Proton transfer anionic polymerization with C–H bond as dormant species

doi:10.21203/rs.3.rs-3589509/v1

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Proton transfer anionic polymerization with C–H bond as dormant species

https://doi.org/10.21203/rs.3.rs-3589509/v1

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published 04 Jul, 2024

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Living anionic polymerization is the most common living polymerization with the longest history whereas it generally requires stringent conditions and one metal initiator per polymer chain. Here, we present proton transfer anionic polymerization using acidic C–H bonds as dormant species and base catalysts for methacrylates via reversible chain transfer or termination of the growing enolate species. A weakly acidic compound such as an alkyl isobutyrate serves as the initiator or chain-transfer agent in the presence of a bulky potassium base catalyst to produce a polymer chain and thereby diminish the metal compound per chain ratio. An added alcohol serves as a reversible terminator to tame the propagation. End-functionalized, star, block, and graft polymers are easily accessible from compounds with C–H bonds.

Physical sciences/Chemistry/Polymer chemistry/Polymer synthesis

Physical sciences/Chemistry/Polymer chemistry/Polymerization mechanisms

Living polymerization has made remarkable progress and is indispensable for the development of functional polymeric materials^1–3. The history began with the discovery of living anionic polymerization of styrene by Szwarc in 1956^4–6. Living anionic polymerizations of styrene and (meth)acrylic monomers often use alkali and other metal-based nucleophiles as initiators, which react with the monomers to produce relatively stable carbanions and enolates^7–10. The propagating anionic species continues living chain growth to afford the polymer chain (Fig. 1A). However, there is still room for improvement of living anionic polymerizations, which generally require one molecule of the metal initiator per polymer chain, the removal of moisture, and stringent conditions at low temperature.

On the other hand, “living” cationic and radical polymerizations, which proceed via less stable propagating species and fast propagation relative to slow initiation, were realized with the presence of dormant species in the 1980s and 1990s, respectively^11–27. In these polymerizations, the dormant species are indispensable in allowing all polymer chains to grow evenly via reversible deactivation of the propagating species^{28, 29}. Thus, these controlled/“living” radical polymerizations should be called reversible deactivation radical polymerization (RDRP) in accordance with IUPAC recommendations³⁰. Reversible deactivation can be attained by halting the propagation of a polymer chain temporally by either reversible termination or reversible chain transfer. A typical example of the former is nitroxide-mediated polymerization (NMP)^17,18, while an example of the latter is reversible addition-fragmentation chain transfer (RAFT)²⁴ polymerization (Fig. 1B). Although dormant species are not necessary present in living anionic polymerizations, the inherent issues mentioned above may be overcome by the presence of appropriate dormant species; reversible chain-transfer agents could diminish the ratio of metal initiators per chain, while reversible termination could tame fast propagation. Whereas dormant species or reversible chain-transfer agents, such as silyl ketene acetals^31,32 and alkylmagnesium species^33,34, have been used in anionic polymerization, these issues remain challenging.

Here, we propose proton transfer anionic polymerization (PTAP) with acidic C–H bonds used as the dormant species activated with base catalysts for anionic polymerization via reversible chain transfer and reversible termination. The key reaction is the 1:1 addition reaction between an olefin and a carbonyl or nitrile compound having a weakly acidic proton (R–H) in the presence of a bulky base catalyst (Mt⁺B^–), as reported by Yamashita and Kobayashi (Fig. 1C)^35–38. In this reaction, the base (B^–) abstracts a proton from R–H to generate an anion (R^–), which adds to the C=C bond of the olefin (C=CR’) to form an anion (R–C–C(R’)^–). The anion then abstracts a proton from R–H or B–H to produce the adduct (R–C–C(R’)–H) and another anion (R^–) or base (B^–), respectively. The anion or base subsequently joins another cycle to enable the catalytic 1:1 addition reaction.

We envisioned that if the proton of the adduct were abstracted by the anion or base, base-catalyzed proton transfer anionic polymerization (PTAP) via reversible activation of the C–H bond would be possible, just as metal-catalyzed Kharasch or atom transfer radical addition (ATRA) has evolved into atom transfer radical polymerization (ATRP)^{20–23, 29}. In this case, one polymer chain is generated from one R–H molecule with an acidic proton, such as alkyl isobutyrate, which reduces the number of metal initiators per chain and will be cost effective and sustainable (Fig. 1D). In particular, there have been no living polymerizations using C–H bonds as the dormant species for vinyl monomers. Furthermore, anionic polymerization could occur even in the presence of alcohols and water because the C–H bond generated by termination can be reactivated to continue anionic propagation, which would enable “living” anionic polymerization under milder conditions.

C–H Dormant Species for Reversible Chain Transfer

In mimicking the condition for 1:1 anionic addition^35–38, we used potassium hexamethyldisilazide (KHMDS: K⁺(Me₃Si)₂N^–) as the base in the presence of 18-crown-6 for anionic polymerization of tert-butyl methacrylate (TBMA) in THF at 0 °C (Fig. 2A). In particular, we chose ethyl isobutyrate (1), which has a “unimer” structure of methacrylate with an acidic proton (Me₂C(CO₂Et)–H), as an R–H that would potentially serve as an initiator or chain-transfer agent (CTA). Without 1, polymers with high molecular weights and broad molecular weight distributions (MWDs) were obtained (M_n = 55700, Ð = 1.96) (entry 1 in Supplementary Table 1, Fig. 2A), indicating that a small amount of the base initiated uncontrolled anionic polymerization. However, upon the addition of 1, the amount of which was 10 equivalents per equivalent of KHMDS ([1]₀/[KHMDS]₀ = 10) or 1/25 of an equivalent per monomer ([M]₀/[1]₀ = 25), the molecular weight and dispersity index were dramatically decreased (M_n = 5300, Ð= 1.23) (entry 2 in Supplementary Table 1, Fig. 2A). Furthermore, the M_n values were easily controlled with the feed ratio of monomer to 1 ([M]₀/[1]₀) and were close to the theoretical values assuming that one molecule of 1 generated one polymer chain (entries 2–4 in Supplementary Table 1, Fig. 2A). Moreover, the MWDs were all narrow (Ð= 1.2–1.3).

Other acidic C–H compounds, such as cumyl isobutyrate (Me₂C(CO₂C(Me)₂Ph)–H; 2), dimethyl 2,2,4-trimethylpentanedioate or the “dimer” of MMA (H–(CH₂C(Me)CO₂Me)₂–H; 3), and 2-phenylpropionitrile (MeC(CN)(Ph)–H; 4), similarly resulted in controlled molecular weights and narrow MWDs (entries 5–7 in Supplementary Table 1, Fig. 2B). The ¹H NMR spectra of the polymers obtained with 1–4 all showed the peaks attributed to the CTA in addition to those assigned to the poly(TBMA) main chains (Supplementary Fig. 1). In particular, the M_n(NMR) values calculated from the integrated ratios of the a-end groups to the pendant tert-butyl esters were close to the M_n(SEC) values. Thus, simple esters and nitriles with an acidic proton (R–H) generated living polymer chains per molecule in the presence of KHDMS and 18-crown-6 in THF at 0 °C.

To evaluate the effects of the base catalysts, we employed lithium and sodium hexamethyldisilazides (LiHMDS and NaHMDS) in conjunction with 12-crown-4 and 15-crown-5, respectively. Despite the presence of 2, the polymer molecular weights obtained with LiHMDS and NaHMDS were high (entries 1 and 2 in Supplementary Table 2, Fig. 2) due to the low basicities of the Li and Na enolates, which made chain transfer via deprotonation of the C–H bonds slow. However, potassium tert-butoxide (KOtBu), a weaker base than KHMDS but with the same counter-cation, also worked efficiently to give controlled molecular weights (Ð= 1.18) (entry 4 in Supplementary Table 2, Fig. 2). Furthermore, we used KH to clarify whether reversible chain transfer or reversible termination was predominant (Supplementary Fig. 3) because KH reacts with R–H to produce R^–K⁺ and H₂, and H₂ does not work as a reversible terminator. The resulting polymers also exhibited controlled molecular weights and chain-end groups originating from R–H (entry 5 in Supplementary Table 2, Supplementary Fig. 4). Thus, reversible chain transfer of the potassium enolate propagating species to the dormant C–H termini contributed to the controlled polymerization, although reversible termination cannot be completely ruled out, as shown below (see the polymerization in the presence of alcohol). Finally, KHMDS without 18-crown-6 resulted in almost the same molecular weights but slightly broader MWDs (Ð= 1.59) (entry 6 in Supplementary Table 2), suggesting that the crown ether enhanced the reactivity of the propagating enolate species to induce fast chain-transfer reactions.

The conditions were varied to improve the controllability and to see the impact on the polymerization. When toluene was used as the main solvent (toluene/THF = 9/1 v/v) with 4 and KOtBu/18-crown-6, the MWD of the obtained polymer was narrower (Ð = 1.10) (entry 2 in Supplementary Table 2, Supplementary Fig. 5) than that resulting in THF only. Furthermore, the M_n values were closer to the calculated values for any monomer-to-4 feed ratio from 10 to 100 (entries 2–5 in Supplementary Table 3, Supplementary Fig. 6).

The effect of temperature was examined from –78 to 20 °C (entries 6–9 in Supplementary Table 3, Fig. 2C). The M_n values were almost the same and close to the calculated values, indicating that the polymerization was controlled over a wide temperature range. Furthermore, the MWDs were narrow (Ð ~ 1.1) even at 10 and 20 °C, whereas they became broader at –40 and –78 °C due to the slower chain-transfer processes occurring at lower temperatures. Thus, molecular weight control was achievable even at ambient temperatures.

To confirm the living nature of the process, a monomer addition experiment was carried out. When the first feed was consumed quantitatively, a fresh feed was added. The added monomer was also consumed, leading to a shift in the SEC curve to a higher molecular weight while maintaining a narrow MWD (Supplementary Fig. 7A).

In addition, the chain extension reaction was successful when isolated polymers with the dormant C–H terminal were used as the macroinitiator or macroCTA, as in ATRP or RAFT polymerizations. The polymers obtained by anionic polymerization of TBMA with 4 were isolated and purified by preparative SEC. The isolated polymers were then used for anionic polymerization of TBMA under the same conditions as the first run. A similar shift of the narrow SEC curve was observed (Supplementary Fig. 7B), as in the monomer addition experiment run without isolation of the polymer. This is the first successful chain extension reaction using isolated polymers for living anionic polymerization of vinyl monomers.

Alcohols for Reversible Termination

Next, additional control by reversible termination as well as the robustness of the anionic polymerizations were examined by adding an alcohol to the polymerization. Alcohols are generally terminators for anionic polymerizations of vinyl monomers because they react with the anionic species to result in C–H “dead” chain ends. However, the results obtained above suggested that the C–H bonds would serve as dormant species and could be reversibly activated (Fig. 3A). In addition, the reversible termination with alcohols or reversible activation with the conjugate bases, i.e., potassium alkoxides in this case, could improve the controllability if initiation by the alkoxide was negligible.

Thus, 1–3 equivalents of tBuOH to KOtBu were added to an anionic polymerization of TBMA with 2. Even in the presence of tBuOH, polymerization occurred, although the polymerization became slower with increasing amounts of tBuOH (Supplementary Table 4). More surprisingly, irrespective of the amount of alcohol, all the M_n values were consistent with the calculated values assuming that one molecule of 2 generated one polymer chain, and the MWDs were narrow (Fig. 3B, Supplementary Fig. 8). The ¹H NMR spectra of the polymers were similar to that obtained without tBuOH (Fig. 3C vs. Supplementary Fig. 1). The M_n(NMR) value calculated from the phenyl groups originating from 2 was close to the M_n(SEC) value. Furthermore, the MALDI-TOF-MS spectrum showed a series of main peaks, which were assigned to poly(TBMA) with a cumyl ester unit at the a-end and a hydrogen at the w-end without evident formation of a tBuO-initiated polymer chain (Fig. 3D). The virtual lack of tBuO groups at the a-ends was ascribed to the low nucleophilicity and high basicity of the tert-butoxide anion. A minor series of peaks was due to elimination of the tert-butyl groups from the monomer unit during the MS analysis. The molecular weights were controlled well by the monomer-to-2 feed ratio, much as they were in the absence of tBuOH (Supplementary Fig. 9 vs. Fig. 2A).

Thus, the alcohol served neither as an initiator nor as a chain-transfer agent, unlike the “immortal” polymerizations of cyclic ethers and esters^39–41. It functioned as a deactivator or reversible terminator that retarded the polymerization by converting the enolate propagating species into the dormant C–H species and the conjugate base.

The stability of the anionic polymerization toward hydroxyl compounds was particularly interesting. Here, as-purchased toluene, which contained 30–50 ppm of water, as measured by Karl-Fischer titration, was used as the solvent for the polymerization without purification (Supplementary Fig. 10). Despite the slower polymerization, the M_n values increased in direct proportion to monomer conversion while maintaining the narrow MWDs (Ð = 1.1–1.2).

To demonstrate the versatility of various methacrylates, TBMA, benzyl methacrylate (BnMA), and methyl methacrylate (MMA) were polymerized with 4 and KOtBu/18-crown-6 under the same conditions in the absence and presence of tBuOH. In all cases, the polymerizations were slower with tBuOH. While the polymerizations of TBMA and BnMA were controlled irrespective of the presence or absence of tBuOH (Supplementary Fig. 11), tBuOH was required to maintain good control of the MMA polymerization. This was attributed to the higher reactivity of MMA. A more prominent effect of an alcohol was observed when using a stronger base such as KHMDS for MMA polymerization (Supplementary Fig. 12). The bimodal MWDs for the PMMA obtained with KHMDS became monomodal and narrower (Ð = 1.1–1.2) upon addition of 5 equivalents of a bulky alcohol such as diphenylethanol (DPEOH) to the KHMDS. The ¹H NMR (Supplementary Fig. 13) and MALDI-TOF-MS (Supplementary Fig. 14) spectra indicated that PMMA was mainly formed from 4 and not from KHMDS and DPEOH. However, a minor series of peaks in the MS spectrum suggested intramolecular cyclization of the terminal enolate. Further optimization could improve the control of anionic MMA polymerization.

Since the obtained PMMA had a dormant C–H bond at the w-end, the polymer could be used for anionic block polymerization with TBMA (Supplementary Fig. 15). The SEC curve shifted to a high molecular weight while maintaining a narrow MWD, indicating the formation of a block copolymer from MMA and TBMA, although a small shoulder suggested the presence of a small amount of PMMA. The ¹H NMR spectrum also supported formation of the block copolymer (Supplementary Fig. 16).

Precision Polymer Syntheses Based on C–H Bonds

In addition to the methacrylate block copolymers, PTAP also enables facile syntheses of end-functionalized, star, block, and graft polymers just by adding various compounds or polymers with weakly acidic C–H bonds into the base-catalyzed polymerizations. This is a tremendous advantage over conventional living anionic polymerizations, which require difficult and cumbersome syntheses of metal-based functionalized initiators or macroinitiators prior to polymerization.

For the synthesis of end-functionalized polymethacrylates, isobutyrate with a vinyl ether group (5), which was prepared from isobutyroyl chloride and ethyleneglycol monovinyl ether, was used for PTAP of TBMA with KOtBu/18-crown-6. The obtained polymer showed controlled molecular weights with narrow MWDs (Ð = 1.15) (Fig. 4A) and a vinyl ether as the a-end group with high functionality (F_n = 1.01; Supplementary Fig. 17A). Another end-functionalized poly(TBMA) (Fig. 4B) with a hydroxy group (F_n = 0.96) was prepared by using tert-butyldimethylsilyl-protected 2-hydroxyl ethyl isobutyrate (6) for TBMA followed by deprotection (Supplementary Figs. 17B and 17C).

A 3-arm star polymer was synthesized by using a tri-isobutyrate prepared from 1,3,5-cyclohexane triol and isobutyroyl chloride as the trifunctional initiator or CTA for PTAP of TBMA with KHMDS/18-crown-6. The MWD for the obtained product was monomodal although slightly broad (Ð = 1.45; Fig. 4C). The M_n measured by multiangle light scattering (MALS) was 17100 and higher than that based on the PMMA standard (14400). Formation of the tri-arm star polymers was confirmed by scission of the arm chains from the core. Treatment of the polymer with sodium methoxide led to a shift of the SEC curve to the low molecular weight region and narrowed the MWD (M_n(SEC) = 5200, Ð = 1.36; Fig. 4D). The M_n(MALS) was also decreased to 5000, approximately one third of the previous value, indicating that the obtained polymers were 3-arm star polymers. This was also supported by the presence and absence of the methine protons of the 1,3,5-cyclohexanoxyl units before and after treatment with base, respectively, in the ¹H NMR spectra (Supplementary Fig. 18).

A combination with living cationic polymerization also enabled facile syntheses of unique block copolymers from vinyl ethers and methacrylates. The hydrogen chloride adduct of the vinyl ether with an isobutyrate unit was prepared from 5 and used as the initiator for living cationic polymerization of isobutyl vinyl ether (IBVE) with ZnCl₂ (Supplementary Fig. 19)⁴² to produce a polyvinyl ether with an isobutyrate group at the a-end (Supplementary Fig. 20A) and a narrow MWD (Ð = 1.12; Fig. 4E). The isolated poly(IBVE) with the terminal isobutyrate unit was then used as a macroinitiator or macroCTA for the PTAP of TBMA with KHMDS/18-crown-6. The SEC curve was shifted to a high molecular weight (Fig. 4F), indicating the formation of a block copolymer of IBVE and TBMA. The ¹H NMR spectrum of the product also supported the formation of poly(IBVE-b-TBMA) (Supplementary Fig. 20B).

Finally, graft polymers were synthesized via a grafting-from method. Living cationic copolymerization of IBVE and 5 was conducted with a 1:1 feed ratio by using the HCl adduct of IBVE and ZnCl₂ to yield the copolymer (M_n = 1800, Ð = 1.17; Fig. 4G). The total number-average degree of polymerization (DP_n) was estimated to be 16, and 5 units of isobutyrate were incorporated on average per chain, according to the ¹H NMR analysis (Supplementary Fig. 22A). The isolated copolymer was then used to polymerize TBMA in the presence of KHMDS/18-crown-6 to give a product with a high molecular weight and a monomodal SEC curve (M_n = 22000, Ð = 1.33; Fig. 4H). The ¹H NMR spectrum also indicated the formation of poly(IBVE-g-TBMA) (Supplementary Fig. 22B).

In conclusion, we developed novel living anionic polymerizations of methacrylates using C–H bonds as the dormant species, which interchanged with the growing enolate species via reversible chain-transfer and termination. This method reduces the number of metal initiators needed per polymer chain and enables control of the molecular weights with the feed ratio of the monomer to the acidic C–H compounds. Added alcohols served as reversible terminating agents and contributed additional control under milder conditions. Furthermore, the syntheses of end-functionalized, star, block, and graft polymers were easily explored. The method based on the dormant C–H species creates a new strategy and history for living anionic polymerization and will lead to further advances in precision polymer syntheses and functional polymer materials.

Materials

tert-Butyl methacrylate (TBMA) (TCI, > 98.0%), benzyl methacrylate (BnMA) (TCI, > 98.0%), methyl methacrylate (MMA) (TCI, > 98.0%), ethyl isobutyrate (1) (TCI, > 98.0%), isobutyl vinyl ether (IBVE) (TCI, > 99.0%), 15-crown-5 (TCI, > 97.0%), and 12-crown-4 (TCI, > 95.0%) were distilled from calcium hydride under reduced pressure before use. α-Methylbenzyl cyanide (4) (Aldrich, 96%) was dried via azeotropic distillation with toluene before use. KHMDS (Aldrich; 1.0 M solution in THF), KOtBu (Aldrich; 1.0 M solution in THF), LiHMDS (Aldrich; 1.0 M solution in THF), LiOtBu (Aldrich; 1.0 M solution in THF), NaHMDS (Aldrich; 1.0 M solution in THF), KH (Aldrich, 30 wt% dispersion in mineral oil), isobutyric anhydride (TCI, > 95.0%), 2-phenyl-2-propanol (TCI, > 98.0%), 4-dimethylaminopyridine (DMAP) (TCI, > 99.0%), 18-crown-6 (TCI, > 98.0%), ethylene glycol monovinyl ether (TCI, > 98.0%), isobutyryl chloride (TCI, > 98.0%), isobutyric acid (TCI, > 99.0%), ethylene glycol (TCI, > 99.5%), tert-butyldimethylsilyl chloride (TCI, > 98.0%), 1,3,5-cyclohexanetriol (cis- and trans- mixture) (TCI, > 95.0%), triethylamine (TCI, > 99.0%), 1,4-dioxane (KANTO, > 99.5%), pyridine (KANTO, > 99.5%), and ZnCl₂ (Aldrich; 1.0 mM solution in diethyl ether) were used as received. Cumyl isobutyrate (2), vinyloxyethyl isobutyrate (5), 2-(tert-butyldimethylsiloxy)ethyl isobutyrate (6), 1,3,5-cyclohexanetriol triisobutyrate (7), and HCl adduct of 5 were synthesized according to the procedures in Supplementary Information. Dimethyl 2,2,4-trimethylpentanedioate (3)⁴³ and the HCl adduct of IBVE (IBVE-HCl)⁴⁴ were synthesized according to the literature methods. THF (KANTO, > 99.5%; H₂O < 0.001%), toluene (KANTO, > 99.5%; H₂O < 1 ppm), n-hexane (KANTO, > 96.0%; H₂O < 1 ppm) and Et₂O (KANTO, > 99.5%; H₂O < 0.005%) were dried and deoxygenized by passing them through the columns of a Glass Contour systems before use.

Anionic polymerization of TBMA in the presence of CTA

Anionic polymerization of TBMA was carried out by the syringe technique under dry argon in a baked glass tube equipped with a three-way stopcock. A typical example of the polymerization procedure is given below. A solution containing a chain-transfer agent and a base catalyst was prepared by mixing KHMDS (0.40 mL of 58.1 mM in THF solution, 23.2 µmol), 18-crown-6 (0.16 mL of 158 mM in THF solution, 25.3 µmol), 2 (0.30 mL of 775 mM THF solution, 0.23 mmol), and THF (5.9 mL). The anionic polymerization was initiated by dropwise addition of the TBMA (0.94 mL, 5.78 mmol) at 0°C over a period of 705 sec with stirring. After 10 min, the polymerization was terminated with degassed methanol (2.0 mL). The quenched reaction mixture was washed with dilute hydrochloric acid and distilled water to remove the residual catalyst, evaporated to dryness under reduced pressure, and vacuum-dried to give the polymer product (M_n = 4700, Ð = 1.25). The monomer conversion rate was determined by gravimetric analysis (conv. >99%).

Synthesis of end-functionalized poly(TBMA)

The synthesis of end-functionalized poly(TBMA) was carried out by the syringe technique under dry argon in a baked glass tube equipped with a three-way stopcock. A typical example of the polymerization procedure is given below. A solution containing a chain-transfer agent and a base catalyst was prepared by mixing KOtBu (0.51 mL of 60 mM in THF/toluene solution, 30.6 µmol), 18-crown-6 (0.22 mL of 150 mM in THF solution, 33.0 µmol), 6 (0.45 mL of 674 mM THF solution, 0.30 mmol), and toluene (4.15 mL). The anionic polymerization was initiated by the dropwise addition of TBMA (0.74 mL, 4.55 mmol) at 0°C over a period of 790 s with stirring. After 67 h, the polymerization was terminated with degassed methanol (2.0 mL). The quenched reaction mixture was washed with dilute hydrochloric acid and distilled water to remove the residual catalyst, evaporated to dryness under reduced pressure, and vacuum-dried to give the polymer product (M_n = 3300, Ð = 1.25).

Deprotection of the silyl-protected end-functionalized poly(TBMA)

Deprotection of the silyl-protected end-functional poly(TBMA) was carried out by the syringe technique under dry nitrogen in a baked glass tube equipped with a three-way stopcock. TBAF solution (0.18 ml of 1.0 M in THF solution, 180 µmol) was added to the polymer solution containing the silyl-protected end-functionalized poly(TBMA) (0.2975, silyl unit: 87.5 µmol, M_n = 3400, Ð = 1.26) and THF (2.0 mL). The mixture was stirred for 29 h at r.t. The reaction was quenched by adding methanol (1.0 mL). The quenched solution was diluted with toluene, washed with dilute hydrochloric acid and distilled water, evaporated to dryness under reduced pressure, and vacuum-dried to give the polymer product with a hydroxyl group at the chain end (M_n = 3200, Ð = 1.24).

Synthesis of 3-arm star poly(TBMA)

The 3-arm star polymer was synthesized with the syringe technique under dry argon in a baked glass tube equipped with a three-way stopcock. A typical example of the polymerization procedure is given below. The solution containing a chain-transfer agent and a base catalyst was prepared by mixing KHMDS (0.40 mL of 60 mM in THF solution, 24 µmol), 18-crown-6 (0.19 mL of 138 mM in THF solution, 26 µmol), 7 (0.34 mL of 235 mM in THF solution, 80 µmol), and THF (6.1 mL). The anionic polymerization was initiated by dropwise addition of the TBMA (0.98 mL, 6.0 mmol) at 0°C over a period of 735 sec with stirring. After 110 min, the polymerization was terminated with degassed methanol (2.0 mL). The quenched reaction mixture was washed with dilute hydrochloric acid and distilled water to remove the residual catalyst, evaporated to dryness under reduced pressure, and vacuum-dried to give the 3-arm star polymer (M_n = 14400, Ð = 1.45). The monomer conversion rate was determined by gravimetric analysis (conv. 95%).

Scission of the 3-arm star polymer

Scission of the 3-arm star poly(TBMA) was carried out with the syringe technique under dry nitrogen in a baked glass tube equipped with a three-way stopcock. After sodium methoxide solution (1.64 mL of 5.0 M CH₃OH solution, mmol) was added into a polymer solution containing the 3-arm star poly(TBMA) (0.165 g, 8.00 µmol mmol, M_n = 14500, M_w/M_n = 1.42), THF (4.1 mL), and CH₃OH (10.6 mL), the mixture was stirred for 115 h at 50°C. The reaction was quenched by cooling the reaction solution to room temperature. The quenched reaction solution was diluted with toluene and washed with dilute hydrochloric acid and distilled water, evaporated to dryness under reduced pressure, and vacuum-dried to give the product polymers (M_n = 5200, Ð = 1.36).

Living cationic polymerization of IBVE with the HCl adduct of 5

Living cationic polymerization of IBVE was carried out by the syringe technique under dry nitrogen in a baked glass tube equipped with a three-way stopcock. A typical example of the polymerization procedure is given below. The cationic polymerization was initiated by sequential addition of the HCl adduct of 5 (1.50 mL of a 200 mM toluene solution, 0.30 mmol) and ZnCl₂ (1.50 mL of a 20 mM Et₂O solution, 30 µmol) via a dry syringe into a monomer solution containing IBVE (0.98 mL, 7.51 mmol) and 1,2-dichlorobenzene (0.08 mL) as an internal standard in toluene (10.9 mL) at 0°C. At predetermined intervals, the polymerization was terminated with methanol (5.0 mL) containing a small amount of triethylamine. The monomer conversion rate was determined from the concentration of residual monomer measured by ¹H NMR with 1,2-dichlorobenzene as an internal standard (20 sec, conv. = 95%). The quenched reaction mixture was washed with dilute hydrochloric acid and distilled water to remove the residual catalyst, evaporated to dryness under reduced pressure, and vacuum-dried to give the product polymers (M_n = 1500, Ð = 1.10). The obtained poly(IBVE) was further purified by preparative SEC (M_n = 1500, Ð = 1.09 for the PSt standard, M_n = 1700, Ð = 1.12 for the PMMA standard) and used as a macro CTA after azeotropic distillation with toluene.

Synthesis of TBMA and IBVE block copolymers

The anionic block copolymerization was carried out by the syringe technique under dry argon in sealed glass tubes. A typical example of the block polymerization procedure is given below. A solution including poly(IBVE) with an isbutyrate terminal as a macroCTA and a base catalyst was prepared by mixing KHMDS (0.72 mL of 60 mM in THF solution, 43 µmol), 18-crown-6 (0.34 mL of 138 mM in THF solution, 47 µmol), poly(IBVE) (0.2134 g, 0.14 mmol, M_n = 1500, Ð = 1.09 for PSt standard, M_n = 1700, Ð = 1.12 for PMMA standard), and THF (3.0 mL). The anionic polymerization was initiated by dropwise addition of the TBMA (0.59 mL, 3.63 mmol) at 0°C over a period of 472 sec with stirring. After 8 h, the polymerization was terminated with degassed methanol (2.0 mL). The monomer conversion rate was determined by ¹H NMR analysis (conv. 85%). The quenched reaction mixture was washed with dilute hydrochloric acid and distilled water to remove the residual catalyst, evaporated to dryness under reduced pressure, and vacuum-dried to give poly(IBVE-b-TBMA) (M_n = 5000, Ð = 1.28).

Living cationic copolymerization of IBVE and 5

Living cationic copolymerization of IBVE and 5 was carried out by the syringe technique under dry nitrogen in a baked glass tube equipped with a three-way stopcock. A typical example of the polymerization procedure is given below. The cationic polymerization was initiated by sequential addition of the HCl adduct of IBVE (0.80 mL of 201 mM in toluene solution, 161 µmol) and ZnCl₂ (0.80 mL of 20 mM in Et₂O solution, 16 µmol) via a dry syringe into a monomer solution containing IBVE (0.27 mL, 2.07 mmol), 5 (0.34 mL, 2.05 mmol), and 1,2-dichlorobenzene (0.05 mL) as an internal standard in toluene (5.67 mL) at 0°C. At predetermined intervals, the polymerization was terminated with methanol (3.0 mL) containing a small amount of triethylamine. The monomer conversion rate was determined from the concentration of the residual monomer measured by ¹H NMR with 1,2-dichlorobenzene as the internal standard (3.5 min, 81% for IBVE and 57% for 5). The quenched reaction mixture was washed with dilute hydrochloric acid and distilled water to remove the residual catalyst, evaporated to dryness under reduced pressure, and vacuum-dried to give the product copolymers (M_n = 1800, Ð = 1.17). The obtained poly(IBVE-co-5) was purified by preparative SEC (M_n = 1900, Ð = 1.13 for the PSt standard, M_n = 2100, Ð = 1.15 for the PMMA standard) and used as a macroCTA after azeotropic distillation with toluene.

Synthesis of IBVE and TBMA graft copolymers

The anionic graft copolymerization was carried out by the syringe technique under dry argon in sealed glass tubes. A typical example of the block polymerization procedure is given below. A solution including poly(IBVE-co-5) as a macroCTA and a base catalyst was prepared by mixing KHMDS (0.40 mL of 60 mM in THF solution, 24 µmol), 18-crown-6 (0.19 mL of 136 mM in THF solution, 26 µmol), poly(IBVE-co-5) (1.65 mL of 145 mM for isobutyrate units in THF solution 0.24 mmol, M_n = 1900, Ð = 1.13 for the PSt standard, M_n = 2100, Ð = 1.15 for the PMMA standard), and THF (4.80 mL). The anionic polymerization was initiated by dropwise addition of the TBMA (0.98 mL, 6.0 mmol) at 0°C over a period of 720 sec with stirring. After 18.5 h, the polymerization was terminated with degassed methanol (2.0 mL). The monomer conversion rate was determined by ¹H NMR analysis (conv. 97%). The quenched reaction mixture was washed with dilute hydrochloric acid and distilled water to remove the residual catalyst, evaporated to dryness under reduced pressure, and vacuum-dried to give poly(IBVE-graft-TBMA) (M_n = 21400, Ð = 1.34).

Measurements

¹H and ¹³C NMR spectra were recorded with a JEOL ECS-400 spectrometer operating at 400 MHz. The number-average molecular weights (M_n) and the molecular weight distributions (Ð) of the product polymers were determined by size-exclusion chromatography (SEC) in THF at 40°C on two polystyrene gel columns [TSKgel MultiporeH_XL-M (7.8 mm i.d. × 30 cm) × 2; flow rate 1.0 mL/min] connected to a JASCO PU-2080 precision pump and JASCO RI-2031 detector. The columns were calibrated against 10 standard poly(methyl methacrylate) samples (Agilent Technologies; M_p = 202–1677000, Ð = 1.02–1.09) and 10 standard polystyrene samples (Agilent Technologies; M_p = 575–2783000, Ð = 1.02–1.23). MALLS analyses were conducted with a DAWN HELEOS photometer (Wyatt Technology; 1–633 nm) and Optilab rEX (Wyatt Technology; 1–633 nm) in THF. MALDI-TOF-MS was performed with a Bruker Autoflex max (linear mode) with trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as the ionizing matrix and sodium trifluoroacetate as the ion source.

Data Availability

All data are available in the main text or Supplementary Information.

Acknowledgments

This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) (No. JP22K05209) and Transformative Research Areas (A) “Green Catalysis Science for Renovating Transformation of Carbon-Based Research” (No. JP23H04915) for M.U. and Scientific Research on Innovative Areas “Hybrid Catalysts for Enabling Molecular Synthesis on Demand” (No. JP20H04819) and Scientific Research (A) (No. JP22H00333) for M.K. We thank Professors Yasuhiro Yamashita, Hironobu Watanabe, and Chihiro Homma for fruitful discussions and valuable comments on this research.

Author contributions

M.U., N.O., K.Y. and M.K. designed the experiments. N.O., K.Y., and K.S. conducted and analyzed the experiments. M.U. and M.K. prepared the manuscript.

Competing interests: Authors declare that they have no competing interests.

Webster, O. W. Living Polymerization Methods. Science 251, 887–893 (1991).
Müller, A. H. E. & Matyjaszewski, K. Eds., Controlled and Living Polymerization: From Mechanisms to Materials (Wiley-VCH, 2009).
Matyjaszewski, K. & Möller M. Eds., Polymer Science: A Comprehensive Reference, Vol. 3. Chain Polymerization of Vinyl Monomers (Elsevier, 2012).
Szwarc, M. ‘Living’ Polymers. Nature 178, 1168–1169 (1956).
Szwarc, M., Levy, M. & Milkovich, R. Polymerization initiated by electron transfer to monomer a new method of formation of block polymers. J. Am. Chem. Soc. 78, 2656–2657 (1956).
Szwarc, M. Living polymers. Their discovery, characterization, and properties. J. Polym. Sci. Part A: Polym. Chem. 36, ix–xv (1998).
Hsieh, H. L. & Quirk, R. P. Anionic Polymerization (Marcel Dekker, 1996).
Baskaran, D. & Muller, A. Anionic vinyl polymerization–50 years after Michael Szwarc. Prog. Polym. Sci. 32, 173–219 (2007).
Hirao, A., Goseki, R. & Ishizone, T. Advances in Living Anionic Polymerization: From Functional Monomers, Polymerization Systems, to Macromolecular Architectures. Macromolecules 47, 1883–1905 (2014).
Ntetsikas, K., Ladelta, V., Bhaumik, S. & Hadjichristidis, N. Quo Vadis Carbanionic Polymerization? ACS Polym. Au 3, 158–181 (2023).
Miyamoto, M., Sawamoto, M. & Higashimura, T. Living polymerization of isobutyl vinyl ether with hydrogen iodide/iodine initiating system. Macromolecules 17, 265–268 (1984).
Faust, R. & Kennedy, J. P. Living carbocationic polymerization. III. Demonstration of the living polymerization of isobutylene. Polym. Bull. 15, 317–323 (1986).
Aoshima, S. & Kanaoka, S. A Renaissance in Living Cationic Polymerization. Chem. Rev. 109, 5245–5287 (2009).
Chen, Y., Zhang, L., Jin, Y., Lin, X. & Chen, M. Recent Advances in Living Cationic Polymerization with Emerging Initiation/Controlling Systems. Macromol. Rapid Commun. 42, e2100148 (2021).
Uchiyama, M., Satoh, K. & Kamigaito, M. Cationic RAFT and DT polymerization. Prog. Polym. Sci. 124, 101485 (2022).
Wu, L. et al. Regulating cationic polymerization: From structural control to life cycle management. Prog. Polym. Sci. 145, 101736 (2023).
Georges, M. K., Veregin, R. P. N., Kazmaier, P. M. & Hamer, G. K. Narrow molecular weight resins by a free-radical polymerization process. Macromolecules 26, 2987–2988 (1993).
Hawker, C. J. Molecular Weight Control by a "Living" Free-Radical Polymerization Process. J. Am. Chem. Soc. 116, 11185–11186 (1994).
Wayland, B. B., Poszmik, G., Mukerjee, S. L. & Fryd, M. Living Radical Polymerization of Acrylates by Organocobalt Porphyrin Complexes. J. Am. Chem. Soc. 116, 7943–7944 (1994).
Kato, M., Kamigaito, M., Sawamoto, M. & Higashimura, T. Polymerization of Methyl Methacrylate with the Carbon Tetrachloride/Dichlorotris- (triphenylphosphine)ruthenium(II)/Methylaluminum Bis(2,6-di-tert-butylphenoxide) Initiating System: Possibility of Living Radical Polymerization. Macromolecules 28, 1721–1723 (1995).
Wang, J. S. & Matyjaszewski, K. Controlled/"living" radical polymerization. atom transfer radical polymerization in the presence of transition-metal complexes. J. Am. Chem. Soc. 117, 5614–5615 (1995).
Patten, T. E., Xia, J., Abernathy, T. & Matyjaszewski, K. Polymers with Very Low Polydispersities from Atom Transfer Radical Polymerization. Science 272, 866–868 (1996).
Percec, V. & Barboiu, B. "Living" Radical Polymerization of Styrene Initiated by Arenesulfonyl Chlorides and Cu^I(bpy)_nCl. Macromolecules 28, 7970–7972 (1995).
Chiefari, J. et al. Living Free-Radical Polymerization by Reversible Addition−Fragmentation Chain Transfer: The RAFT Process. Macromolecules 31, 5559–5562 (1998).
Yamago, S., Iida, K. & Yoshida, J. Organotellurium Compounds as Novel Initiators for Controlled/Living Radical Polymerizations. Synthesis of Functionalized Polystyrenes and End-Group Modifications. J. Am. Chem. Soc. 124, 2874–2875 (2002).
Truong, N. P., Jones, G. R., Bradford, K. G. E., Konkolewicz, D. & Anastasaki, A. A comparison of RAFT and ATRP methods for controlled radical polymerization. Nat. Rev. Chem. 5, 859–869 (2021).
Corrigan, N. et al. Reversible-deactivation radical polymerization (Controlled/living radical polymerization): From discovery to materials design and applications. Prog. Polym. Sci. 111, 101311 (2020).
Kamigaito, M., Satoh, K. & Uchiyama, M. Degenerative chain‐transfer process: Controlling all chain‐growth polymerizations and enabling novel monomer sequences. J. Polym. Sci. Part A: Polym. Chem. 57, 243–254 (2019).
Kamigaito, M. & Sawamoto, M. Synergistic Advances in Living Cationic and Radical Polymerizations. Macromolecules 53, 6749–6753 (2020).
Jenkins, A. D., Jones, R. G. & Moad, G. Terminology for reversible-deactivation radical polymerization previously called "controlled" radical or "living" radical polymerization (IUPAC Recommendations 2010). Pure Appl. Chem. 82, 483–491 (2010).
Webster, O. W., Hertler, W. R., Sogah, D. Y., Farnham, W. B. & RajanBabu, T. V. Group-transfer polymerization. 1. A new concept for addition polymerization with organosilicon initiators. J. Am. Chem. Soc. 105, 5706–5708 (1983).
Webster, O. W. The discovery and commercialization of group transfer polymerization. J. Polym. Sci. Part A: Polym. Chem. 38, 2855–2860 (2000).
Desbois, P. et al. Towards the control of the reactivity in high temperature bulk anionic polymerization of styrene, 1. Influence ofn,s-dibutylmagnesium on the reactivity of polystyryllithium species. Macromol. Chem. Phys. 200, 621–628 (1999).
Carlotti, S., Desbois, P., Warzelhan, V. & Deffieux, A. Retarded anionic polymerization (RAP) of styrene and dienes. Polymer 50, 3057–3067 (2009).
Yamashita, Y., Sato, I., Suzuki, H. & Kobayashi, S. Catalytic Asymmetric 1,4-Addition Reactions of Simple Alkylnitriles. Chem. Asian. J. 10, 2143–2146 (2015).
Yamashita, Y. & Kobayashi, S. Catalytic Carbon-Carbon Bond-Forming Reactions of Weakly Acidic Carbon Pronucleophiles Using Strong Bronsted Bases as Catalysts. Chem. Eur. J. 24, 10–17 (2018).
Yamashita, Y., Igarashi, R., Suzuki, H. & Kobayashi, S. Catalytic alkylation reactions of weakly acidic carbonyl and related compounds using alkenes as electrophiles. Org. Biomol. Chem. 16, 5969–5972 (2018).
Kobayashi, S. & Yamashita, Y. New Dimensions of Brønsted Base Catalyzed Carbon–Carbon Bond-Forming Reactions. Synlett 32, 14–22 (2020).
Asano, S., Aida, T. & Inoue, S. 'Immortal' Polymerization. Polymerization of Epoxide catalysed by an Aluminium Porphyrin-Alcohol System. J. Chem. Soc., Chem. Commun., 1148–1149 (1985).
Aida, T. Living and Immortal Polymerizations. Prog. Polym. Sci. 19, 469–528 (1994).
Inoue, S. Immortal Polymerization: The Outset, Development, and Application. J. Polym. Sci. Part A: Polym. Chem. 38, 2861–2871 (2000).
Kamigaito, M., Sawamoto, M. & Higashimura, T. Living cationic polymerization of isobutyl vinyl ether by protonic acid/zinc halide initiating systems: evidence for the halogen exchange with zinc halide in the growing species. Macromolecules 25, 2587–2591 (1992).
Ando, T., Kamigaito, M. & Sawamoto, M. Reversible Activation of Carbon−Halogen Bonds by RuCl₂(PPh₃)₃: Halogen Exchange Reactions in Living Radical Polymerization. Macromolecules 33, 2819–2824 (2000).
Kamigaito, M., Maeda, Y., Sawamoto, M. & Higashimura, T. Living cationic polymerization of isobutyl vinyl ether by hydrogen chloride/Lewis acid initiating systems in the presence of salts: in-situ direct NMR analysis of the growing species. Macromolecules 26, 1643–1649 (1993).

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Proton transfer anionic polymerization with C–H bond as dormant species

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Abstract

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Main

Method

Materials

Anionic polymerization of TBMA in the presence of CTA

Synthesis of end-functionalized poly(TBMA)

Deprotection of the silyl-protected end-functionalized poly(TBMA)

Synthesis of 3-arm star poly(TBMA)

Scission of the 3-arm star polymer

Living cationic polymerization of IBVE with the HCl adduct of 5

Synthesis of TBMA and IBVE block copolymers

Living cationic copolymerization of IBVE and 5

Synthesis of IBVE and TBMA graft copolymers

Measurements

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References

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