiblock copolymers are highly demanding materials due to their unique properties in comparison to corresponding di- or triblock copolymers.41−44 Initially, the initiator C reacted with 40 equiv of the most reactive monomer 1. Subsequently, 40 equiv of monomer 9 and 2 were added in 10 s intervals. Each monomer was fully converted into polymers in just 10 s, and diblock (P140-b-P940) and triblock copolymer (P140-b-P940-b-P240) were prepared sequentially. The further addition of 30 equiv of 4 produced the tetrablock copolymer (P140-b-P940-b-P240-b-P430) in 7 s, and finally, the reaction with 30 equiv of 5 for 10 s completed the ABCDE pentablock copolymer (P140-b-P940-b-P240-b-P430-bP530) (Figure 3a). Only 1 min was required to synthesize this complex block copolymer, and the resulting polymer with a narrow Đ of 1.10 was isolated in high yield (89%). Importantly, with the addition of each monomer, the SEC traces cleanly shifted to the higher molecular weight region, from Mn of 7.4 kDa to 12.0, 17.5, 19.7, and finally 23.0 kDa while maintaining narrow Đ values (1.03−1.10) (Figure 3b,c). This work highlights the great utility of fast living polymerization using initiator C to produce poly(aryl isocyanide)s with precise control and broad monomer scope.

On the other hand, the polymerization of 4-n-octyloxyphenyl isocyanide (6) containing a stronger electron-donating alkoxy group was less reactive and led to the partially successful control, with only low M/I ratios of 25 and 50 (Mn = 5.7 kDa, Đ = 1.08 and Mn = 11.9 kDa, Đ = 1.25, respectively) (Table 2, entries 21 and 22). In contrast, the less electron-rich monomer 7 containing a meta-octyloxy substitution could be polymerized with much higher M/I ratios up to 500, producing a linear increase in Mn from 7.6 to 102.6 kDa with narrow Đ values of 1.03−1.15 (Table 2, entries 23−26, and Figure 2c). This direct comparison between the polymerization of 6 and 7 confirmed that the electron-rich monomers were more challenging to polymerize, but still with the highly active complex C, both monomers containing strong electron-donating groups underwent living polymerizations in 5 min. Although the ortho-substituted aryl isocyanide is an attractive monomer, because the stability of the helical conformation of polyisocyanide depends on the steric effect of side chains,6,7,39 only few Pd13 and Rh20 initiators succeeded in its living polymerization because of the steric hindrance. With the initiator C, the sterically hindered monomer 8 was tested, and it required a longer reaction time (3−100 min) to reach a full conversion depending on the M/I ratios. As a result, P8 having various Mn ranging from 9.1 to 541.1 kDa with narrow Đ values (1.02−1.10) were prepared by M/I ratios from 50 to 1500 in high yields (Table 2, entries 27−33, and Figure 2d). In addition, a relatively sharp imino carbon peak was observed at 161.1 ppm in the 13C NMR spectrum of P8 as compared to that of P1 (Figure S4), which indicates that the ortho-substituent on the phenyl ring helped to form more stereoregular conformation.40 Furthermore, we polymerized chiral 4-((l)-menthoxycarbonyl)phenyl isocyanide (9) with various M/I ratios from 50 to 500, and the controlled polymers having Mn of 6.8−105.1 kDa with narrow Đ values of 1.03−1.10 were obtained (Table 2, entries

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CONCLUSION In conclusion, using an air-stable o-Tol(dppe)NiCl initiator (C) with high activity, we greatly expanded the monomer scope of aryl isocyanide polymerizations and demonstrated fast living E

DOI: 10.1021/acs.macromol.8b01090 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

(8) Deming, T. J.; Novak, B. M. Mechanistic Studies on the NickelCatalyzed Polymerization of Isocyanides. J. Am. Chem. Soc. 1993, 115, 9101−9111. (9) Metselaar, G. A.; Schwartz, E.; de Gelder, R.; Feiters, M. C.; Nikitenko, S.; Smolentsev, G.; Yalovega, G. E.; Soldatov, A. V.; Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M. X-Ray Spectroscopic and Diffraction Study of the Structure of the Active Species in the NiII-Catalyzed Polymerization of Isocyanides. ChemPhysChem 2007, 8, 1850−1856. (10) Suginome, M.; Ito, Y. Transition Metal-Mediated Polymerization of Isocyanides. Adv. Polym. Sci. 2004, 171, 77−136. (11) Onitsuka, K.; Ogawa, H.; Joh, T.; Takahashi, S.; Yamamoto, Y.; Yamazaki, H. Reactions of μ-Ethynediyl Complexes of Transition Metals: Selective Double Insertion of Isocyanides and Molecular Structure of [Cl(Et3P)2PdC≡CC(=NPh)C(=NPh)Pd(PEt3)2Cl]. J. Chem. Soc., Dalton Trans. 1991, 1531−1536. (12) Onitsuka, K.; Yanai, K.; Takei, F.; Joh, T.; Takahashi, S. Reactions of Heterodinuclear μ-Ethynediyl Palladium-Platinum Complexes with Isocyanides: Living Polymerization of Aryl Isocyanides. Organometallics 1994, 13, 3862−3867. (13) Xue, Y.-X.; Zhu, Y.-Y.; Gao, L.-M.; He, X.-Y.; Liu, N.; Zhang, W.Y.; Yin, J.; Ding, Y.; Zhou, H.; Wu, Z.-Q. Air-Stable (Phenylbuta-1,3diynyl)palladium(II) Complexes: Highly Active Initiators for Living Polymerization of Isocyanides. J. Am. Chem. Soc. 2014, 136, 4706− 4713. (14) Xue, Y.-X.; Chen, J.-Li.; Jiang, Z.-Q.; Yu, Z.; Liu, N.; Yin, J.; Zhu, Y.-Y.; Wu, Z.-Q. Living Polymerization of Arylisocyanide Initiated by the Phenylethynyl Palladium(II) Complex. Polym. Chem. 2014, 5, 6435−6438. (15) Chen, J.-Li.; Su, M.; Jiang, Z.-Q.; Liu, N.; Yin, J.; Zhu, Y.-Y.; Wu, Z.-Q. Facile Synthesis of Stereoregular Helical Poly(phenyl isocyanide)s and Poly(phenyl isocyanide)-block-poly(L-lactic acid) Copolymers Using Alkylethynylpalladium(II) Complexes as Initiators. Polym. Chem. 2015, 6, 4784−4793. (16) Jiang, Z.-Q.; Xue, Y.-X.; Chen, J.-L.; Yu, Z.-P.; Liu, N.; Yin, J.; Zhu, Y.-Y.; Wu, Z.-Q. One-Pot Synthesis of Brush Copolymers Bearing Stereoregular Helical Polyisocyanides as Side Chains through Tandem Catalysis. Macromolecules 2015, 48, 81−89. (17) Liu, C.; Mi, Y.-X.; Wang, R.-H.; Jiang, Z.-Q.; Zhang, X.-Y.; Liu, N.; Yin, J.; Wu, Z.-Q. Facile Synthesis of Well-Defined ABC Miktoarm Star Terpolymers Bearing Poly(ε-caprolactone), Polystyrene and Stereoregular Helical Poly(phenyl isocyanide) Blocks. Polym. Chem. 2016, 7, 2447−2451. (18) Wang, Q.; Chu, B.-F.; Chu, J.-H.; Liu, N.; Wu, Z.-Q. Facile Synthesis of Optically Active and Thermoresponsive Star Block Copolymers Carrying Helical Polyisocyanide Arms and Their Thermo-Triggered Chiral Reslution Ability. ACS Macro Lett. 2018, 7, 127−131. (19) Yamamoto, M.; Onitsuka, K.; Takahashi, S. Polymerization of Aryl Isocyanides Possessing Bulky Substituents at an ortho Position Initiated by Organorhodium Complexes. Organometallics 2000, 19, 4669−4671. (20) Onitsuka, K.; Yamamoto, M.; Mori, T.; Takei, F.; Takahashi, S. Living Polymerization of Bulky Aryl Isocyanide with Arylrhodium Complexes. Organometallics 2006, 25, 1270−1278. (21) Deming, T. J.; Novak, B. M. Oragnometallic Catalysis in Air and Water: Oxygen-Enhanced, Nickel-Catalyzed Polymerizations of Isocyanides. Macromolecules 1991, 24, 326−328. (22) Deming, T. J.; Novak, B. M. Polyisocyanides Using [(η3C3H5)Ni(OC(O)CF3)]2: Rational Design and Implementation of a Living Polymerization Catalyst. Macromolecules 1991, 24, 6043−6045. (23) Yamada, T.; Suginome, M. Synthesis of Helical Rod-Coil Multiblock Copolymers by Living Block Copolymerization of Isocyanide and 1,2-Diiscyanobenzene Using Arylnickel Initiators. Macromolecules 2010, 43, 3999−4002. (24) Yamada, T.; Noguchi, H.; Nagata, Y.; Suginome, M. Chiral Arylnickel Complexes as Highly Active Initiators for Screw-Sense Selective Living Polymerization of 1,2-Diisocyanobenzenes. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 898−904.

polymerizations of electron-rich or sterically hindered aryl isocyanides, which were known to be very challenging monomers with other Ni initiators. The detailed investigations by chain extension experiments and 31P NMR studies revealed that the tightly bound chelating ligand on C was crucial in stabilizing the chain-ends to achieve living polymerizations. Furthermore, by comparing the polymerization results from two monomers, which have the same substituents at different positions on the phenyl ring, we found that the more electronrich aryl isocyanide with a para-substitution led to a lower polymerization efficiency than that with a meta-substitution. Finally, we achieved the synthesis of ABCDE pentablock copolymer containing various functional groups on the side chains.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01090. Experimental procedures, characterizations, NMR spectra for new compounds and polymers, SEC traces, UV−vis and CD spectra, and other supporting experiments (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.-L.C.). ORCID

Jaeho Lee: 0000-0001-7584-920X Tae-Lim Choi: 0000-0001-9521-6450 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support from the Creative Research Initiative Grant and the Nano-Material Technology Program through NRF, Korea. We also thank NCIRF and NICEM at SNU for supporting 13C NMR and CD measurements.

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REFERENCES

(1) Schwartz, E.; Koepf, M.; Kitto, H. J.; Nolte, R. J. M.; Rowan, A. E. Helical Poly(isocyanides): Past, Present and Future. Polym. Chem. 2011, 2, 33−47. (2) Nolte, R. J. M.; van Beijnen, A. J. M.; Drenth, W. Chirality in Polyisocyanides. J. Am. Chem. Soc. 1974, 96, 5932−5933. (3) Van Beijnen, A. J. M.; Nolte, R. J. M.; Drenth, W.; Hezemans, A. M. F. Screw Sense of Polyisocyanides. Tetrahedron 1976, 32, 2017− 2019. (4) Drenth, W.; Nolte, R. J. M. Poly(iminomethylenes): Rigid Rod Helical Polymers. Acc. Chem. Res. 1979, 12, 30−35. (5) Van Beijnen, A. J. M.; Nolte, R. J. M.; Drenth, W.; Hezemans, A. M. F.; van de Coolwijk, P. J. F. M. Helical Configuration of Poly(iminomethylenes). Screw Sense of Polymers Derived from Optically Active Alkyl Isocyanides. Macromolecules 1980, 13, 1386− 1391. (6) Kamer, P. C. J.; Nolte, R. J. M.; Drenth, W. Screw Sense Selective Polymerization of Achiral Isocyanides Catalyzed by Optically Active Nickel(II) Complexes. J. Am. Chem. Soc. 1988, 110, 6818−6825. (7) Kamer, P. C. J.; Cleij, M. C.; Nolte, R. J. M.; Harada, T.; Hezemans, A. M. F.; Drenth, W. Atropisomerism in Polymers. ScrewSense Selective Polymerization of Isocyanides by Inhibiting the Growth of One Enantiomer of a Racemic Pair of Helices. J. Am. Chem. Soc. 1988, 110, 1581−1587. F

DOI: 10.1021/acs.macromol.8b01090 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01090 Macromolecules XXXX, XXX, XXX−XXX