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Helix-Sense-Specific and Enantiomer-Specific Living Polymerizations of Phenyl Isocyanides Using Chiral Palladium(II) Catalysts Lei Xu,†,§ Li Yang,†,§ Zongxia Guo,‡,§ Na Liu,† Yuan-Yuan Zhu,† Zhibo Li,*,‡ and Zong-Quan Wu*,† †

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Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, and Anhui Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, Hefei University of Technology, Hefei 230009, Anhui Province, China ‡ School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, Shandong Province, China S Supporting Information *

ABSTRACT: Controlled synthesis of a single-handed helical polymer from an achiral monomer is still a great challenge. In this work, we report two chiral Pd(II) catalysts bearing commercially available R- or S-2,2′-bis(diphenylphosphino)-1,1′binaphthalene (R- or S-BINAP) ligands. Polymerization of achiral phenyl isocyanide (A-1) by S-BINAP/Pd(II) afforded a single right-handed helical poly(phenyl isocyanide)s in high yields with controlled molar mass (Mns) and narrow molar mass distributions (Mw/Mns). While the R-BINAP/Pd(II) catalyst leads to the formation of a single left-handed helix under the same conditions. The single-handed helices were determined by circular dichroism, UV−vis analyses, and direct atomic force microscopy (AFM) observations as well. Moreover, the chiral Pd(II) catalysts showed enantiomer specificity on polymerizations of isocyanide enantiomers (L- and D-1), although the chiral center was remote from the polymerization site. S-BINAP/Pd(II) can effectively promote the living polymerization of L-1, afforded a right-handed helical poly-L-1m(S), while it failed on the polymerization of the D-1 enantiomer and the D/L-1 racemate under the same conditions. Accordingly, R-BINAP/ Pd(II) can only catalyze the polymerization of the D-1 enantiomer.

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INTRODUCTION Helix is one of the most important secondary structures in biomacromolecules and plays indispensable roles in the living system such as molecular recognition, catalytic activity, genetic function, and so on.1,2 Inspired by the exquisite helical structures and functions in nature, tremendous efforts have been devoted on artificial helical polymers in recent years.3−8 However, control on the helix-sense of synthetic polymers still remains a formidable challenge.9−15 Optically active helical polymers were usually prepared either by the polymerization of specific nonracemic monomers16−20 or by the helix-senseselective polymerization of achiral or prochiral bulky monomers with chiral catalysts/initiators21−27 or additives.28 Nevertheless, the reported helix-sense-selective polymerizations usually produce polymers with low optical activity because they may contain both right- and left-handed helices just with a bias of the two polymers. Controlled synthesis of a single-handed helical polymer from an achiral monomer is still a great challenge. For example, living polymerization of achiral © XXXX American Chemical Society

phenyl isocyanide by the alkyne−Pd(II) catalyst using a chiral lactide as the additive just give polyisocyanide with 24% excess of a one-handed helix.28 One excellent example is the living polymerization of triphenylmethyl methacrylate using 9fluorenyllithium complexed with chiral (−)-sparteine (Sp) ligands, which results in an isotactic helical polymer with a large optical rotation.29 On the other hand, asymmetric polymerization of the chiral monomer usually generated helical polymers with complicated chirality because they contain more than one chiral source.30−32 For example, controlled synthesis of optically active helical polyisocyanide via the polymerization of enantiopure phenyl isocyanide containing an optically active helical backbone and chiral pendants come from the monomer.17 This complicated chirality may cause a complex in the applications dealing with chirality.33,34 To the best of Received: May 5, 2019 Revised: July 8, 2019

A

DOI: 10.1021/acs.macromol.9b00926 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules our knowledge, a single-handed helical polymer without coexisting of any mirror image helix impurity, and the chirality solely arises from the helical main chain and have rarely been reported. On the other hand, optically active helical polymers prepared from nonracemic monomers usually cost a large amount of chiral materials. As we know, chiral materials are expensive and very limited except for some natural products. Thus, development of the novel method for precise synthesis of single-handed helical polymers from achiral monomers is of great desired. Through which, a large amount of chiral materials with high optical activity can be facilely obtained from a tiny amount of chiral catalysts. Motivated by the enantioselective reactions of enzymes in nature, increasing attention has been paid on the enantiomerselective polymerizations,28,35−39 in which, one of the two enantiomers was preferentially polymerized over the antipode one. As a result, both optically active polymer and enantioenriched monomer were simultaneously obtained from a single reaction.40,41 Recently, some excellent chiral catalysts have been reported to exhibit high enantioselective polymerization on racemic monomers.42−45 Most of the reported enantiomerselective polymerizations were realized through the different polymerization rates of the two enantiomers, while each enantiomer can be polymerized indeed.27,28 Moreover, the chiral center of the enantiomers was very close to the polymerization moiety, generally located at the α- and βpositions, and few on the γ-position.38,46,47 When the chiral center is remote from the reaction site, it is very difficult to realize a good enantioselectivity, especially for acyclic monomers.48,49 Yashima et al. have reported an excellent enantiomer-selective polymerization of phenyl isocyanide by using a pre-prepared single-handed helical polyisocyanide bearing a living chain end as a macroinitiator, the selectivity on the two enantiomers is ca. 4.0. While Nolte and Rowan et al. achieved a stereospecific copolymerization of chiral isocyanopeptides using trifluoroacetic acid as an initiator with one chiral center close to the polymerizable isocyanide (β-position).50 Enantiomer-specific polymerization indicates an extremely high enantiomer-selective polymerization, in which only one the two enantiomers can be polymerized. While the enantiomeric antipode and racemate cannot be polymerized under the same conditions. To the best of our knowledge, enantiomer-specific polymerization of enantiomeric monomers with a chiral center remote from the reaction site has rarely been reported so far. Recently, we have developed a family of alkyne−Pd(II) catalysts which can promote living polymerization of various isocyanides, yield well-defined helical polyisocyanides with controlled molar masses (Mns), narrow molar mass distributions (Mw/Mns), and very importantly high tacticity.21 In this contribution, we report on chiral palladium(II) catalysts by introducing commercially available R- or S-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (R- or S-BINAP) ligands onto the alkyne−Pd(II) complex (Scheme 1). Such catalysts can promote living polymerization of phenyl isocyanides, affording well-defined helical polyisocyanides in high yields with controlled Mns and narrow Mw/Mns. Remarkably, living polymerization of achiral 4-isocyanobenzoyl-2-aminoisobutyric acid decyl ester (A-1) by the chiral catalysts lead to the formation of the single right- or lefthanded helix depending on the chirality of the catalysts. The isolated polyisocyanides showed high optical activity solely owing to the single-handed helical main chain, no chiral

Scheme 1. Helix-Sense-Specific Polymerization

moieties reside on the pendants nor on the polymer chain ends. Moreover, the R- and S-BINAP/Pd(II) catalysts showed an enantiomer-specific manner on the polymerization of isocyanide enantiomers although the chiral center is eight atom interval far from the polymerization site. Only one enantiomer can be polymerized by the R- or S-BINAP/Pd(II) catalyst, yielding a single-handed helical polyisocyanides, while the enantiomeric antipode or the racemic mixtures cannot be polymerized under the same conditions.

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RESULTS AND DISCUSSION Helix-Sense-Specific Polymerization. As shown in Scheme 1, a series of chiral R- and S-BINAP/Pd(II) catalysts in different ratios of the S-BINAP ligand to Pd(II) catalyst was prepared through the reaction of the alkyne−Pd(II) complex with the R- or S-BINAP ligand in CHCl3 at room temperature, followed the procedure reported by us previously.26 The resulting catalyst solutions were directly used in the isocyanide polymerization without further isolation and purification. The polymerization of achiral monomer A-1 with S-BINAP/Pd(II) was first performed in CHCl3 at 55 °C ([S-BINAP]0/[Pd]0 = 3, [A-1]0/[Pd]0 = 100/1, and [A-1]0 = 0.2 M). The generated poly-A-1100(S) (the footnote indicates the initial feed ratio of the monomer to Pd(II) catalyst) showed a symmetrical and unimodal elution curve on size exclusion chromatography (SEC). The Mn and Mw/Mn were, respectively, determined to be 39.4 kDa and 1.22, with equivalent to polystyrene standards. The optical activity of the isolated poly-A-1100(S) was investigated by circular dichroism (CD) and UV−vis spectroscopies. As shown in Figure 1a, an intense positive CD around 364 nm that reflect the helix-sense of the backbone was clearly observed.27,28 The molar CD intensity at 364 nm (Δε364) was estimated to be +6.4, which was further supported by polarimetry ([α]D25 = +668, 1.0, CHCl3). This result suggested the polymerization yielded a predominantly right-handed helix. It has been reported that the Δε364 value for a single-handed helical poly(phenyl isocyanide) is around 20.0 by Yashima and co-workers.27 Thus, the enantiomeric excess of the one-handed helix (eeh) of the generated polymer was determined to be ca. 32%. Interestingly, the Δε364 value was depending on the composition of the catalyst. Increase the ratio of S-BINAP to Pd(II) in the catalyst used in the polymerization, the Δε364 value of the yielded poly-A-1100(S) increased accordingly (Figure 1a). While the composition of the catalyst almost has no influence on the Mn and Mw/Mn values of the produced polymers. The isolated poly-A-1100(S)s prepared by different catalysts possess similar Mn (∼39.0 kDa) and narrow Mw/Mn (99%, which is the highest value of helical polyisocyanide prepared from achiral isocyanide to date. Because there is no other asymmetric element involved in the polymerization, it can be safely concluded that the single right-handed helix was specifically produced through the asymmetric induction of the chiral catalyst. This is the first single-handed helical polyisocyanide without containing any chiral moieties on the pendants nor on the polymer chain ends. Note that the chiral Pd(II) complex on the chain end was removed during the post-polymerization workup as confirmed by 31P NMR (see below).

Figure 2. (a) CD and UV−vis spectra of poly-A-1m(S) and poly-A1m(R) prepared with different initial feed ratios of the monomer to the catalyst (recorded in THF at 25 °C). (b) Plots of the Δε364 and optional rotation values of poly-A-1m(S) and poly-A-1m(R) as a function of the Mn values.

CD in the absorption region of the polyisocyanide backbone. While it seems the CD intensity is depending on the Mn values. The Δε364 value increased with the increase of Mn until it reached to 31.2 kDa, at which point a maximum Δε364 value of +20.3 was obtained. Further increase of the Mn could not enhance the optical activity. The optical rotations of the isolated polymers showed a similar Mn-dependent relationship,

Table 1. Characterization Data for Poly-A-1m(S) and Poly-A-1m(R)a run

polymerb

Mnc (kDa)

Mw/Mnc

Mnd (kDa)

Mw/Mnd

yielde (%)

Δε364f

[α]D25g

1 2 3 4 5 6 7 8 9

poly-A-130(S) poly-A-140(S) poly-A-150(S) poly-A-160(S) poly-A-170(S) poly-A-180(S) poly-A-1100(S) poly-A-180(R) poly-A-1100(R)

11.2 16.7 19.8 24.4 26.6 31.2 39.4 30.8 38.6

1.25 1.25 1.23 1.24 1.21 1.20 1.20 1.23 1.21

32.2 48.5 60.2 71.4 80.5 91.6 117.1 87.5 110.5

1.09 1.11 1.07 1.23 1.19 1.20 1.16 1.17 1.18

83 85 82 85 89 82 83 83 82

+8.47 +12.67 +15.61 +17.98 +19.89 +20.06 +20.30 −20.10 −20.40

+754 +1095 +1288 +1504 +1690 +1798 +1804 −1800 −1820

a

These polymers were prepared according to Scheme 1. bThe footnote indicates the initial feed ration of the monomer to the catalyst. cThe Mn and Mw/Mn values were determined by SEC with reference to polystyrene standards. dThese Mn and Mw/Mn values were determined by SEC using multi-angle light scattering detector (SEC-MALS) with THF as the eluent, dn/dc values of poly-A-1m(S)s were 0.1369. eThe isolated yields. fThe Δε364 were measured in THF at 25 °C (c = 0.2 mg/mL). gThe optical rotations were recorded in CHCl3 at 25 °C (c = 1.0). C

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of the achiral monomer is carrying out in CHCl3 at 55 °C using the R- or S-BINAP/Pd(II) catalyst. To get great insights of the helix-sense specific polymerization, the polymerizations of A-1 were carried out with the presence of R- and S-BINAP/Pd(II) mixture in different enantiopurities in CHCl3 at 55 °C with the [A-1]0/[Pd]0 fixed at 100 (Figure S3 and Table S2, Supporting Information). It was found that the optical activity of the produced polyisocyanides was strongly correlated to the enantiomeric excess (ee) of the catalysts. Increased ee of the catalyst, helical polyisocyanides with enhanced optical activity can be obtained (Figure S4, Supporting Information). This study further supported that the single-handed helix of the obtained helical poly(phenyl isocyanide) came from the asymmetric induction of the chiral catalyst during the process of the living polymerization. The structures of the single-handed helices were further verified by 1H, 31P NMR, and Fourier transform infrared spectroscopies (Figures S5−S7, Supporting Information). No signals could be detected on the 31P NMR of the isolated polyisocyanides (Figure S6, Supporting Information), suggesting the Pd(II) complex on the polymer terminus is unstable, and was decomposed during the post-polymerization process.51 These studies further support that the optical activity of the isolated poly-A-1100(S) was solely come from the helical main chain, not the chiral chain ends. Because the Pd(II) unit was decomposed in the post-polymerization workup, the Rand S-BINAP ligand may be recovered and reused in the helixsense-specific polymerizations. After the polymerization solution was precipitated into excess of methanol, the produced polymer was precipitated, while the S-BINAP remain in the solution. S-BINAP can be recovered in ca. 80% yield and the structure was confirmed by 1H NMR (Figure S8, Supporting Information). The optical activity of the recovered chiral S-BINAP was fully maintained as revealed by polarimetry ([α]D25 = −220, 0.5, benzene). The recovered S-BINAP was then reused in the S-BINAP/Pd(II) catalyst to promote the polymerization of A-1, following the procedure described above. As displayed in Figure S9 in the Supporting Information, the CD and UV−vis of the generated poly-A1100(S*) (the asterisk indicates the using of the recovered ligand) are almost the same to those prepared using a fresh SBINAP ligand in the catalyst. To get more details, the Δε364 and the optical rotation of the produced poly-A-1100(S*) were plotted against the recycling times, and was displayed in Figure 3b. It was found that the chiral S-BINAP ligand could be recovered and reused for at least 4 cycles with maintaining chiral induction ability. Thus, it can be concluded that almost no chiral resource was lost during the polymerization, and the chiral economy of the polymerization was high. Although the isolated single-handed helical polyisocyanides did not contain any chiral groups on the pendants nor on the polymer chain ends, their helical structures are quite stable. No obvious changes could be discerned on the CD and UV−vis spectra of poly-A-1100(R) and poly-A-1100(S) at the temperature range from −5 to 50 °C in THF and other common organic solvents (Figure S10, Supporting Information). The eeh value of poly-A-1100(R) and poly-A-1100(S) was further confirmed by high-resolution atomic force microscopy (AFM) observations. Diluted THF solutions of poly-A-1100(R) and poly-A-1100(S) were casted onto highly oriented pyrolytic graphite (HOPG).27 After being slowly evaporated to dryness in air, they were subjected to AFM observations. As shown in

implying that the polyisocyanides formed a stable singlehanded helix when the degree of the polymerization reached to ca. 80. To further confirm the helix-sense-specific polymerization of the achiral A-1 monomer was indeed induced by the chiral Pd(II) catalyst, the polymerization of A-1 by the enantiomeric antipode R-BINAP/Pd(II) catalyst was performed under the same experimental conditions. As expected, the polymerizations produced poly-A-1m(R)s with desired Mns and narrow Mw/Mns (Table 1). The isolated poly-A-1m(R) exhibited a negative CD at 364 nm with a mirror image to that of poly-A-1m(S), while the absorption spectra were almost the same (Figure 2a). The Δε364 of poly-A-1100(R) was estimated to be −20.4, almost the same to that of the poly-A-1100(S) in absolute value but opposite in sign. The CD and optical activity of poly-A-1m(R) were also dependent on its Mn, similar to that of the poly-A-1m(S) (Figure 2a,b). These studies further confirmed the helix-sense-specific manner of the living polymerization of achiral phenyl isocyanide by a chiral Pd(II) catalyst. As we know, the Pd(II)-mediated isocyanide polymerization give helical polyisocyanides with almost fully isotacticity.21 Thus it is worthy to note that poly-A-1m(R) and poly-A-1m(S) are enantiomeric to each other, while the most reported helical polymers with opposite handedness prepared from chiral monomers are diastereoisomers.7 To disclose the solvent effect on the polymerization, a series of polymerizations of A-1 using S-BINAP/Pd(II) as the catalyst were carried out in various organic solvents with different polarities including tetrahydrofuran (THF), dimethylformamide (DMF), and toluene ([A-1]0/[Pd]0 = 100). As expected, the polymerizations produced desired poly-A1100(S)s with expected Mns and narrow Mw/Mns (Figure S2 and Table S1, Supporting Information). All the isolated polymers showed intense positive CD in the absorption region of the polyisocyanides backbone. While the CD intensity is remarkably different. As displayed in Figure 3a, the polymers

Figure 3. (a) CD and UV−vis spectra of poly-A-1100(R) and poly-A1100(S) obtained from polymerizations in different solvents. (b) Plots of CD and optical rotation values of poly-A-1100(S*) prepared using recovered S-BINAP in the catalyst with recycling time.

obtained from THF, DMF, and toluene show attenuated CD as compared with those obtained in CHCl3. This behavior can be possibly ascribed to the weakened intramolecular hydrogen bonding between the adjacent repeating units of polyisocyanide in polar solvents.7 The reason for the lower optical activity of polyisocyanides from toluene is not very clear at the current stage. Probably, because of the noncovalent interaction between the aromatic solvent with the aromatic S-BINAP ligand. Thus, it can be concluded that the optimized experimental conditions for helix-sense specific polymerization D

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images. However, the uncrystallized polyisocyanides with opposite handedness on HOPG cannot be completely ruled out, because it is hard to distinguish the uncrystallized structure by AFM. Enantiomer-specific Polymerization. The living polymerization of phenyl isocyanide using the R- or S-BINAP/ Pd(II) catalyst was then extended to the polymerization of phenyl isocyanide enantiomers bearing L- and D-alanine decyl ester via an amide linkage (L- and D-1), respectively corresponding to the S- and R-configuration of the carbon chiral center. As shown in Scheme 2, the polymerizations of L-

Figure 4a of the taping model AFM height images, poly-A1100(S) was found to possess a rigid rod helical conformation

Scheme 2. Enantiomer-Specific Polymerizations

Figure 4. AFM height image of poly-A-1100(S) (a). AFM phase image of poly-A-1100(S) (b), and poly-A-1100(R) (d), and the expanded images of poly-A-1100(S) (c) and poly-A-1100(R) (e). (f) Schematic representations of the right-handed helical poly-A-1100(S) (right) and left-handed helical poly-A-1100(R) (left).

and self-assembled into well-defined two-dimensional helical bundles. The average constant height of a single helical chain was estimated to be ca. 2.18 nm (Figure S11, Supporting Information) with a diameter of ca. 2.21 nm. The approximately equal size of the height and diameter may be ascribed to the rigid rod helical conformation of the poly-A1100(S) backbone. The high-resolution AFM images clearly showed a one-handed helical array of the pendants with clockwise at 65 ± 2°, respect to the axis of the polymer backbone. Fortunately, the helix-sense of each polymer chain can be clearly distinguished on the zoomed in AFM phase images. As displayed in Figures 4c and S12 in the Supporting Information, only a right-handed helix can be observed on the AFM image of poly-A-1100(S) based on an estimation of more than 1000 helical blocks. That means that the eeh of poly-A1100(S) is relatively high and almost up to 100%. Accordingly, the AFM image of the poly-A-1100(R) enantiomer showed a similar morphology to that of poly-A-1100(S), but the helical array of the pendant is anticlockwise at 65 ± 2° (Figure 4f). The structural parameters obtained from AFM images are very similar to those reported in the literature with analogue chemical structures.17,27 Moreover, only the left-handed helix was observed on the AFM images of poly-A-1100(R) (Figures 4d,e, and S13 in Supporting Information), no opposite righthanded helix could be discerned under the current experimental conditions. These results suggested that only the single left- or right-handed helix was produced from the polymerization, and further confirmed the helix-sense-specific polymerization of the achiral phenyl isocyanide using the chiral Pd(II) catalyst bearing the R- or S-BINAP ligand. It should be noted that, the AFM images of the helices were depending on the surface-crystallization of the polymers. Because the leftand right-handed helices having the same chemical structure, they may possess a similar crystallization ability. Thus both left- and right-handed helices should be observed on the AFM

and D-1 were paralleled carried out in CHCl3 at 55 °C using SBINAP/Pd(II) as the catalyst ([1]0 = 0.2 M, [1]0/[Pd]0 = 100). Although the experimental condition was the same, the polymerization behavior of the two enantiomers were completely different. For L-1, the polymerization solution gradually turned to viscously dark brown with the progress of polymerization, similar to that of the achiral A-1 (Figure S14a, Supporting Information). However, the solution remained light yellow for the D-1 enantiomer throughout the polymerization progress. After the polymerization solutions were poured into methanol, a large amount of yellow polymeric materials was precipitated from the L-1 solution, while no polymer could be isolated from that of D-1 (Figure S14b, Supporting Information). Recorded SEC of the isolated poly-L1100(S) showed a symmetric and unimodal elution peak (Figure S15, Supporting Information). The Mn and Mw/Mn were estimated to be 38.7 kDa and 1.25, respectively, by SEC with reference to the polystyrene standards. This result suggested that the polymerizations of L- and D-1 enantiomers using the S-BINAP/Pd(II) catalyst may proceed in an enantiomer-specific manner. That is, the L-1 enantiomer was specifically polymerized over the enantiomeric antipode D-1 under the same experimental conditions. To confirm this hypothesis, the polymerizations of both L-1, A-1, and D-1 monomers using S-BINAP/Pd(II) were performed under identical experimental conditions with the presence of polystyrene (Mn = 2620, Mw/Mn = 1.06) as the internal standard. The polymerizations were traced by SEC. Timedependent SEC of L-1 was shown in Figure 5a. It clearly revealed that the polymerization of L-1 proceed in a living/ controlled chain-growth manner. A unimodal and symmetric E

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Supporting Information), while L-1 cannot be polymerized. This result further demonstrated the enantiomer-specific polymerization feature and supported that the specificity of the polymerization originated from the chirality of the catalyst. For comparison, the polymerization of achiral A-1 was also conducted and monitored by SEC under the same conditions. As outlined in Figures 5b and 6b, the conversion of A-1 was slower than that of L-1. It takes more than 28 h to consume 80% of A-1. The rate constant was estimated to be 4.23 × 10−5 s−1, about 2.90 times lower than that of L-1. There were almost no difference on the polymerization behavior of A-1 initiated by R- and S-BINAP/Pd(II) catalysts. This result indicated that the chirality of L-1 agreed with S-BINAP/Pd(II), and the synergistic effect enhanced the polymerization rate. While the chirality of D-1 disagreed with the chirality of the S-BNAP/ Pd(II) catalyst, and the polymerization was suppressed because of the contradictory effect (Figure S16, Supporting Information). Note that no polymerization occurred on the racemic D/L-1 mixture, when R- or S-BINAP/Pd(II) was used. Probably, the coordination of a disagreed enantiomer on the chiral Pd(II) center terminated the polymerization. The optical activities of the afforded poly-L-1100(S) and polyD-1 100 (R) were then investigated by CD and UV−vis spectroscopies (Figure 7). Both the polymers showed intense

Figure 5. Time-dependent SEC chromatograms for the polymerization of L-1 (a) and A-1 (b) using polystyrene (Mn = 2620, Mw/Mn = 1.06) as the internal standard initiated by S-BINAP/Pd(II) in CHCl3 at 55 °C.

elution peak of the produced poly-L-1m(S) was clearly observed, and continually shifted to the higher Mn-region with the progress of L-1 consumption. Plot of the conversion of L-1 with the polymerization time suggested that the polymerization was relatively fast; more than 90% of L-1 was consumed within 18 h (Figure 6a). The polymerization obeys the first

Figure 7. CD and UV−vis spectra of poly-L-1100(S), poly-D-1100(R), poly-L-1100, and poly-D-1100, poly-A-1100(S), and poly-A-1100(R) recorded in THF at 25 °C.

Figure 6. (a) Plots of conversion of L-1 and A-1 with the polymerization time using the S-BINAP/Pd(II) catalyst. (b) Firstorder kinetic plots for the polymerizations of L-1 (square, black) and A-1 (triangle, red) using the S-BINAP/Pd(II) catalyst in CHCl3 at 55 °C.

CD in the absorption region of the main chain. The Δε364 for poly-L-1100(S) and poly-D-1100(R) were estimated to be +19.2 and −19.4, implying the formation of right- and left-handed helixes, respectively. The CD and UV−vis spectra of the two polymers are almost the same to those of poly-A-1100(S) and poly-A-1100(R), but the CD intensity is a little bit lower. On the basis of the CD intensity, the eeh for the two polymers was estimated to be ca. 98%. It is worthy to note that poly-L-1100 prepared from the polymerization of L-1 using an achiral Pd(II) initiator (without any chiral ligand) showed negative CD at 364 nm, opposite to that of poly-L-1100(S). The Δε364 of poly-L-1100 is −15.0, indicating that an excess of the lefthanded helix was selectively produced due to the asymmetric induction of the chiral monomer. Thus, it can be concluded that the helicity of the produced polyisocyanides using the Ror S-BINAP/Pd(II) catalyst was solely determined by chirality of the catalyst, regardless of the monomer’s chirality. By using this method, a series of poly-L-1m(S)s with different Mn and narrow Mw/Mns were facilely prepared just through the variation on the initial feed ratio of the monomer to catalyst

reaction rate law. The appearance rate constant for L-1 was estimated to be 1.23 × 10−4 s−1 based on the kinetic plot (Figure 6b). However, in sharp contrast to L-1, no high-Mn elution peak could be discerned on the time-dependent SEC of the polymerization of D-1 (Figure S16, Supporting Information), suggesting that the polymerization of D-1 by S-BINAP/ Pd(II) did not take place. Actually, more than 85% of the D-1 monomer can be recovered from the polymerization solution by chromatography. These studies indicate the polymerizations of L- and D-1 enantiomers using the S-BINAP/Pd(II) catalyst were proceed in an enantiomer-specific manner. To further confirm the enantiomer-specific manner, the polymerizations of L- and D-1 were then performed by using the enantiomeric antipode R-BINAP/Pd(II) as the catalyst, under the same conditions to that of S-BINAP/Pd(II). As anticipated, D-1 was specifically polymerized and yield the expected helical poly-D-1m in high yields (>90%) with expected Mns and narrow Mw/Mns (Figure S15 and Table S3, F

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visualized by AFM. As displayed in Figure 9a,d of the tapingmodel AFM phase images of the poly-L-1100(S) and poly-D-

(Table 2, and Figure S17 in Supporting Information). The optical activity of the poly-L-1m(S)s with different Mn was Table 2. Characterization Data for Poly-L-1m(S) and Poly-D1m(R)a run

polymerb

Mnc (kDa)

Mw/Mnc

yieldd (%)

Δε364e

[α]D25f

1 2 3 4 5 6 7 8

poly-L-110(S) poly-L-120(S) poly-L-130(S) poly-L-140(S) poly-L-160(S) poly-L-180(S) poly-L-1100(S) poly-D-1100(R)

3.7 6.5 10.8 17.1 24.6 32.4 38.7 36.4

1.16 1.18 1.21 1.25 1.28 1.24 1.25 1.25

89 87 85 82 83 85 86 85

−3.02 −5.66 −2.51 +5.52 +12.61 +18.38 +19.20 −19.40

−260 −478 −215 +454 +1150 +1578 +1630 −1604

Figure 9. AFM phase image of self-assembled poly-L-1100(S) (a) and poly-D-1100(R) (d), and the expanded images of poly-L-1100(S) (b) and poly-D-1100(R) (e) on HOPG. Schematic representations of the right-handed helical poly-L-1100(S) (c), and left-handed helical poly-D1100(R) (f). Some defections of opposite handedness were marked with circles.

a These polymers were prepared according to the Scheme 2. bThe footnote indicates the initial feed ration of the monomer to catalyst. c The Mn and Mw/Mn values were determined by SEC with reference to polystyrene standards. dThe isolated yields. eThe Δε364 were measured in THF at 25 °C (c = 0.2 mg/mL). fThe optical rotations were recorded in CHCl3 at 25 °C (c = 1.0).

1100(R). The helical polymer chains were self-assembled into well-defined two-dimensional helical bundles and parallel stacked closely to each other due to the strong intermolecular interactions on the HOPG substrate. The height of the bundle is of ca. 2.20 nm and with good homogeneity (Figure S18, Supporting Information), consistent to the single layer of the polymers. To our delight, the helix-sense, helical pitch, and the diameter of the two helical polymers can also be clearly distinguished from the zoomed high-resolution AFM phase images (Figure 9b,e). As displayed in Figure 9b, it showed a large area of periodic oblique strips ascribed to the one-handed helical array of the pendants. The diameter of the polymer chain was estimated to be 2.23 nm for poly-L-1100(S), and 2.21 nm for poly-D-1100(R), respectively. Because of the one-handed helical array, the pendants of poly-L-1100(S) were tilted clockwise at ca. 67°, while poly-D-1100(R) were tilted anticlockwise at ca. 68° (Figure 9c,f). The average helical pitch of poly-L-1100(S) was estimated to be 1.34 nm, almost the same as that of poly-D-1100(R) (1.33 nm). The high-resolution AFM phase images revealed that poly-L-1100(S) possess a predominantly right-handed helix, while a small number of lefthanded helices can also be observed on the AFM images. Based on the evaluation of more than 1000 helical blocks, the eeh was determined to be 98% for both poly-L-1100(S) and poly-D-1100(R) (Figures S19 and S20, Supporting Information), and were consistent to the CD and optical rotation values. As we described above that the helicity of the synthetic helical polyisocyanides was quite stable, and can be maintained in various organic solvents at a wide temperature range (−5 to 50 °C), thus it is hard to change the helicity during the process of AFM observation. It was found that the eeh value of poly-L1100(S) and poly-D-1100(R) is lower than that of poly-A-1100(S) and poly-A-1100(R) prepared from the achiral monomer using the same chiral catalysts. Probably the chiral monomer has a negative effect on the helix-sense selectivity during the living polymerizations. For example, the polymerization L-1 by the achiral Pd(II)-catalyst usually lead to the formation of the lefthanded helix, while the polymerization of L-1 using chiral SBINAP/Pd(II) lead to the formation of the right-handed helix. The chirality of the monomer and catalyst showed the opposite direction on the helix-sense selectivity, which may decreased eeh of the resulting helical polymers.

investigated by CD and UV−vis absorption spectroscopies (Figure 8). To our surprise, isolated poly-L-1m(S)s with lower

Figure 8. (a) CD and UV−vis spectra of poly-L-1m(S) prepared with different initial feed ratios of the monomer to catalyst. (b) Plots of Δε364 and optional rotation values of poly-L-1m(S) as a function of the Mn values. The CD and UV−vis were recorded in THF at 25 °C.

Mn showed a negative CD at 364 nm, not a positive one as that of poly-L-1100(S) with higher Mn, suggesting that a left-handed helix was preferentially formed. The negative CD increased with the increase of Mn, and reached to a maximum at Mn = 7.5 kDa. After that, the negative CD decreased with the increase of Mn and turned positive when the Mn reached to 11.0 kDa. Then, the positive CD continually increased and became constant until the Mn reached to ca. 38.7 kDa, corresponding to the degree of the polymerization is 100, at which point a maximum Δε364 of +19.2 was obtained. Further increased the Mn could not enhance the optical activity (Figure 8a,b), indicating a right-handed helix was selectively produced. The optical rotation values of the isolated poly-L-1m(S)s with different Mn showed almost the same relationship with Mn as that obtained from the CD investigation. The reason of the helicity inversion is not very clear at the current stage. Probably, a left-handed helix was preferentially formed due to the chirality of the L-1 enantiomer at the beginning stage of the polymerization. While it was self-terminated by the formed helicity when the polymer chain increased to a certain length. In addition to CD and polarimetry analyses, the helix-sense of the afforded poly-L-1100(S) and poly-D-1100(R) was further

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CONCLUSIONS In conclusion, we have developed a family of chiral catalysts of R- or S-BINAP/Pd(II). The polymerization of achiral phenyl G

DOI: 10.1021/acs.macromol.9b00926 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

(3) Yashima, E.; Ousaka, N.; Taura, D.; Shimomura, K.; Ikai, T.; Maeda, K. Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions. Chem. Rev. 2016, 116, 13752. (4) Shen, J.; Okamoto, Y. Efficient Separation of Enantiomers Using Stereoregular Chiral Polymers. Chem. Rev. 2016, 116, 1094. (5) Zhang, D.-W.; Zhao, X.; Hou, J.-L.; Li, Z.-T. Aromatic Amide Foldamers: Structures, Properties, and Functions. Chem. Rev. 2012, 112, 5271. (6) Akagi, K. Helical Polyacetylene: Asymmetric Polymerization in a Chiral Liquid-Crystal Field. Chem. Rev. 2009, 109, 5354. (7) Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Helical Polymers: Synthesis, Structures, and Functions. Chem. Rev. 2009, 109, 6102. (8) Freire, F.; Quiñoá, E.; Riguera, R. Supramolecular Assemblies from Poly(phenylacetylene)s. Chem. Rev. 2016, 116, 1242. (9) Maeda, K.; Hirose, D.; Okoshi, N.; Shimomura, K.; Wada, Y.; Ikai, T.; Kanoh, S.; Yashima, E. Direct Detection of Hardly Detectable Hidden Chirality of Hydrocarbons and Deuterated Isotopomers by a Helical Polyacetylene through Chiral Amplification and Memory. J. Am. Chem. Soc. 2018, 140, 3270. (10) Tian, G.; Lu, Y.; Novak, B. M. Helix-Sense Selective Polymerization of Carbodiimides: Building Permanently Optically Active Polymers from Achiral Monomers. J. Am. Chem. Soc. 2004, 126, 4082. (11) Aoki, T.; Kaneko, T.; Maruyama, N.; Sumi, A.; Takahashi, M.; Sato, T.; Teraguchi, M. Helix-Sense-Selective Polymerization of Phenylacetylene Having Two Hydroxy Groups Using a Chiral Catalytic System. J. Am. Chem. Soc. 2003, 125, 6346. (12) Ikai, T.; Yoshida, T.; Shinohara, K.-i.; Taniguchi, T.; Wada, Y.; Swager, T. M. Triptycene-Based Ladder Polymers with One-Handed Helical Geometry. J. Am. Chem. Soc. 2019, 141, 4696. (13) Suzuki, N.; Fujiki, M.; Kimpinde-Kalunga, R.; Koe, J. R. Chiroptical Inversion in Helical Si−Si Bond Polymer Aggregates. J. Am. Chem. Soc. 2013, 135, 13073. (14) Gudeangadi, P. G.; Sakamoto, T.; Shichibu, Y.; Konishi, K.; Nakano, T. Chiral Polyurethane Synthesis Leading to π-Stacked 2/1Helical Polymer and Cyclic Compounds. ACS Macro Lett. 2015, 4, 901. (15) Huang, H.; Yuan, Y.; Deng, J. Helix-Sense-Selective Precipitation Polymerization of Achiral Monomer for Preparing Optically Active Helical Polymer Particles. Macromolecules 2015, 48, 3406. (16) Wang, S.; Feng, X.; Zhao, Z.; Zhang, J.; Wan, X. Reversible CisCisoid to Cis-Transoid Helical Structure Transition in Poly(3,5disubstituted phenylacetylene)s. Macromolecules 2016, 49, 8407. (17) Kajitani, T.; Okoshi, K.; Sakurai, S.-i.; Kumaki, J.; Yashima, E. Helix-Sense Controlled Polymerization of a Single Phenyl Isocyanide Enantiomer Leading to Diastereomeric Helical Polyisocyanides with Opposite Helix-Sense and Cholesteric Liquid Crystals with Opposite Twist-Sense. J. Am. Chem. Soc. 2006, 128, 708. (18) Kouwer, P. H. J.; Koepf, M.; Le Sage, V. A. A.; Jaspers, M.; van Buul, A. M.; Eksteen-Akeroyd, Z. H.; Woltinge, T.; Schwartz, E.; Kitto, H. J.; Hoogenboom, R.; Picken, S. J.; Nolte, R. J. M.; Mendes, E.; Rowan, A. E. Responsive biomimetic networks from polyisocyanopeptide hydrogels. Nature 2013, 493, 651. (19) Hu, G.; Li, W.; Hu, Y.; Xu, A.; Yan, J.; Liu, L.; Zhang, X.; Liu, K.; Zhang, A. Water-Soluble Chiral Polyisocyanides Showing Thermoresponsive Behavior. Macromolecules 2013, 46, 1124. (20) Asaoka, S.; Joza, A.; Minagawa, S.; Song, L.; Suzuki, Y.; Iyoda, T. Fast Controlled Living Polymerization of Arylisocyanide Initiated by Aromatic Nucleophile Adduct of Nickel Isocyanide Complex. ACS Macro Lett. 2013, 2, 906. (21) 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 (Phenylbuta1,3-diynyl)palladium(II) Complexes: Highly Active Initiators for Living Polymerization of Isocyanides. J. Am. Chem. Soc. 2014, 136, 4706.

isocyanides by the chiral catalysts afforded single-handed helical poly(phenyl isocyanide)s with controlled Mns and narrow Mw/Mns. The chirality of the catalyst determined the whole handedness of the afforded helical polymers. The helicity was undisputed and confirmed by CD, UV−vis, optical rotation, and direct AFM observations as well. To the best of our knowledge, this is the first report on the controlled synthesis of single-handed helical polyisocyanides from an achiral monomer. The high optical activity of the resulting polymer was solely came from the helical backbone, without coexistence of any other chiral moieties on pendants or on the polymer chain ends. Moreover, the R- and S-BINAP/Pd(II) catalysts also showed interesting enantiomer-specific manners on the polymerization of phenyl isocyanide enantiomers, although the chiral center is far from the polymerization site. Only one of the two enantiomers can be specifically polymerized under identical experimental conditions, leading to the formation of single-handed helical polymers. We believe that the present study provides a useful method for the precise synthesis of high optically active materials from achiral materials. Considering the modification on the structure of monomers, a variety of chiral functional materials can be facilely produced, which may have great potential in enantiomer separation, asymmetric catalysis, liquid crystallization, and so on.

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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.9b00926. Synthetic procedure and additional spectroscopies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.L.). *E-mail: [email protected] (Z.-Q.W.). ORCID

Yuan-Yuan Zhu: 0000-0002-3142-0396 Zhibo Li: 0000-0001-9512-1507 Zong-Quan Wu: 0000-0001-6657-9316 Author Contributions §

L.X., L.Y. and Z.G. authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC, nos. 21622402, 21574036, 51673057, and 21871073). Z.Q.W. thanks financial supports from the Thousand Young Talents Program of China, and the fundamental research funds for the central universities.

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DOI: 10.1021/acs.macromol.9b00926 Macromolecules XXXX, XXX, XXX−XXX