Understanding the effects of nucleophiles in fast living cationic polymerisation of isobutyl vinyl ether in a microflow system from stability and activity of propagating chains

Abstract

Previous studies on fast living cationic polymerisation have rarely focused on the effects of nucleophiles, which are crucial in determining a suitable initiation system. In this work, taking the polymerisation of isobutyl vinyl ether (IBVE) as an example, the effects of various nucleophiles, namely ethyl acetate, 1,4-dioxane, 1,3-dioxolane, diethyl ether, and ethyl propionate, on the activity and stability of the propagating chains were systematically examined in a microflow system, in which IBVE-HCl/SnCl₄ was selected as the initiation system and 2-hydroxyethyl methacrylate was employed as an effective end-capping agent for ¹H NMR spectroscopy measurements. The characteristic time of activity (t_act) and the half-life of stability (t_decay) exhibited almost linear proportional relationships, revealing that the nucleophiles did not alter the relative values of activity and stability, acting simply to improve the controllability of the cationic polymerisation process. Furthermore, a density functional theory simulation method was established to calculate the dissociation energy of IBVE/SnCl₄/Nus (ΔE). Approximately linear relationships between ln(t_act) or ln(t_decay) and ΔE were found, indicating the potential of theoretical calculations to estimate the activities and stabilities of the propagating chains in this process.

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1. Introduction

The propagating chains initiated by carbocations in conventional cationic polymerisation processes are usually unstable and tend to undergo irreversible termination and chain transfer reactions via proton elimination.^1,2 In the last few decades, researchers have achieved living cationic polymerisation via the introduction of nucleophiles, thereby providing a fundamental base for the preparation of functional living polymers with a desired sequence distribution and well-defined architectures.^1,3–12 For example, the living cationic polymerisation process enabled the preparation of various functional poly(vinyl ether)s,¹³ including thermo-sensitive polymers,^14–17 pH/temperature responsive polymers,^18,19 amphiphilic block copolymers,²⁰ star-shaped polymers,^7,21–23 and stimuli-responsive polymers.^24–28 The living initiation system of vinyl ethers usually employed adducts of isobutyl vinyl ether (IBVE) and HCl or acetic acid as initiators, a series of Lewis acids as co-initiators, and ethers or esters as nucleophiles.^29,30 The functions of such nucleophiles include stabilisation of the propagating chains, adjusting the acidity of the co-initiator (Lewis acid), and the formation of monomeric Lewis acidic species, which ultimately result in uniform chain initiation and chain propagation.^2,31–36 From another point of view, these nucleophiles can affect the formation and tightness of the various active species, such as ion pairs. Typically, loose ion pairs result in lower stabilities and higher activities for the propagating chains, while tight ion pairs result in the opposite effects.

Recently, various initiation systems have been developed, including the FeCl₃/cyclic ether (1,4-dioxane/1,3-dioxolane/THF),³⁷ SnCl₄/cyclic ether (1,4-dioxane) or ethyl acetate,^38,39 Et_xAlCl_3−x/ethyl acetate,^21,40 FeBr₃/THF, InCl₃/1,4-dioxane (DO) or ethyl acetate (EA), ZnCl₂/EA or DO, HfCl₄/EA, ZrCl₄/EA, BiCl₃/EA, GaCl₃/THF, GeCl₄/DO, and TiCl₄/EA³⁰ systems. However, the main challenge associated with living cationic polymerisation is that the polymerisation rate is quite slow, which directly restricts its application in industry.³² For example, the TiCl₄ and EtAlCl₂ system suitable for rapid cationic polymerisation (i.e., 1 s, 100% conversion) can achieve the living cationic polymerisation of IBVE in the presence of ethyl acetate, but it requires 120 and 22 h to reach complete conversion, respectively.^21,30,40 Efforts have also been channelled into fast living initiation systems,^2,38,39,41 such as the FeCl₃/cyclic ether (15 s, 100%) and EtAlCl₂/SnCl₄/ethyl acetate (2 min, 100%) systems. Furthermore, our previous studies show that accelerated living cationic polymerisation with the reduced addition of nucleophiles can be achieved in a microflow system, since the microflow can ensure the stability of the produced propagating chains over a limited time and space. We also demonstrated that the activity of the propagating chains can be measured using the apparent rate constant (k_app) and characteristic time (t_1/2),^42,43 and the dissociation energies of the active species combining the nucleophile, calculated by density functional theory (DFT) simulations, showed a relationship with living performance.^43,44

In this work, we aim to elucidate the effects of different nucleophiles on the activities and stabilities of the growing propagating chains. For this purpose, five different nucleophiles, namely 1,4-dioxane (DO), 1,3-dioxolane (DOL), ethyl acetate (EA), diethyl ether (Et₂O), and ethyl propionate (EP), are used to prepare initiation systems containing SnCl₄ and IBVE-HCl, which are suitable for the fast living polymerisation of IBVE in a microflow reaction system. The activities and stabilities of various active species are then measured and the relationship between them determined. In addition, the dissociation energies between SnCl₄ and various nucleophiles are determined by DFT simulations to reveal the key role of the nucleophile and how they influence the activity (or the stability) of the propagating chains quantitatively.

2. Materials and methods

2.1 Reagents

IBVE (>99.0%, TCI) was purified by heating under reflux for several hours and distilling over calcium hydride prior to use. Toluene, 1,4-dioxane, 1,3-dioxolane, ethyl acetate, diethyl ether (A.R. grade, Beijing Lanyi Chemical Products) and ethyl propionate (99%, TCI) were subjected to reflux for several hours, distilled over calcium hydride, and stored over 4 Å molecular sieves. 2-Hydroxyethyl methacrylate (97%, Innochem) and SnCl₄ (99%, anhydrous, Acros) were used as received without further purification. The adduct of IBVE with HCl (IBVE-HCl) was prepared from the addition reaction of IBVE with HCl.⁴⁵ All chemicals were stored in a glove box under dry argon.

2.2 Characterisation

The number-average molecular weight (M_n), weight-average molecular weight (M_w), and molecular weight distribution (MWD, M_w/M_n) of poly isobutyl vinyl ether (PIBVE) were measured by gel permeation chromatography (GPC). GPC was carried out using a Waters 1515 isocratic HPLC pump connected to three Waters Styragel HT2, HT3, and HT4 columns and a Waters 2414 Refractive Index Detector at 30 °C [Waters; Styragel HT2 (THF), Styragel HT3 (THF), and Styragel HT4 (THF); molecular weight range = 100–10k, 500–30k, 5–600k; particle size = 10 μm; column size = 7.8 mm (internal diameter) × 300 mm; flow rate = 1.0 mL min⁻¹]. ¹H NMR spectra were recorded on a Bruker AVANCE III HD 400 spectrometer (400 MHz).

2.3 Computational methods

The B3LYP DFT functional was used for all quantum chemical (QC) calculations, which were performed using the Gaussian 16 program. All geometries were optimised using the 6-311G(d,p) basis set, with the exception where the SDD set was used for the Sn atoms. Besides, the addition of a polarisation function (+) for anions (i.e., the SnCl₄/Nu complex) was required. The effect of a toluene environment was approximated using the SMD solvation model. The final structure was determined based on analytical frequency calculations carried out on the optimised geometries.

2.4 Polymerisation procedure

The polymerisation procedure was carried out in a microflow system. As described in our previous work,⁴² the flow rates for the monomer, Lewis acid, and MeOH (or 2-hydroxyethyl methacrylate (HEMA)) were 8, 8, and 9.6 mL min⁻¹, respectively. The quenched mixture was washed with dilute hydrochloric acid, an aqueous NaOH solution, and then deionised water to remove the initiator residues. The volatiles were then removed under reduced pressure at room temperature (30 °C) and the residue was vacuum-dried overnight to yield a gummy polymer. The monomer conversions were determined gravimetrically.

3. Results and discussion

3.1 Determination of the activities and stabilities of the propagating chains with the addition of various nucleophiles

As mentioned above, the activities of the propagating chains can be represented by the apparent rate constant (k_app) and the characteristic time (t_1/2, referred to as t_act in this work),^42,43 whereby this method requires the monomer concentration data to be obtained at different time intervals prior to reaching 100% monomer conversion. Thus, to obtain the kinetic data for activity determination, the polymerisation reaction of isobutyl vinyl ether (IBVE) with SnCl₄ in conjunction with IBVE-HCl in the presence of a low concentration of EA, DO, DOL, Et₂O or EP (0.1 M) as the nucleophile was investigated in detail. The results are presented in Table 1, and the obtained GPC profiles are shown in Fig. 1.

Fig. 1 (a) Time–conversion curves for the polymerisation reaction of IBVE with SnCl₄, (b) M_n and M_w/M_n for the polymerisation reaction of IBVE (polymerisation conditions: [IBVE]₀ = 0.38 M, [IBVE-HCl]₀ = 10 mM, [SnCl₄]₀ = 5.0 mM, [EA/DO/DOL/Et₂O/EP] = 0.1 M, in toluene at 0 °C).

Table 1 Living cationic polymerisation of IBVE using SnCl₄/X (X = EA, DO, DOL, Et₂O or EP)

Entry	Nus	t/s	Conv./%	M n,cal/g mol^−1	M n/g mol^−1	M w/Mn	ln([M0]/[M])
All experiments: [IBVE]0 = 0.38 M, [IBVE-HCl]0 = 10 mM, [SnCl4]0 = 5 mM, nucleophile: [EA]0 = 0.1 M for entries 1–5, [DO]0 = 0.1 M for entries 6–11, [DOL]0 = 0.1 M for entries 12–16, [Et2O]0 = 0.1 M for entries 17–22, [EP]0 = 0.1 M for entries 23–28, in toluene at 0 °C.
1	EA	0.75	33	1260	1610	1.14	0.3988
2		1.51	56	2170	2520	1.14	0.8321
3		3.02	79	3020	3480	1.10	1.5476
4		6.03	95	3640	3840	1.09	2.9499
5		7.54	97	3740	4070	1.09	3.6088
6	DO	0.60	45	1710	1980	1.15	0.5894
7		0.90	56	2170	2550	1.14	0.8315
8		1.21	67	2580	2930	1.13	1.1112
9		1.51	74	2860	3220	1.11	1.3606
10		2.26	89	3420	3970	1.07	2.2154
11		3.02	94	3620	4010	1.09	2.8416
12	DOL	0.15	55	2120	2270	1.15	0.8049
13		0.30	78	2990	3000	1.13	1.5115
14		0.45	89	3420	3690	1.10	2.2059
15		0.60	96	3690	3960	1.09	3.2560
16		0.75	98	3750	4180	1.08	3.7206
17	Et2O	0.16	31	1200	1483	1.16	0.3747
18		0.32	48	1852	2200	1.18	0.6583
19		0.53	70	2677	3230	1.13	1.1945
20		0.80	80	3076	3519	1.11	1.6147
21		1.06	91	3482	4100	1.09	2.3727
22		1.33	95	3643	4220	1.09	2.9700
23	EP	0.32	31	1180	1490	1.16	0.3671
24		0.53	46	1784	2140	1.14	0.6247
25		0.8	54	2076	2430	1.13	0.7779
26		1.33	79	3047	3420	1.10	1.5774
27		1.86	87	3332	3840	1.09	2.0227
28		2.12	90	3450	3940	1.09	2.2871

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As shown in Table 1 entries 1–5, the polymerisation using the SnCl₄/EA initiator system gave complete conversion at ∼7.54 s, while the SnCl₄/DO, SnCl₄/EP, SnCl₄/Et₂O and SnCl₄/DOL initiating systems resulted in complete conversion at 3.02 s, 2.12 s, 1.33 s and 0.75 s, respectively. In addition, it should be emphasised that polymers with a narrow MWD (M_w/M_n = 1.08–1.18) can be produced under all conditions in a microflow system, and as shown in Fig. 1, the M_n value of the polymer is directly proportional to the monomer conversion. This is in good agreement with the calculated value, assuming that the initiator species corresponds to the step-by-step growth of the polymer chain. These results confirmed that under the investigated conditions, the SnCl₄/Nu-mediated IBVE polymerisation reactions are invariably living processes. In addition, a nucleophile concentration of 0.1 M was sufficient, and the obtained data were valid for determining the activity of the propagating chains.

In terms of the stability, end-capping analysis is required for the living cationic polymerisation process of IBVE initiated by the SnCl₄/X (X = EA, DO, DOL, Et₂O or EP) system after reaching 100% monomer conversion. In a pioneering work conducted by Higashimura et al. in 1990, the concentration of living polymers ([P*], the sum of the concentrations of active chains and dormant chains) was determined using sodiomalonic ester as a quencher in the HI/ZnX₂ and HI/I₂ systems.^46,47 Subsequently, they used ln[P*] as a function of time to measure the lifetime of a growing species. However, the synthetic route to sodiomalonic ester is expensive and complex, and no systematic investigations were carried out into the effects of different factors on the lifetime. Since HEMA was reported to be an effective and cheap quencher for living poly(MOVE)s,⁴⁸ we expected that it could be a suitable and cheap substitute for sodiomalonic ester.

Thus, the reaction was quenched at varying intervals using an excess of HEMA. Fig. 2 shows a typical ¹H NMR spectrum for a HEMA-terminated PIBVE, where peaks corresponding to the methacrylate protons at 6.1, 5.6, and 1.9 ppm are clearly visible. The peak intensity ratio of the terminal protons (e) to those of the methylene and methine units of poly(IBVE) (f and g) corresponds to the concentration of living ends. The living polymer chains can be calculated as follows: [living polymer] = [P*] = [M]₀ × 10³ × conv. × 3e/(f + g), where [M]₀ is the initial monomer concentration. In addition, all spectra revealed several chain end structures of side reactions: internal olefin, aldehyde, alkanal, and the structure resulting from β-proton elimination.⁴⁹ The results of the end-capping analysis and related data are presented in Table 2. It was found that independent of the type of nucleophile, the MWDs of the polymers were always very narrow (M_w/M_n = 1.08–1.13), thereby indicating that the termination process using HEMA was also quantitative and instantaneous.

Fig. 2 ¹H NMR spectrum of the HEMA-terminated PIBVE species ([IBVE]₀ = 0.38 M, [IBVE-HCl]₀ = 10 mM, [SnCl₄]₀ = 5 mM, nucleophile: [DOL]₀ = 0.1 M for entry 18 in Table 2).

Table 2 Cationic polymerisation of IBVE with SnCl₄/X (X = EA, DO, DOL, Et₂O or EP)

Entry	Nus	t/s	Conv./%	M n,cal/g mol^−1	M n/g mol^−1	M w/Mn	ln[P*]
All experiments: [IBVE]0 = 0.38 M, [IBVE-HCl]0 = 10 mM, [SnCl4]0 = 5 mM, nucleophile: [EA]0 = 0.1 M for entries 1–6, [DO]0 = 0.1 M for entries 7–12, [DOL]0 = 0.1 M for entries 13–18, [Et2O]0 = 0.1 M for entries 19–26, [EP]0 = 0.1 M for entries 26–34, in toluene at 0 °C. [P*] = 0.38 × 10^3 × conv. × 3e/(f + g).
1	EA	60.32	100	3840	4540	1.09	2.2495
2		75.40	100	3840	4450	1.09	2.1952
3		96.51	100	3840	4460	1.09	2.0766
4		120.64	100	3840	4220	1.10	1.9695
5		156.83	100	3840	3850	1.10	1.8061
6		180.96	100	3840	3960	1.11	1.7147
7	DO	24.13	100	3840	4590	1.07	2.2447
8		36.19	100	3840	4550	1.08	2.1676
9		48.25	100	3840	4340	1.11	2.0979
10		60.32	100	3840	4590	1.08	1.9357
11		90.48	100	3840	4410	1.09	1.7039
12		120.64	100	3840	4560	1.07	1.3522
13	DOL	1.51	100	3840	3830	1.09	2.1743
14		3.02	100	3840	3680	1.10	2.1396
15		6.03	100	3840	3740	1.09	2.0119
16		9.05	100	3840	4020	1.08	1.9099
17		12.06	100	3840	3810	1.09	1.8135
18		15.08	100	3840	3660	1.09	1.6899
19	Et2O	6.03	100	3840	4670	1.10	2.2946
20		9.05	100	3840	4640	1.12	2.2185
21		12.06	100	3840	4670	1.11	2.1790
22		15.08	100	3840	4640	1.11	2.0487
23		24.13	100	3840	4690	1.11	1.9765
24		36.19	100	3840	4690	1.11	1.8265
25		48.25	100	3840	4630	1.13	1.9520
26		60.32	100	3840	4620	1.12	2.2575
27	EP	12.06	100	3840	4550	1.10	1.9577
28		15.08	100	3840	4610	1.10	2.2822
29		24.13	100	3840	4590	1.09	2.1181
30		36.19	100	3840	4450	1.10	2.0506
31		48.25	100	3840	4500	1.11	1.9316
32		60.32	100	3840	4530	1.10	1.8922
33		90.48	100	3840	4490	1.10	1.5774
34		120.64	100	3840	4470	1.10	1.5489

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3.2 Quantitative relationship between the activities and stabilities of the propagating chains

A first-order kinetic model was constructed (Fig. 3) by calculating the apparent polymerisation rate (k_app) from the plot of ln([M]₀/[M]) vs. the reaction time. For the SnCl₄/EA, SnCl₄/DO, SnCl₄/EP, SnCl₄/Et₂O and SnCl₄/DOL living initiation systems, k_app was determined to be 0.4871 s⁻¹, 0.9450 s⁻¹, 1.0962 s⁻¹, 2.1973 s⁻¹ and 5.0997 s⁻¹, respectively. Accordingly, t_act (i.e., the time required to reach ∼50% conversion at a constant k_app) was found to be 1.42 s, 0.73 s, 0.63 s, 0.32 s and 0.14 s, respectively. In the other words, k_app,DOL ≈ 2.32k_app,Et2O ≈ 4.65k_app,EP ≈ 5.40k_app,DO ≈ 10.47k_app,EA.

Fig. 3 Plot of ln([M]₀/[M]) versus time for the SnCl₄/X (X = EA, DO, EP, Et₂O or DOL) system (polymerisation conditions: [IBVE]₀ = 0.38 M, [IBVE-HCl]₀ = 10 mM, [SnCl₄]₀ = 5.0 mM, [EA/DO/DOL/Et₂O/EP] = 0.1 M, in toluene at 0 °C).

The ln[P*] values, obtained from these ¹H NMR spectra, were then plotted against time, as shown in Fig. 4. The straight ln[P*]–t plots revealed that these decay processes were of the first order in ln[P*], and so can be expressed as follows: −d[P*]/dt = k_decay[P*]. The lifetime (or the half-life, t_decay) of the living polymer ends was then calculated from the slope (k_decay), t_decay = (ln 2)/k_decay (t_decay refers to the time at which [P*] is reduced to one-half of the original [IBVE-HCl] in value). In detail, the t_decay is 154.03 s for SnCl₄/EA, 74.53 s for SnCl₄/DO, 79.67 s for SnCl₄/EP, 38.94 s for SnCl₄/Et₂O, and 19.36 s for SnCl₄/DOL, respectively. All of them are significantly shorter than the duration that the stable (living) growing propagating chains present in the HI/I₂ system, estimated to range from 40 to 1000 min (17 h).⁴⁶ It can be noticed that t_decay,EA ≈ 2.07t_decay,DO ≈ 1.93t_decay,EP ≈ 3.96t_decay,Et2O ≈ 7.96t_decay,DOL, which is almost reverse with the quantitative relationship of t_act. These results therefore indicate that HEMA reacts quantitatively and specifically with the living chain ends, and is confirmed to be an effective end-capping reagent adaptive to [P*] determination for the first time. It was speculated that the ester oxygen atom of EA stabilised the growing end to a greater extent than cyclic ethers due to stronger coordination or solvation, corresponding to the longest t_act being recorded for the SnCl₄/EA initiation system.

Fig. 4 Plot of ln[P*] as a function of time in the IBVE polymerisation reaction initiated by the SnCl₄/X (X = EA, DO, EP, Et₂O or DOL) system (polymerisation conditions: [IBVE]₀ = 0.38 M, [IBVE-HCl]₀ = 10 mM, [SnCl₄]₀ = 5.0 mM, [EA/DO/DOL/Et₂O/EP] = 0.1 M, in toluene at 0 °C).

The characteristic time of stability (t_decay) against the characteristic time of activity (t_act) can further illustrate the relationship between the activity and stability of the propagating chains intuitively and reveal the effect of the nucleophile essentially, as shown in Fig. 5. The obvious almost linear proportional relationship indicates that for an initiation system the activity and stability values can be calculated from one another, and the effects of nucleophiles on the activities and stabilities may abide by a common rule. This is the first time that the quantitative relationship between these values and the influence of nucleophiles on the activities and stabilities of the propagating chains have been demonstrated experimentally.

Fig. 5 Plot of t_decay as a function of t_act in the IBVE polymerisation reactions initiated by the SnCl₄/X (EA, DO, DOL, Et₂O or EP) system (polymerisation conditions: [IBVE]₀ = 0.38 M, [IBVE-HCl]₀ = 10 mM, [SnCl₄]₀ = 5.0 mM, [EA/DO/DOL/Et₂O/EP] = 0.1 M, in toluene at 0 °C).

3.3 Correlation between the experimental and simulated data

Our previous report showed that from the perspective of prediction, the dissociation energy of different active species can correspond to the activity (or stability) to some extent.^43,44 Herein, we wished to quantify the relationship between the DFT calculation results and experimental values of the activity (or stability), and clarify the nature of different IBVE-HCl·SnCl₄·ether ion pairs. Herein, we attempted to establish a more suitable calculation model in DFT simulations with the considerations of more accurate basis sets and the polarization of negative ions, etc. Then the last half of the activation step in the dormant-active equilibrium was calculated, that is, the chain initiation step ([IBVE-HCl·SnCl₄·Nu] → [IBVE-HCl·Nu]⁺ + [SnCl₅·Nu]⁻). In detail, the ionization process of the C–Cl bond (dormant species, IBVE-HCl·SnCl₄·Nu complexes) in the growing end produced [IBVE-HCl·Nu]⁺ and [SnCl₅·Nu]⁻ active ions. Then, accompanied by the consumption of monomers, the living chains continue to grow until the terminator is added.

As shown in Table 3 and Fig. 6, an ab initio calculation was conducted using Gaussian 16, where it was suggested that the combination of SnCl₄ with a nucleophile leads to the formation of a large and stable hexacoordinated anion consisting of five chlorine anions and one additional nucleophile. Due to the large steric hindrance, the introduction of DOL may weaken the interactions with SnCl₄ (or IBVE-HCl) to some extent. Accordingly, EA binds more tightly with SnCl₄ (or IBVE-HCl), thereby resulting in the most pronounced effects on the activity and stability of the propagating chains in the IBVE-HCl/SnCl₄ initiating system, and then a longer lifetime and a lower polymerisation rate. In addition, DFT calculations carried out in a toluene system indicate that the complex of IBVE-HCl·SnCl₄·EA has the highest dissociation energy (30.76 kcal mol⁻¹), while the IBVE-HCl·SnCl₄·DOL complex exhibits the smallest bond dissociation energy (i.e., 22.65 kcal mol⁻¹). These results indicate that the equilibrium shifts to a greater extent toward the ion pair side in the case of the SnCl₄/DOL system, which is in accord with the experimental results. In order to establish a quantitative relationship for prediction from the calculated dissociation energies, an approximately linear kinetic model was obtained by plotting ln(t_act) or ln(t_decay) vs. the dissociation energy (Fig. 7). The parallel relationship observed for the ln(t)–ΔE curves shows that the nucleophiles change the absolute values of activity and stability without changing the relative values. It further reveals that nucleophiles mainly affect the frequency of carbocation generation, and the quantitative effects can be calculated well.

Fig. 6 Dissociation energies and optimised structures of IBVE-HCl·SnCl₄·Nus calculated using Gaussian 16. (Calculation method: B3LYP/genecp for SnCl₅⁻·Nus, 6-311+G(d,p) for C, H, O, and Cl atoms, SDD for Sn; B3LYP/6-311G(d,p) for IBVE+·Nus; B3LYP/genecp for IBVE-HCl·SnCl₄·Nus, 6-311G(d,p) for C, H, O, and Cl, SDD for Sn; em = gd3bj and the SMD solvent model were used for all calculations.)

Fig. 7 Plots of ln(t_act) or ln(t_decay) as a function of the dissociation energy for the SnCl₄/X (X = EA, DO, EP, Et₂O or DOL) system.

Table 3 Dissociation energies with IBVE-HCl·SnCl₄·Nus^a

Energy	E (Hartree)	E (kcal mol^−1)
a B3LYP/genecp for SnCl5^−·Nus, 6-311+G(d,p) for C, H, O, and Cl, SDD for Sn; B3LYP/6-311G(d,p) for IBVE^+·Nus; B3LYP/genecp for IBVE-HCl·SnCl4·Nus, 6-311G(d,p) for C, H, O, and Cl, SDD for Sn; em = gd3bj and the SMD solvent model were used for all calculations.
SnCl5^−·EA	−2612.7259	−1 639 511.6239
IBVE^+·EA	−619.4267	−388 696.4441
IBVE-HCl·SnCl4·EA	−3232.2016	−2 028 238.8279
Dissociation energy	0.04902	30.7599
SnCl5^−·DO	−2612.6838	−1 639 485.2164
IBVE^+·DO	−619.3790	−388 666.4862
IBVE-HCl·SnCl4·DO	−3232.1043	−2 028 177.7837
Dissociation energy	0.0416	26.0812
SnCl5^−·DOL	−2573.3490	−1 614 802.2184
IBVE^+·DOL	−580.0466	−363 985.0131
IBVE-HCl·SnCl4·DOL	−3153.4316	−1 978 809.8815
Dissociation energy	0.0361	22.6500
SnCl5^−·Et2O	−2538.6458	−1 593 025.6153
IBVE^+·Et2O	−545.3621	−342 220.1438
IBVE-HCl·SnCl4·Et2O	−3084.0463	−1 935 269.8994
Dissociation energy	0.0385	24.1403
SnCl5^−·EP	−2652.0534	−1 664 190.0033
IBVE^+·EP	−658.7570	−413 376.6327
IBVE-HCl·SnCl4·EP	−3310.8514	−2 077 592.3758
Dissociation energy	0.0410	25.7398

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The significance of the above work lies in providing a predictable method for pursuing living/controlled systems. A living system needs not only an inherent controlled reaction, but also a match between mixing characteristic time and reaction characteristic time. The reaction characteristic time can be regulated by the introduction of nucleophiles. In some cases, a conventional system can meet the demand of living polymerisation at the cost of a low production efficiency; however, with stricter requirements, it is necessary to use process enhancing equipment, such as a microflow device with a short mixing time, which results in an improved production efficiency. We also note that nucleophile types do not alter the degree to which the polymerisation reaction deviates from living cationic polymerisation under ideal mixing, and so if such alteration is required, other factors need to be considered. Our ongoing work mainly focuses on the concentration of nucleophiles, solvent type, co-initiator type and monomer type to further reveal the complex effects on the activity and stability of propagating chains.

4. Conclusion

We herein reported the convenient flow determination of the activities and stabilities of propagating chains in fast living cationic polymerization processes in a microflow system, and then investigated the effects of different nucleophiles systematically. More specifically, five nucleophiles, namely ethyl acetate (EA), 1,4-dioxane (DO), diethyl ether (Et₂O), ethyl propionate (EP), and 1,3-dioxolane (DOL), were introduced into the IBVE-HCl/SnCl₄ initiation system to conduct the accelerated living cationic polymerisation of IBVE in a microflow system. ¹H NMR spectroscopy confirmed that 2-hydroxyethyl methacrylate was an effective end-capping agent that can quantitatively terminate and combine with living polymers. Subsequently, the activities and stabilities of the growing propagating chains were presented based on the characteristic time (t_act) and half-life (t_decay), respectively. It was found that the IBVE-HCl/SnCl₄/EA initiation system resulted in the longest half-life (t_decay,EA = 154.03 s) and the lowest frequency of carbocation generation. In contrast, the IBVE-HCl/SnCl₄/DOL initiation systems exhibited shorter half-lives (i.e., t_decay,DOL = 19.36 s). The characteristic time (t_act) and the half-life (t_decay) exhibited an approximately linear proportional relationship, revealing that the nucleophiles did not change the relative values of the activity and stability, but simply improved the controllability of the cationic polymerisation process through prolonging t_act under limited mixing performance. Furthermore, a density functional theory simulation method was established to calculate the dissociation energy (ΔE) of IBVE-HCl·SnCl₄·Nus. An approximately linear relationship between ln(t_act) or ln(t_decay) and ΔE was found, indicating the potential of theoretical calculations in estimating the activity and stability of the propagating chains. These findings therefore identify the role of nucleophiles in living cationic polymerisation processes, and may provide a predictive method for establishing an appropriate initiation system.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors are grateful for the financial support of the National Natural Science Foundation of China (21422603, U1662120, 21978152).

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