Cationic Polymerization of Vinyl Monomers in Aqueous Media: From Monofunctional Oligomers to Long-Lived Polymer Chains

Abstract

Polymer latexes are easily prepared on a multimillion ton scale in industry using free radical initiated emulsion and suspension polymerizations in water, a cheap, nonviscous, heat-controlling, and environmentally benign solvent. Until recently, researchers had done little investigation into ionic polymerization because even a small amount of water would easily deactivate the conventional catalysts used in these processes. In the last decade, however, cationic polymerization in aqueous media has emerged as a new and attractive method for controlling the polymerization reactions using mild experimental conditions. This Account reviews the current science of and future outlook for cationic polymerization of vinyl monomers in aqueous media. We particularly emphasize the design and evolution of catalytic systems and the precision synthesis of functional polymers. Early work to tailor the suspension and emulsion cationic polymerizations of reactive monomers such as p-methoxystyrene and vinyl ethers used long-chain strong acids, called INISURF for their dual roles as initiators and surfactants, and lanthanide triflates. These polymerization processes shared two main features: (i) all reactions (initiation, propagation, and termination) occurred at the particle interface; (ii) synthesized polymers have limits on their molecular weight, attributed to the "critical DP" effect, related to the entry of oligomers inside the particles as they become increasingly hydrophobic. The next generation of catalysts, named "Lewis acid-surfactant combined catalysts" (LASC), shifted the polymerization locus from the interface to the inside of the monomer droplets, allowing for the production of long polymer chains. Recently, catalytic systems based on boranes, (BF(3)OEt(2), B(C(6)F(5))(3), (C(6)F(4)-1,2-[B(C(6)F(5))(2)]), and (C(6)F(4)-1,2-[B(C(12)F(8))](2))), have shown great potential in controlling the cationic polymerization in "wet" solution, containing an excess of water relative to Lewis acid, or aqueous media of such industrially important monomers as styrene, cyclopentadiene, and even isobutylene.

A Catalyst Platform for Unique Cationic (Co)Polymerization in Aqueous Emulsion

“…Besides,t he occurrence of relatively broad but monomodal MWD (Figure 2a)e vidences that branching occurs under these conditions.T he polymer microstructure analyzed by 1 Ha nd 13 CNMR spectroscopy ( Figure S10) revealed predominantly trans-1,4-(72 %) and cis-1,4-units (21 %), whereas 1,2-and 3,4-structures were present only as minor components (4 and 3%,r espectively), in agreement with the cationic polymerization mechanism. [10] Several features observed in previous works [2,18] and reproduced here further advocate for ac ationic polymeri- [30] by metallocenep olymerization (*, &)( the dot line is the fit of Foxequation with T g 'sof102 and À58 8 8Cf or polystyrene and polyisoprene, respectively); c) olefinic part of 13 CNMR spectra (for detailed assignments see Figure S13). …”

“…[1] Different generations of catalysts have been tested over the last 15 years,m ost of which were shown to efficiently polymerize p-methoxystyrene (pMOS). [2] Yb(OTf) 3 was the first water-tolerant Lewis acid successfully used to catalyze the slow (several days) cationic polymerization of pMOS in aqueous suspension [3] or emulsion at 30 8 8C.…”

“…. zation process.1 -(4-Methoxyphenyl) ethanol (pMOS-OH), an initiator in B(C 6 F 5 ) 3 -mediated cationic polymerization, [2] does not initiate the polymerization of pMOS (Table S2, Figure S11). On the other hand, the addition of this compound to styrene or isoprene polymerization led to as trong decrease of the reaction rate and M n (entries S17, S18, and S21 in Table S2).…”

Characteristics and Mechanism of Styrene Cationic Polymerization in Aqueous Media Initiated by Cumyl Alcohol/B(C6F5)3

“…first reported on the cationic polymerization of p‐methoxystyrene (pMOS) in aqueous media using Yb(OTf) 3 as a water‐tolerant LA in 1999, cationic polymerization in aqueous media as a new and attractive polymer synthesis method has emerged . With the continuous studies on polymerization in this cheapest non‐toxic reaction medium, cationic polymerization techniques in aqueous media have made a considerable progress since the twenty‐first century . At present, this type of polymerization is also the important development direction of cationic polymerization industry.…”

“…Lanthanide triflates as water‐tolerant LAs were the first reported co‐initiators to induce cationic polymerizations of pMOS at 30 °C in aqueous suspension and emulsion . Due to the polymerizations proceeded at the droplet interface, synthesized polymers had limits on their molecular weight because of the “critical DP” effect . Subsequent works on a new initiating system named “Lewis acid surfactant combined catalyst” (LASC), which is made of lanthanide derivatives and surfactants, were investigated in the emulsion cationic polymerization of pMOS .…”