Sulfonic Acids as Water-Soluble Initiators for Cationic Polymerization in Aqueous Media with Yb(OTf)3* KOTARO SATOH, MASAMI KAMIGAITO, MITSUO SAWAMOTO Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan Received 4 April 2000; accepted 5 May 2000 ϭ ABSTRACT: Aqueous sulfonic acids (HOSO2R; R CH3, Ph-p-CH3, and Ph-p-NO2), coupled with a water-tolerant Lewis acid, ytterbium triflate [Yb(OTf)3; OTf ϭ O OSO2CF3], initiate the cationic suspension polymerization of p-methoxystyrene (pMOS) in heterogeneous aqueous media. They induce controlled polymerization of pMOS at 30 °C, and the molecular weights of the polymers (weight-average molecular weight/number-average molecular weight ϳ 1.7) increase with conversion. These sus- pension polymerizations are initiated by the entry of sulfonic acid from the aqueous phase into the organic phase and proceed via reversible activation of the sulfonyl terminus by the Lewis acid. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2728–2733, 2000 Keywords: sulfonic acid; ytterbium triflate; cationic polymerization; p-methoxysty- rene; living polymerization INTRODUCTION lymerization of p-methoxystyrene (pMOS) in an aqueous suspension (eq 1).4–6 When coupled with the pMOS–HCl adduct or aqueous hydro- Organic reactions and polymerizations in water gen chloride (1) as the initiator, for example, and related aqueous media have been drawing Yb(OTf) gives long-lived poly(pMOS) whose much attention not only from mechanistic but 3 number-average molecular weight (Mn)in- also from environmental and industrial view- creases with monomer conversion, and the mo- 1,2 We are interested in reactions medi- points. lecular weight distributions (MWDs) are rela- ated by rare earth metal salts. In contrast to tively narrow [weight-average molecular highly moisture-sensitive conventional Lewis weight/number-average molecular weight (Mw/ acids, they are water-tolerant, retain acidity M ) ϳ 1.4].4 This polymerization most likely in water, and catalyze Aldol, Diels–Alder, and n 3 proceeds in organic droplets suspended in water Michael reactions even in aqueous media. via a carbocationic growing species generated Recently, we found that ytterbium triflate by the Yb(OTf) -catalyzed reversible activation [Yb(OTf) ; OTf ϭ OOSO CF ] and related rare 3 3 2 3 of the COCl bond at the dormant polymer ter- earth metal salts also mediate the cationic po- minal. Evidently, the carbocationic intermedi- ates survive and propagate in the water-satu- rated organic phase (although not in bulk wa- *This work was presented in part at the 47th Symposium on Macromolecules, Society of Polymer Science, Nagoya, Ja- ter) and maintain long lifetimes to provide pan, October 1998; Paper 1Pf026. See Satoh, K.; Kamigaito, molecular weight control. These features are M.; Sawamoto, M. Polym Prepr Jpn 1998, 47, 1239. clearly different from those of conventional Correspondence to: M. Sawamoto ionic polymerizations performed in extremely Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 2728–2733 (2000) © 2000 John Wiley & Sons, Inc. dry solvents: 2728 CATIONIC POLYMERIZATION IN AQUEOUS MEDIA WITH SULFONIC ACIDS 2729 (1) The suspension cationic polymerizations are the selection and combination of the acidities of unique and different from conventional cationic the protonic and Lewis acids. For example, the systems and also from recently developed living living cationic polymerization of styrene with 7 cationic processes. However, like the living cat- SnCl4/nBu4NCl was achieved through combina- ionic processes that are effected in anhydrous or- tion with 18(a) or methanesulfonic acid (2)8(b) as ganic solvents, they are based on two-component the initiator (1 and 2 have similar acidities; see eq initiating systems in which a protonic acid (or its 2);9 under similar conditions, trifluoromethane- adduct with a monomer) serves as the initiator sulfonic acid, a much stronger acid, fails to initi- and a Lewis acid serves as the activator/catalyst ate living polymerization. Similarly, the selection that assists carbocation formation from the initi- of protonic acids (initiators) is important for the 7 ator and triggers controlled propagation. A key suspension polymerization with Yb(OTf)3 in to living cationic polymerization, therefore, lies in water: (2) This article concerns the use of water-soluble sul- the presence of Yb(OTf)3 (Scheme 1). In radical fonic acid (HOSO2R) initiators for the cationic emulsion polymerization, a water-soluble initia- suspension polymerization of pMOS in water in tor generates a radical in the aqueous phase, Scheme 1 2730 SATOH, KAMIGAITO, AND SAWAMOTO lower acidity of the former (higher pKa). This is probably due to their substituents, which facili- tate the entry of the initiators into the organic phase and, in turn, initiation. These sulfonic acids afforded polymers of con- trolled molecular weights and MWDs, similar to the results obtained with the HCl–pMOS adduct 4 and 1. Figure 2 shows Mn and MWD curves of 12 poly(pMOS)s produced in water. The Mn in- creased with monomer conversion, and the ϳ MWDs were unimodal but broad (Mw/Mn 1.8) Figure 1. Time–conversion curves for the polymer- throughout the polymerization. During the early stages of the polymerizations, observed molecular ization of pMOS with initiator/Yb(OTf)3 in water at 30 ϭ ϭ °C: [pMOS]0 3.0 M; [initiator]0 60 mM; [Yb(OTf)3]0 weights were higher than calculated values, as- ϭ 400 mM; aqueous/organic ϭ 1/1 (initiators: E, 1; F, 2; suming that one protonic acid generates one poly- (M) 3;(N) 4). mer chain; the deviation indicates slow initiation which subsequently enters a micelle to react with monomer in the oil phase and triggers radical 10 propagation. In contrast, in Yb(OTf)3-mediated cationic suspension polymerization, a water-solu- ble initiator forms a cationic initiating species after diffusing into a droplet of pMOS. The poly- merization with aqueous protonic acids may then be initiated simply by the addition of monomer into an aqueous solution of the protonic acid and the Lewis acid [Yb(OTf)3 is highly water-soluble]. RESULTS AND DISCUSSION Polymerization with Aqueous HOSO2R ϭ Three sulfonic acids [HOSO2R; R CH3 (2), Ph- p-CH3 (3), and Ph-p-NO2 (4)] were employed as initiators for the cationic polymerization of pMOS in water. The polymerization was initiated by the addition of pMOS to an aqueous solution of Yb(OTf)3 and one of the sulfonic acids under vig- orous stirring at 30 °C; the reagent concentra- ϭ tions were [pMOS]0 6.0 M in the organic phase ϭ ϭ and [Yb(OTf)3]0 800 mM and [HOSO2R]0 120 mM in the aqueous phase.11 No dispersants were employed. The experiments were carried out un- der air without a vacuum or an inert gas. pMOS consumption was quantitative with a Figure 2. M , M /M , and MWD curves of poly(p- short induction phase, probably because of slow n w n MOS) obtained with sulfonic acid (2–4)/Yb(OTf)3 in initiation due to the transfer of the initiator from ϭ ϭ water at 30 °C: [pMOS]0 3.0 M; [sulfonic acid]0 60 the aqueous phase to the organic phase (Fig. 1). ϭ ϭ mM; [Yb(OTf)3]0 400 mM; aqueous/organic 1/1 The rate depended on the substituents in the (sulfonic acids: F and Œ, 2;(M, ) 3;(N, ‡) 4. The initiators: p-substituted benzenesulfonic acids (3 diagonal bold line indicates the Mn calculated on the and 4) induced faster polymerizations than the assumption of the formation of one living polymer per more hydrophilic acids (1 and 2), regardless of the initiator molecule. CATIONIC POLYMERIZATION IN AQUEOUS MEDIA WITH SULFONIC ACIDS 2731 and the latter dominates the former. This is prob- ably due to quenching by dilution with excess water. However, this in turn suggests that the growing carbocationic species reacts with water to form the hydroxyl terminus, which can regen- erate a cationic species during the polymeriza- tion. Note that (the terminal) benzyl alcohol can- not always be cleaved into a carbocation, depend- ing on which Lewis acid is employed. The polymers obtained with sulfonic acids 3 and 4 gave similar spectra. Table I summarizes ␣ O ␻ the functionalities of these (CH3 ) and ter- 1 O O Figure 3. H NMR spectrum of poly(pMOS) (Mn mini ( OH and OSO2R). The functionality of ϭ ϭ ␣ 1910; Mw/Mn 1.43) obtained with 2/Yb(OTf)3 in the CH3 group ( end) was close to unity (Fn ϭ ϭ ϳ ␻ ␻ water at 30 °C: [pMOS]0 3.0 M; [2]0 60 mM; 1.0) in all cases, whereas those of the 1 and 2 ϭ ϭ [Yb(OTf)3]0 400 mM; aqueous/organic 1/1. ends were much lower, which suggests some chain transfer. This is also indicated by the mo- lecular weights of the products being lower than by the diffusion of sulfonic acids from the aqueous the calculated values. phase to the organic phase. The lower Mn in the Figure 4 shows typical distributions of two later stages indicates chain transfer as observed samples obtained by matrix-assisted laser-de- in the polymerization with the HCl–pMOS ad- sorption-ionization time-of-flight mass spectrom- 4 14 duct/Yb(OTf)3. The use of 4 led to Mn’s closer to etry (MALDI-TOF-MS). The spectra consist of a the calculated values. These results indicate that main and minor series of peaks; in each series, water-soluble sulfonic acids, in the presence of the signals are separated by 134.2 Da, or the Yb(OTf)3, induce a controlled suspension cationic molecular weight of pMOS. The molecular polymerization of pMOS. Most likely, initiation is weights of each main-series peak were very close O O due to acid–monomer adducts via activation by to the calculated values for H (pMOS)nϩ1 OH ϩ ϩ Yb(OTf)3 of the sulfonyl ester linkage, even in the Na , that is, HO-capped poly(pMOS).