Propagation-Inspired Initiation of an Aliphatic Sodium Amidate for the

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Propagation-Inspired Initiation of an Aliphatic Sodium Amidate for the Living Anionic Homo- and Copolymerization of Isocyanates: Access to the Multiblocky Sequence Distribution of Binary Comonomers Chang-Geun Chae, In-Gyu Bak, and Jae-Suk Lee*

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School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea S Supporting Information *

ABSTRACT: We report the propagation-inspired initiation of sodium N-phenethyl-3-phenylpropanamide (NaPEPPA), an aliphatic sodium amidate, for the living anionic homo- and copolymerization of isocyanates. This initiator was compared with sodium benzanilide (NaBA), an aromatic sodium amidate, in the living anionic homopolymerization of n-hexyl isocyanate (HIC). Only NaPEPPA attained the initiation efficiencies close to unity at the early stage of propagation. The homopolymerization with [HIC]0/[NaPEPPA]0 = 38.9/85.1/203 led to poly(n-hexyl isocyanate)s (PHICs) with predictable MWs and low dispersities (Mn,theo = 5.12/10.7/24.7 kDa; Mn = 5.22/11.1/27.4 kDa; Đ = 1.11/1.10/1.06). NaPEPPA was also used to initiate the living anionic copolymerization of HIC and furfuryl isocyanate (FIC). As a result, poly(furfuryl isocyanateblock-n-hexyl isocyanate) (P(FIC-b-HIC)) was afforded by the blocky monomer sequence distribution. Based on the copolymerization kinetics, a series of polyisocyanate-based multiblock copolymers, P(FIC-b-HIC)1/P(FIC-b-HIC)2/P(FIC-bHIC)3/P(FIC-b-HIC)4 (Mn,theo = 5.47/10.6/15.8/20.8 kDa; Mn = 5.59/11.5/16.3/20.3 kDa; Đ = 1.10/1.03/1.03/1.03), were afforded by the repetitive sequential addition of comonomers.

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INTRODUCTION

exciting challenge for polyisocyanates to adopt novel structures. For this purpose, the copolymerization of at least two monomers can be a promising approach. The common synthetic methods for polyisocyanates are the anionic polymerization33−40 and organotitanium(IV)-catalyzed coordination polymerization of isocyanates.41−44 We consider that the former is better suited in controlling the monomer sequence of copolymer by the choice of comonomers.45−52 The kinetics of propagation reaction depends greatly on the structural factors of monomer, such as steric hindrance and electron-donating/withdrawing effect of substituent. The propagating ion pairs of polyisocyanates (amidate ion pairs) are tolerant to a wide variety of functional groups except protic

Three-dimensional organization of multiple secondary structural peptides is a crucial factor for the emergence of various biological structures and functions in proteins.1−6 This has motivated the development of peptide-mimic macromolecules and the exploration of their novel applications.7−16 In polymer chemistry, the synthesis of artificial helical polymers with their main-chain conformations originating from the adequate steric restriction has been widely investigated.17−20 Specially, polyisocyanates, which have dynamic helical conformations,21−29 are considered as promising models to mimic the peptide dynamics due to their feature of reversible transformation between helices and sheets under thermodynamic control.30 The structure of each protein is associated with its amino acid sequence.31,32 Accordingly, the unique sequence distribution of functionalized side groups along the main chain is an © XXXX American Chemical Society

Received: September 25, 2018 Revised: December 3, 2018

A

DOI: 10.1021/acs.macromol.8b02052 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules groups due to their weak nucleophilicity.33,36 This factor allows the anionic polymerization of many types of functional isocyanates.53−61 In order for the propagating species to remain living in the anionic polymerization, the equilibrium shift from multiple active centers (free ions, unimeric/associated ion pairs) to single-site ones with adequate reactivity is often necessary.62−65 The types of active centers involving the anionic polymerization of isocyanates are unstable free ions and highly living amidate ion pairs.66 The free ion formation is prevented by adding sodium tetraphenylborate (NaBPh4) as a common ion in order that the active chain ends can be protected from the unexpected termination.66−68 From a kinetic study of living anionic polymerization using fast-initiating sodium diphenylmethane (NaDPM), we have found that the rate of propagation reaction follows the first-order kinetics with respect to the initial concentration of initiator.66 Many classes of organosodium compounds with a wide range of nucleophilicities have been available as initiators for the living anionic polymerization of isocyanates.69−73 Those include sodium amidate compounds that possess the same kinds of active centers as the propagating amidate ion pairs.69,72 The first amidate initiator used is sodium benzanilide (NaBA), which has quantitatively yielded well-defined polyisocyanate products.69 However, this initiator has seemed to have too low nucleophilicity to perform the fast initiation reaction with aliphatic isocyanates. It is important for an initiating species to have nucleophilicity comparable to that of propagating species to perform the uniform monomer sequence distribution during copolymerization. The use of a sodium amidate with aliphatic substituents that can resemble aliphatic isocyanate monomers is considered as a reasonable way to satisfy the requirement of fast initiation in the anionic copolymerization of isocyanates. This can be proven by confirming the different effects of aromatic and aliphatic sodium amidates on the kinetic behavior of homopolymerization of isocyanates. As a proof of this concept, in this work, we develop a new sodium amidate, sodium N-phenethyl-3-phenylpropanamide (NaPEPPA), which can rapidly initiate the anionic polymerization of isocyanates. NaPEPPA is an aliphatic sodium amidate as it contains two phenethyl substituents, and thus is different from NaBA, an aromatic sodium amidate, in terms of reactivity. In NaPEPPA, the loss of the resonance effect of two phenyl groups due to the two intervening ethylene linkers localizes the negative charge on the amidate active center, thereby increasing its nucleophilicity. This work deals with a comparative kinetic study of the anionic polymerizations of isocyanates initiated by NaBA and NaPEPPA to evaluate the initiation ability. Then, a kinetic study of the NaPEPPAinitiated copolymerization of binary isocyanate comonomers is attempted to investigate its novel monomer sequence distribution.

Scheme 1. (a) Chemical Structures of NaBA and NaPEPPA, (b) Anionic Homopolymerization of HIC Initiated by NaBA or NaPEPPA in the Presence of NaBPh4 ([NaBPh4]0/ [NaBA or NaPEPPA]0 = 5), and (c) Anionic Homopolymerization of FIC Initiated by NaPEPPA in the Presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr

isocyanate (HIC) to afford poly(n-hexyl isocyanate) (PHIC). HIC has been proven to be a model monomer suitable to investigate the kinetic behavior of anionic polymerization.66 The anionic polymerization of HIC initiated by NaBA or NaPEPPA was typically performed in THF at −98 °C under 10−6 Torr, in which each initiator was mixed with 5 mol equiv of NaBPh4 ([NaBPh4]0/[NaBA or NaPEPPA]0 = 5) (Scheme 1b). Under similar conditions, NaPEPPA was also utilized in the anionic polymerization of furfuryl isocyanate (FIC) to afford poly(furfuryl isocyanate) (PFIC) (Scheme 1c). In fact, the use of NaBA has enabled the living anionic polymerization of HIC without the aid of common-ion effect.69 This system has quantitatively yielded lowly dispersed PHICs with MWs 5 times as high as ones expected from [HIC]0/ [NaBA]0 × conversion to polymer. The improved livingness and low initiation efficiencies (f ≈ 0.20) have been attributed to that a fraction of the total NaBA molecules participate in the initiation reaction and the rest are cross-associated with the propagating ion pairs protecting them from possible side reactions.69 This dual-functional process might have originated from a nature of NaBA for the formation into a highly stabilized cluster, (NaBA)n, in THF. The degree of association has not been determined, but two probable cluster structures,

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RESULTS AND DISCUSSION Kinetic Study of the Living Anionic Homopolymerization of Isocyanates Initiated by NaBA or NaPEPPA. NaBA and NaPEPPA represent the aromatic and aliphatic sodium amidate compounds, respectively (Scheme 1a). Each of them was synthesized by the reaction between sodium naphthalenide (NaNaph) and its conjugate acid (benzanilide (BA) or N-phenethyl-3-phenylpropanamide (PEPPA)) and then was utilized in the anionic homopolymerization of n-hexyl B


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Figure 1. Kinetics profiles of the anionic polymerization of HIC initiated by (a) NaBA or (b) NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/ [NaBA or NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr: (left columns) plots of ln([HIC]0/[HIC]t) versus t, (middle columns) plots of Mn and Đ of PHIC versus conversion to polymer (linear fits of Mn,theo against conversion to polymer are displayed as dashed lines), and (right columns) plots of f versus conversion to polymer

various initial molar concentrations of initiator ([NaBA]0 = 15.3/7.25/3.23/1.71 × 10−3 mol L−1; [NaPEPPA]0 = 12.9/ 6.56/3.02/1.26 × 10−3 mol L−1). Those set values correspond to [HIC]0/[NaBA]0 = 16.9/36.0/79.6/149 and [HIC]0/ [NaPEPPA]0 = 19.8/38.9/85.1/203. The values of conversion to polymer, Mn, and Đ of PHIC at each different time (t, [s]) were determined through size exclusion chromatography− multiangle laser light scattering (SEC-MALLS). In most cases, the quantitative conversions to polymer were achieved without noticeable formation of cyclotrimers. The kinetic profiles revealed that NaBA slowly initiated the anionic polymerization of HIC at [HIC]0/[NaBA]0 = 16.9− 149 ([NaBA]0 = 15.3−1.71 × 10−3 mol L−1) (Figure 1a). The increase of ln([HIC]0/[HIC]t) was accelerated over time, indicating that a number of propagating species were slowly generated from NaBA molecules during propagation (Figure 1a, left column). This tendency was considered to be attributed to the rate of initiation much lower than that of propagation. Accordingly, the Mn values of growing PHICs not only slowly increased throughout the propagation period but also were larger than the values of theoretical Mn (Mn,theo) (Figure 1a, middle column). The initiation efficiencies gradually increased with the conversion to polymer but failed to finally reach ∼1 (Figure 1a, right column). When NaPEPPA was used instead of NaBA, the kinetic profiles showed its initiation much faster than that of NaBA (Figure 1b). Although the initiation period was still largely overlapped with the propagation period at [HIC]0/[NaPEPPA]0 = 19.8 ([NaPEPPA]0 = 12.9 × 10−3 mol L−1), the period of overlap became considerably narrowed at [HIC]0/

especially dimer ((NaBA)2) and tetramer ((NaBA)4), have been determined by density functional theory calculation.73 We revisited the NaBA-initiated anionic polymerization of HIC in the absence of NaBPh4 ([NaBPh4]0/[NaBA]0 = 0]) for the kinetic study (Figure S11a). The living polymerization proceeded only at high [NaBA]0 values (= 13.3−6.30 × 10−3 mol L−1), leading to the low initiation efficiencies (f = 0.182− 0.214) and the quantitative conversion to PHICs (= 97.4− 92.5%) with low dispersities (number-average MW (Mn) = 13.1−22.8 kDa; dispersity (Đ) = 1.07−1.12). The living system was destroyed at lower [NaBA]0 values (= 3.11−1.45 × 10−3 mol L−1). Reducing the initiator concentration not only increased the initiation efficiency (f = 0.308−0.357) but also caused the irreversible termination, which consequently led to the low conversion to PHICs (= 67.0−66.7%) with relatively high dispersities (Mn = 22.7−42.0 kDa; Đ = 1.34−1.37). We presumed that the partial breakage of cross-associated ion pairs in dilute solutions caused the equilibration between the unimeric ion pairs and free ions. When NaPEPPA was used instead of NaBA in the anionic polymerization of HIC ([NaBPh4]0/[NaPEPPA]0 = 0]) for the kinetic study, both slow initiation and irreversible termination occurred at [NaPEPPA]0 = 10.6−1.23 × 10−3 mol L−1 probably due to the partial existence of free ions (Figure S11b). The limited livingness led us to decide to adopt the common-ion strategy using NaBPh4. The kinetic analysis was designed as follows: the anionic polymerization was performed at standard initial molar concentrations of HIC ([HIC]0 = 0.258 ± 0.002 mol L−1 for NaBA and 0.256 ± 0.001 mol L−1 for NaPEPPA) and C


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Table 1. MW Properties of Final PHICs Yielded from the Anionic Polymerization of HIC Initiated by NaBA or NaPEPPA in the Presence of NaBPh4 ([NaBPh4]0/[NaBA or NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr entry

[HIC]0 (mol L−1)

[I]0 (mol L−1)

1 2 3 4

0.258 0.261 0.257 0.255

15.3 7.25 3.23 1.71

× × × ×

10−3 10−3 10−3 10−3

16.9 36.0 79.6 149

5 6 7 8

0.256 0.255 0.257 0.256

12.9 6.56 3.02 1.26

× × × ×

10−3 10−3 10−3 10−3

19.8 38.9 85.1 203

[HIC]0/[I]0

Mn,theoc (kDa)

Mnd (kDa)

Đd

fe

98.4/0 98.8/0.1 98.1/0.4 93.3/0.6

2.31 4.72 10.1 17.9

5.13 8.10 14.7 23.2

1.09 1.11 1.16 1.20

0.428 0.572 0.683 0.770

99.2/0 98.3/0 96.5/0 94.7/0

2.50 5.12 10.7 24.7

3.25 5.22 11.1 27.4

1.04 1.11 1.10 1.06

0.750 0.980 0.963 0.901

conv (%) (Pa/CTb)

t (s)

1: NaBA 450 600 1200 1800 I: NaPEPPA 450 600 1200 2400

a

Conversion to polymer determined from the gravimetric yield of polymer. bConversion to cyclotrimer determined from 1H NMR. cMn,theo = MW of terminal residue + [HIC]0/[I]0 × MW of HIC × conv to polymer/100%. dDetermined from SEC-MALLS using a typical dn/dc value of PHIC = 0.090 mL g−1. ef = (Mn,theo − MW of terminal residue)/(Mn − MW of terminal residue).

Figure 2. (a, b) SEC-dRI traces and (c, d) plots of Mn and Đ versus [HIC]0/[NaBA or NaPEPPA]0 for final PHICs yielded from the anionic polymerization of HIC initiated by NaBA or NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/[NaBA or NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr. Initiator: (a, c) NaBA and (b, d) NaPEPPA.

[NaPEPPA]0 = 38.9−203 ([NaPEPPA]0 = 6.56−1.26 × 10−3 mol L−1). In this range, ln([HIC]0/[HIC]t) linearly increased with t (Figure 1b, left column). The Mn values of growing PHICs almost linearly increased with the conversion to polymer, well in agreement with the scaling of Mn,theo values (Figure 1b, middle column). The plots of f versus conversion to polymer revealed that the conversion of NaPEPPA reached the completion points close to 1 at the early stage of propagation (Figure 1b, right column). In the plots of ln([HIC]0/[HIC]t) versus t, the slopes of the three linear fits thus could be considered the same as the values of apparent rate constant (kapp, [s−1]). The final results of the anionic polymerization of HIC initiated by NaBA or NaPEPPA are listed in Table 1. The Mn

of PHIC increased from 5.13 to 23.2 kDa by increasing [HIC]0/[NaBA]0 from 16.9 to 149 and increased from 3.25 to 27.4 kDa by increasing [HIC]0/[NaPEPPA]0 from 19.8 to 203. The Đ values of final PHICs were as low as 1.04−1.20 in both cases of initiation. Noticeably, the fast and complete initiation was accomplished only in the polymerization at [HIC]0/[NaPEPPA]0 = 38.9−203. In this case, the concentration of effectively converted NaPEPPA ([NaPEPPA] = [NaPEPPA]0 × f) was close to [NaPEPPA]0 whereby the final PHICs retained the Mn values (= 5.22/11.1/27.4 kDa) well accorded with the Mn,theo values (= 5.12/10.7/24.7 kDa). The SEC difference refractive index (dRI) traces of PHICs did not show any noticeable initiator dependence on the MW distribution of PHIC (Figure 2a,b). However, the NaPEPPA D


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by the time-varying polymerization with [FIC]0/[NaPEPPA]0 = 99.6 ([FIC]0 = 0.252 M; [NaPEPPA]0 = 2.53 × 10−3 mol L−1) (Figure 4). The conversion of FIC passed through 77.4% in just 30 s and finally reached 99.3% in 120 s. The livingness of polymerization was verified by the nearly linear increase of ln[FIC]0/[FIC]t with t (Figure 4a). However, the initiation was incompletely finished probably due to its competition with very fast propagation. As a result, the PFICs at the conversions to polymer within 77.4−99.3% exhibited the MWs slightly larger than the theoretical ones (Mn,theo = 9.36−12.4 kDa; Mn = 11.4−14.4 kDa; Đ = 1.07−1.11) and the relatively low initiation efficiencies (f = 0.817−0.859) (Figure 4b,c). As the slow initiation seemed not to be serious, the average f, [NaPEPPA], and kapp for FIC were successfully calculated. Then, we determined a typical value of k∓p as average kapp/ average [NaPEPPA]. The k∓p value for FIC was found to 21.0 L mol−1 s−1. This value is 20.4 times as high as that for HIC. Kinetic Study of the Living Anionic Copolymerization of HIC:FIC Initiated by NaPEPPA. The terminal model theory can describe the kinetic behavior of the NaPEPPAinitiated anionic copolymerization of a binary mixture of isocyanates (M1:M2) as the contribution of the self- and crosspropagation reactions of two kinds of terminal ion pairs (M∓1 and M∓1 ) with four kinds of rate constants (k∓11, k∓12, k∓22, and ∓ k21 ). The copolymer equation can be given using two comonomer reactivity ratios (r1 = k∓11/k∓12; r2 = k∓22/k∓21):74,75

initiation showed an obvious advantage in the control of MW of PHIC based on the feed ratio of monomer to initiator (Figure 2c,d). The initiation of NaBA offered the Mn values of PHICs that are larger than the Mn,theo values and not linearly dependent on [HIC]0/[NaBA]0 due to too slow initiation of NaBA (Figure 2c). When the fast and complete initiation succeeded by NaPEPPA at [HIC]0/[NaPEPPA]0 = 38.9−203, the Mn value of PHIC increased linearly with [HIC]0/ [NaPEPPA]0, well in agreement with the scaling of Mn,theo at 100% conversion to polymer (Mn,theo(100%)) (Figure 2d). For [NaPEPPA]0 = 6.56−1.26 × 10−3 mol L−1 where the fast and complete initiation was attained, we calculated the average values of f, [NaPEPPA], and kapp. We plotted the kapp/ s−1 values against the [NaPEPPA]/(mol L−1) values on the double-logarithmic scale and finally obtained a linear fit with a slope of 1 and a intercept of 0.0333. This result indicates that the rate of propagation exhibits a first-order dependence on [NaPEPPA]. A similar trend has been found in our previous kinetic study of the anionic polymerization of HIC initiated by NaDPM in the presence of NaBPh4,66 in which we have found that the propagating amidate ion pairs of PHIC do not have any self-association behavior that may lead to their conversion to dormant species. Therefore, the main propagating species existing in the NaPEPPA-initiated anionic polymerization of HIC would be PEPPA(HIC)nNa (n: degree of polymerization).

F1 =

We developed a rate law of the anionic polymerization of HIC initiated by NaPEPPA. At [NaPEPPA]0 = 6.56−1.26 × 10−3 mol L−1, the molar concentration of PEPPA(HIC)nNa at t ([PEPPA(HIC)nNa]t) is close to [NaPEPPA]0. Thus, the rate of propagation, Rp, is given by R p = k p∓[PEPPA(HIC)n Na]t [HIC]t ≈ k p∓[NaPEPPA]0 [HIC]t (1) −1

r1f12 + 2f1 f2 + r2f2 2

(2)

where f1 and f 2 are the instantaneous mole fractions of M1 and M2, respectively, in the feed, and F1 and F2 are the instantaneous mole fractions of M1 and M2, respectively, in the copolymer. Several classes of copolymerizations, such as random, alternating, gradient, and blocky copolymerizations, are defined according to their monomer sequences. The relation of four rate constants is a clue for the kinetic behavior of monomer sequence distribution in the copolymerization. Therefore, the determination of the comonomer reactivity ratios is an essential step to predict the monomer sequence of a copolymer (random sequence: r1 and r2 ≈ 1; alternating sequence: r1 and r2 ≪ 1; gradient sequence: r1 < 1 < r2; blocky sequence: r1 and r2 ≫ 1). There are many types of determinations for reactivity ratios, which include Mayo−Lewis,74,75 Fineman−Ross,76 Kelen−Tüdös,77 and nonlinear least-squares curve fitting methods.78 Previously, we have studied the kinetics of the anionic copolymerization of HIC:allyl isocyanate (AIC) initiated by NaDPM in the presence of NaBPh4 in THF at −98 °C under 10−6 Torr.66 Those comonomers has generated the tapered block copolymers containing a broad gradient interface. Now, we decided to change the combination of binary comonomers to achieve a novel sequence between functional and nonfunctional side groups. We have used FIC in our previous study due to the significance of furan ring for the postpolymerization modification through the Diels−Alder click chemistry.79 We envisioned that the replacement of AIC with FIC in the copolymerization with HIC may alter the nature of monomer sequence distribution. We investigated the kinetics of the anionic copolymerization of HIC and FIC initiated by NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C

Figure 3. Plot of ln(kapp/s−1) versus ln{[NaPEPPA]/(mol L−1)} for the anionic polymerization of HIC initiated by NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr.

k∓p

r1f12 + f1 f2

−1

where ([L mol s ]) is the rate constant of propagation for the amidate ion pair. The k∓p value for HIC was determined to be 1.03 L mol−1 s−1, which is comparable to a value determined from the anionic polymerization of HIC initiated by NaDPM in the presence of NaBPh4 (k∓p = 1.24 L mol−1 s−1). We employed NaPEPPA to initiate the anionic polymerization of FIC in the presence of NaBPh4 ([NaBPh4]0/ [NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr to afford PFIC (Scheme 1c). The kinetic analysis was performed E


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Figure 4. Kinetics profiles of the anionic polymerization of FIC initiated by NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr: (a) plot of ln([FIC]0/[FIC]t) versus t, (b) plots of Mn and Đ of PFIC versus conversion to polymer (a linear fit of Mn,theo against conversion to polymer is displayed as a dashed line), and (c) plot of f versus conversion to polymer.

under 10−6 Torr. The copolymerization was performed by adding a binary mixture of HIC:FIC with a molar feed composition, f HIC:f FIC = 0.502:0.498, to the initiating compound (1:5 NaPEPPA:NaBPh4). Every copolymerization with [HIC:FIC]0/[NaPEPPA]0 = 202 ([HIC:FIC]0 = 0.249 mol L−1; [NaPEPPA]0 = 1.23 × 10−3 mol L−1) was terminated at a different times to observe the change of molar composition between HIC and FIC units in copolymer at each total conversion to copolymer. Surprisingly, the following kinetic evidence revealed that the anionic copolymerization of HIC:FIC led to the highly blocky monomer sequence distribution in order from FIC to HIC to generate poly(furfuryl isocyanate-block-n-hexyl isocyanate) (P(FIC-b-HIC)) containing a very narrow gradient interface (Scheme 2). The propagation reaction was very fast in the first 30 s, in which the total conversion to copolymer reached 42.2%. After that time, the total conversion of copolymer slowly increased

from 42.2% to 95.0% as the copolymerization time increased from 30 to 2400 s. The chemical compositions of those copolymers were characterized by their 1H NMR spectra (Figure 5). Their molar compositions between HIC and FIC units were determined by the integral ratios of proton resonances for the α-carbons of two kinds of side chains. Until the conversion to copolymer reached 49.7%, FIC units were almost the whole units comprising the copolymers. When the conversion to copolymer increased from 49.7% to 95.0%, the mole fraction of HIC units in copolymer gradually increased. The results of the time-varying anionic copolymerization of HIC:FIC with f HIC:f FIC = 0.502:0.498 are listed in Table 2. The conversions of HIC and FIC at each total conversion to copolymer were determined from the characterized molar composition between HIC and FIC. Then, the Mn,theo value at each total conversion to copolymer was calculated based on the conversions of HIC and FIC. When the SEC-MALLS measurement was conducted, most of the copolymers at the total conversions within 42.2−95.0% exhibited the Mn values (= 11.1−24.6 kDa) which well accorded with the Mn,theo values (= 10.8−24.2 kDa) and very low Đ values (= 1.04−1.06). The cumulative mole fractions of HIC and FIC units in copolymer (Fcum,HIC and Fcum,FIC) at each total conversion to copolymer were determined from the 1H NMR results. Then, the instantaneous mole fractions of HIC and FIC (Finst,HIC and Finst,FIC) at each total conversion to copolymer were calculated according to the following equations:80

Scheme 2. Living Anionic Copolymerization of HIC:FIC Initiated by NaPEPPA in the Presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr To Afford P(FIC-b-HIC)

Finst,HIC(i) =

total conv(i) × Fcum,HIC(i) − total conv(i − 1) × Fcum,HIC(i − 1) total conv(i) − total conv(i − 1)

(3) Finst,FIC(i) = 1 − Finst,HIC(i)

(4)

The kinetic behavior of the living anionic copolymerization of HIC:FIC with f HIC:f FIC = 0.502:0.498 initiated by NaPEPPA was presented in the plots of [HIC]t and [FIC]t versus total conversion to copolymer and the plots of Finst,HIC and Finst,FIC in copolymer versus total conversion to copolymer (Figure 6). The FIC monomers preferentially reacted with the propagating species until those were almost consumed. Subsequently, the HIC monomers began the propagation reaction (Figure 6a). As a result, the homogeneous F


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Figure 5. 1H NMR spectra of P(FIC-b-HIC)s yielded from the time-varying anionic copolymerization of HIC:FIC with f HIC:f FIC = 0.502:0.498 initiated by NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr. t = (a) 30, (b) 60, (c) 150, (d) 300, (e) 600, (f) 1200, and (g) 2400 s.

Table 2. Time-Varying Anionic Copolymerization of HIC:FIC with f HIC:f FIC = 0.502:0.498 Initiated by NaPEPPA in the Presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torra conv (%) b

entry

t (s)

total

9 10 11 12 13 14 15

30 60 150 300 600 1200 2400

42.2 49.7 54.0 60.0 75.5 82.4 95.0

HICc

FICc

dn/dcd (mL g−1)

Mn,theoe (kDa)

Mnf (kDa)

Đf

FcumHICg

Fcum,FICg

Finst,HICh

Finst,FICh

1.7 2.3 8.5 20.6 51.3 65.2 90

83.0 97.5 99.9 99.8 99.9 99.8 99.9

0.120 0.120 0.118 0.115 0.110 0.108 0.105

10.8 12.6 13.7 15.3 19.2 21.0 24.2

11.1 12.8 14.1 15.7 19.7 21.5 24.6

1.04 1.04 1.04 1.04 1.04 1.06 1.05

0.021 0.023 0.079 0.171 0.341 0.396 0.476

0.979 0.977 0.921 0.829 0.659 0.604 0.524

0.021 0.034 0.356 0.999 0.999 0.998 0.999

0.979 0.966 0.644 0.001 0.001 0.002 0.001

[HIC:FIC]0 = 0.249 mol L−1; [NaPEPPA]0 = 1.23 × 10−3 mol L−1; [HIC:FIC]0/[NaPEPPA]0 = 202; f HIC and f FIC: mole fractions of HIC and FIC in the feed. bTotal conversion to copolymer determined from the gravimetric yield of copolymer. cConversions of HIC and FIC to copolymer determined from the 1H NMR spectrum of copolymer. ddn/dc of copolymer = weight fraction of HIC units in copolymer × 0.090 mL g−1 + weight fraction of FIC units in copolymer × 0.120 mL g−1. eMn,theo = MW of PEPPA + [HIC:FIC]0/[NaPEPPA]0 × (f HIC × conv of HIC/100% × MW of HIC + f FIC × conv of FIC/100% × MW of FIC). fDetermined from SEC-MALLS. gFcum,HIC and Fcum,FIC: cumulative mole fractions of HIC and FIC units in copolymer determined from the 1H NMR spectrum of copolymer. hFinst,HIC and Finst,FIC: instantaneous mole fractions of HIC and FIC units in copolymer calculated from eqs 3 and 4. a

incorporation for the two comonomers predominantly proceeded in order from FIC to HIC, while the compositional drift occurred at the narrow conversions to copolymer within 49.7−60.0% (Figure 6b). These results indicate that the combination of HIC and FIC in the copolymerization allows the highly blocky monomer sequence distribution to generate a well-defined diblock copolymer, P(FIC-b-HIC). To more comprehensively understand the copolymerization kinetics for the blocky monomer sequence distribution, we attempted to determine the reactivity ratios of HIC and FIC (rHIC = k∓HIC‑HIC/k∓HIC‑FIC; rFIC = k∓FIC‑FIC/k∓FIC‑HIC) using the Mayo−Lewis plotting method. An equation is derived from the rearrangement of eq 2 to present rFIC as a linear function of rHIC:

rFIC

ÄÅ ÉÑ ÑÑ yz fHIC ÅÅÅÅ Finst,FIC ijj fHIC zz − 1ÑÑÑ jj1 + ÅÅ r = z ÑÑ HIC zz ÑÑ fFIC ÅÅÅÅ Finst,HIC jj fFIC ÑÖ k { Ç

(5)

We conducted the low-conversion anionic copolymerization of HIC:FIC with f HIC:f FIC across the values of 0.837:0.163, 0.741:0.259, 0.597:0.403, 0.389:0.611, 0.256:0.744, and 0.155:0.845 for 15 s. The total conversions to copolymer were observed to be 5.1, 9.1, 23.5, 35.5, 47.5, and 63.6%, respectively. From the 1H NMR spectra of the copolymers, the Fcum,HIC:Fcum,FIC values were determined to be 0.068:0.932, 0.031:0.969, 0.023:0.977, 0.010:0.990, 0.006:0.994, and 0.006:0.994, respectively (Table S8 and Figure S13). We made an assumption that the Fcum,HIC:Fcum,FIC values are close to the Finst,HIC:Finst,FIC values at the given low conversions. Using eq 5, we plotted rFIC values against arbitrary rHIC values. G


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Figure 7. Mayo−Lewis plot for the low-conversion anionic copolymerization of HIC:FIC mixtures with f HIC:f FIC = 0.837:0.163, 0.741:0.259, 0.597:0.403, 0.389:0.611, 0.256:0.744, and 0.155:0.845 initiated by NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/ [NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr.

rHIC and rFIC values well reflect the tendency toward the highly blocky monomer sequence distribution. The kinetic preference of FIC consumption in both of the self- and cross-propagation reactions might be due to the reactivity of FIC much higher than HIC. The origin for the large reactivity difference between HIC and FIC is still vague. We believe that this will be revealed by a study of the molecular orbitals of isocyanate monomers. The kinetic confirmation of the highly blocky monomer sequence distribution led us to investigate the repetitive sequential addition of binary comonomers toward well-defined multiblock copolymers. Frey’s group has approached this strategy and obtained tapered multiblock copolymers from binary mixtures of isoprene:4-methylstyrene.52 We attempted the repetitive sequential addition of HIC:FIC mixtures with f HIC:fAIC = 0.502:0.498 in the anionic copolymerization initiated by NaPEPPA in the presence of NaBPh 4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10 −6 Torr (Scheme 3). The first mixture of binary comonomers with [HIC:FIC]0/[NaPEPPA]0 = 41.8 was added to a initiator solution and then the same mixture was repeatedly added whenever the prior comonomer conversion was completed. The number of comonomer addition varied from 1 to 4 to generate P(FIC-b-HIC)1, P(FIC-b-HIC)2, P(FIC-b-HIC)3, and P(FIC-b-HIC)4 as di-, tetra-, hexa-, and octablock copolymers, respectively. Each of the comonomer additions reached the quantitative total conversion to copolymer (= 99.7−98.3%). As a result, the 1 H NMR spectroscopy proved that four final copolymers possess nearly identical molar compositions between HIC and FIC units (Figure 8). The values of Mn, Đ, and Fcum,HIC:Fcum,FIC for the four final products are listed in Table 3. Both HIC and FIC monomers were quantitatively incorporated to the copolymer chains (conversion of HIC = 99.5−96.7%, conversion of FIC = 99.9%). The copolymers, denoted as P(FIC-b-HIC)m (m = 1/2/3/4), exhibited the Mn values (= 5.59/11.5/16.3/20.3 kDa) well accorded with the Mn,theo values (= 5.47/10.6/15.8/20.8 kDa) and the very low Đ values (= 1.10/1.03/1.03/1.03). The Fcum,HIC:Fcum,FIC values (= 0.501:0.499−0.494:0.506) were nearly identical to the f HIC:f FIC value (= 0.502:0.498).

Figure 6. Kinetics profiles of the living anionic copolymerization of HIC:FIC with f HIC:f FIC = 0.502:0.498 initiated by NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr ([HIC:FIC]0 = 0.249 mol L−1; [NaPEPPA]0 = 1.23 × 10−3 mol L−1; [HIC:FIC]0/[NaPEPPA]0 = 202): (a) plots of [HIC]t and [FIC]t versus total conversion to copolymer and (b) plots of Finst,HIC and Finst,FIC in copolymer versus total conversion to copolymer.

This process generated six different linear fits depending on the f HIC:f FIC value (Figure 7). In the Mayo−Lewis plot, the reactivity ratios are generally determined from the range of points where linear fits intersect. In our investigation, only two intersection points were obtained ((rHIC, rFIC) = (0.204, 139) and (0.0188, 63.2)). From the average, the typical values of rHIC and rFIC were determined to be 0.111 and 101, respectively. Consequently, the four individual rate constants of propagation were determined as follows: k∓HIC‑HIC = 1.03 L mol−1 s−1, k∓HIC‑FIC = 9.28 L mol−1 s−1, k∓FIC‑FIC = 21.0 L mol−1 s−1, and k∓FIC‑HIC = 0.208 L mol−1 s−1. One may claim that the observed comonomer reactivity ratios are consistent with the typical values for the gradient copolymerization. In our case, the cross-propagation reaction from FIC to HIC much faster than that in reverse order (k∓HIC‑FIC/k∓FIC‑HIC = 44.6) would maximize the probability of the terminal ion pairs from FIC (FIC∓) rather than those from HIC (HIC∓) at the initial stage of copolymerization. Afterward, the self-propagation for FIC much faster than that ∓ ∓ for HIC (k FIC‑FIC /k HIC‑HIC = 20.4) would allow the homopolymerization of FIC at first and the homopolymerization of HIC at second. Therefore, we ensure that the observed H


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Scheme 3. Repetitive Sequential Addition of HIC:FIC Mixtures in the Anionic Copolymerization Initiated by NaPEPPA in the Presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr To Afford P(FIC-b-HIC)1, P(FIC-bHIC)2, P(FIC-b-HIC)3, and P(FIC-b-HIC)4

Figure 8. 1H NMR spectra of (a) P(FIC-b-HIC)1, (b) P(FIC-b-HIC)2, (c) P(FIC-b-HIC)3, and (d) P(FIC-b-HIC)4 yielded from the repetitive sequential addition of HIC:FIC mixures with f HIC:f FIC = 0.502:0.498 in the anionic copolymerization initiated by NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr.

HIC∓ to FIC∓ immediately after the addition of comonomers. The plots of Mn and Đ versus [HIC:FIC]0/[NaPEPPA]0 revealed that the repetitive sequential addition of same HIC:FIC mixtures ensures the precise control over the Mn values of multiblock copolymers within the range of 5.59−20.3 kDa despite the slight pseudolinearity with [HIC:FIC]0/ [NaPEPPA]0 (Figure 9b).

The SEC-dRI traces of the four products showed the uniform trace shift to higher MW region maintaining the low dispersity by the repetitive sequential addition of same HIC:FIC mixtures (Figure 9a). This result proves that those products are indeed di-, tetra-, hexa-, and octablock copolymers. The precise multiblock copolymerization is considered to be attributed to the complete conversion from I


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Table 3. Repetitive Sequential Addition of HIC:FIC Mixures with f HIC:f FIC = 0.502:0.498 in the Anionic Copolymerization Initiated by NaPEPPA in the Presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torra conv (%) polymer P(FIC-b-HIC)1 P(FIC-b-HIC)2 P(FIC-b-HIC)3 P(FIC-b-HIC)4

[HIC:FIC]0/[NaPEPPA]0b 1st

41.8 + 41.82nd + 41.83rd + 41.84th

t (s) 1st

1440 + 20402nd + 27003rd + 33604th

totalc

HICd

FICd

Mn,theoe (kDa)

Mnf (kDa)

Đf

FcumHICg

Fcum,FICg

99.7 99.2 98.9 98.3

99.5 98.5 97.9 96.7

99.9 99.9 99.9 99.9

5.47 10.6 15.8 20.8

5.59 11.5 16.3 20.3

1.10 1.03 1.03 1.03

0.501 0.498 0.497 0.494

0.499 0.501 0.503 0.506

f HIC and f FIC: mole fractions of HIC and FIC in the feed. b[HIC:FIC]01st = 0.174 mol L−1; [NaPEPPA]01st = 4.16 × 10−3 mol L−1; [HIC:FIC]02nd = 0.120 mol L−1; [NaPEPPA]02nd = 2.87 × 10−3 mol L−1; [HIC:FIC]03rd = 0.0914 mol L−1; [NaPEPPA]03rd = 2.19 × 10−3 mol L−1; [HIC:FIC]04th = 0.0738 mol L−1; [NaPEPPA]04th = 1.77 × 10−3 mol L−1. cTotal conversion to copolymer determined from the gravimetric yield of copolymer. d Conversions of HIC and FIC to copolymer determined from the 1H NMR spectrum of copolymer. eMn,theo = MW of PEPPA + [HIC:FIC]0/ [NaPEPPA]0 × (f HIC × conv of HIC/100% × MW of HIC + f FIC × conv of FIC/100% × MW of FIC). fDetermined from SEC-MALLS. gFcum,HIC and Fcum,FIC: cumulative mole fractions of HIC and FIC in copolymer determined from the 1H NMR spectrum of copolymer. a

quantitative production of polyisocyanates with predictable MWs and low dispersities. The use of NaPEPPA was useful in the living anionic copolymerization of binary comonomers. We discovered that the combination of HIC and FIC leads to the highly blocky monomer sequence distribution. Finally, we produced a series of well-defined multiblock copolymers consisting of PHIC and PFIC through the repetitive sequential addition of HIC:FIC mixtures. We consider that those polymers are important helical objects for the study of threedimensional single-chain organization.

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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.8b02052. Description on the experimental details; supplementary data for the kinetic analyses and characterization (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (+82)-62-715-2306. ORCID

Chang-Geun Chae: 0000-0001-8805-6743 Jae-Suk Lee: 0000-0002-6611-2801 Notes

The authors declare no competing financial interest.

Figure 9. (a) SEC-dRI traces and (b) plots of Mn and Đ of versus [HIC:FIC]0/[NaPEPPA]0 for P(FIC-b-HIC)1, P(FIC-b-HIC)2, P(FIC-b-HIC)3, and P(FIC-b-HIC)4 yielded from the repetitive sequential addition of HIC:FIC mixtures with f HIC :f FIC = 0.502:0.498 in the anionic copolymerization initiated by NaPEPPA in the presence of NaBPh4 ([NaBPh4]0/[NaPEPPA]0 = 5) in THF at −98 °C under 10−6 Torr.

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ACKNOWLEDGMENTS This work was supported by “Nobel Research Project” grant for Grubbs Center for Polymers and Catalysis funded by the GIST in 2018.

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CONCLUSIONS We developed an aliphatic sodium amidate, NaPEPPA, as an initiator inspired by propagating active centers in the living anionic polymerization of isocyanates. This compound has the molecular design to overcome the slow initiation of NaBA as a conventional aromatic sodium amidate with insufficient nucleophilicity originating from the charge delocalization and steric hindrance. High nucleophilicity of NaPEPPA remained by two aliphatic substituents. Consequently, NaPEPPA successfully performed the fast and quantitative initiation in the living anionic polymerization of isocyanates in the presence of NaBPh4 in THF at −98 °C under 10−6 Torr, leading to the

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