Perspective Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Carbon Dioxide-Switchable Polymers: Where Are the Future Opportunities? Michael F. Cunningham*,†,‡ and Philip G. Jessop‡ Department of Chemical Engineering and ‡Department of Chemistry, Queen’s University, Kingston, ON, Canada K7L 3N6
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ABSTRACT: Carbon dioxide has emerged as a new and innovative “trigger” for stimuli-responsive materials. It is abundant, inexpensive, nontoxic, and environmentally benign. Carbon dioxide-switchable polymers are a class of stimuli-responsive polymers for which CO2 is a trigger used to reversibly switch the polymer properties, typically from hydrophobic (in the absence of CO2) to hydrophilic (in the presence of CO2). Recent years have witnessed a surge in interest in polymers incorporating CO2-switchable moieties, where the change in properties upon switching has enabled the development of a new generation of stimuli-responsive materials, including switchable particles, surfaces, catalysts, and a plethora of nonspherical polymer particle morphologies such as wormlike micelles, vesicles, tubules, and gels. This Perspective introduces the principles of CO2-switching, highlights recent major developments, and presents our personal insights into challenges with commercialization of CO2-switchable polymers and future opportunities for new research directions and materials applications.
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INTRODUCTION Designing enhanced performance features into polymeric materials includes the capability to “switch” or alter the material’s properties or behavior when desired, effectively yielding a single polymeric structure that can fulfill two or more functions. In some applications, a single switch in one direction may suffice, but the more versatile design lies in a material whose properties can be reversibly switched over numerous cycles without deterioration. Various triggers to initiate switching in polymers have been used, including pH, temperature, light, and voltage. The past decade has seen a surge of interest is gas-responsive polymers, with CO2responsive (CO2-switchable) materials being the most widely studied. CO2-responsiveness is in effect a subset of pHresponsiveness, with the critical distinction that while the vast majority of pH-responsive systems use liquid acids/bases as triggers, CO2-responsive systems use gaseous CO2, most commonly at atmospheric pressure although pressurized systems can also be used. The reverse trigger of CO2 can be any nonacidic gas, including air, nitrogen, or argon. CO2’s attractiveness as a trigger stems from several advantages; it is nontoxic, inexpensive, and abundant. Moreover, when it is added to and removed from any system over one or multiple cycles, it does not accumulate. Contrast this feature with the use of liquid acids and bases to enact a pH switch, where multiple switching cycles result in accumulation of salts and suppression of effective switching after a few cycles. In addition to polymeric materials, CO2-responsiveness has been investigated for many nonpolymeric materials, including for example switchable solvents, surfactants, and surfaces. In several instances there is a direct relationship with polymer science; switchable surfactants have been used to prepare CO2switchable nanoparticle latexes, and CO2-switchable solvents © XXXX American Chemical Society
have been used for catalyst and ligand recovery in ATRP. Surfaces can be made CO2-responsive by binding any appropriate CO2-switchable moiety to the surface, but perhaps the most popular and arguably most effective approach is to graft CO2-switchable polymers to the surface. However, it is the rich variety of polymeric structures that can be made CO2responsive that has captured the imagination of researchers. Nanoparticles, wormlike micelles, vesicles, membranes, and gels have been reported, targeted at applications such as sensing, separation, coatings, encapsulation, CO2 capture and sequestration, forward osmosis, and catalysis. The ability to design materials with nanoscale switchable structure may facilitate a new generation of materials for nanotechnology. This Perspective highlights progress in the development of CO2-switchable polymeric materials and the use of CO2switching principles in polymer science and presents opinions on future opportunities as well as the challenges facing widespread adoption of CO2 -switchable materials and technology. Comprehensive reviews are available to the interested reader.1−4 Comprehensive reviews on CO2-switchable polymers have been published.1−4 In this Perspective we have not attempted to provide the level of detail found in these reviews nor to be as comprehensive in scope. Rather, we have chosen to highlight recent progress in what we believe are the key areas in the development of CO2-switchable polymeric materials, emphasizing those with the greatest potential for commercial implementation or for enabling the most impactful new materials. For readers not familiar with the field, we introduce the chemical principles of the switching process. Received: May 3, 2019 Revised: August 2, 2019
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DOI: 10.1021/acs.macromol.9b00914 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. CO2-Switching of a Polymeric Amine with n Protonatable Sites Relies upon the Addition of CO2 Causing Most of the Sites To Be Protonated and the Removal of CO2 Causing Those Sites To Become Largely Unprotonateda
Finally, we present our own personal opinions on the most promising future opportunities as well as the challenges facing widespread adoption of CO 2-switchable materials and technology. A multitude of CO2-switchable materials have been reported in recent years, including for example solvents, inorganic nanoparticles, quantum dots, carbon nanotubes, and graphene. We have chosen to limit this Perspective to macromolecular materials, even if some of the examples noted above have been modified with grafted switchable polymers.
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PRINCIPLES OF CO2-SWITCHING CO2-switching is based on the reversible conversion of basic or acidic compounds upon exposure to CO2 in the presence of a protic species (usually water but could also be an alcohol or a primary or secondary amine). Most CO2-switchable small molecules and almost all CO2-switchable polymers have been neutral nitrogen-containing compounds such as amines, amidines, and guanidines that are converted to their bicarbonate salts upon exposure to CO2 in the presence of water (eq 1). Compounds with N−H bonds may also be converted to carbamate salts upon exposure to CO2 (eq 2, shown for a secondary amine). Some weak organic acids (e.g., carboxylic and phenolic) can also be CO2-switchable, depending on their pKa values, where the anions can be reversibly converted to the neutral species by CO2 (eq 3). B + CO2 + H 2O F [BH+][HCO3−]
(1)
2R 2NH + CO2 F [R 2NH 2+][R 2NCO2−]
(2)
A + CO2 + H 2O F HA +
(3)
−
HCO3−
a
The pH must alternate between a value below the system midpoint and a value above that point.
In addition to the pKaH (and/or system midpoint), the concentration of switchable species plays a critical and often unappreciated role in the effectiveness of switching. (Note that the word “concentration” in this discussion refers to the moles of protonatable sites per liter of aqueous solution, not the moles of polymer chains per liter.) For example, consider a switchable polymer that is fully dissolved in water under either air or CO2, so that the system midpoint is the same as the pKaH. If the pKaH is 8.0 and the concentration is 10 mM under air, then the solution would have an elevated pH of 10 due to the high concentration of basic groups in solution. Because the pH would be significantly above the system midpoint, the degree of protonation would be very low (Table 1). Upon Table 1. pH Values Obtained with an Amine of pKaH 8.0 at Selected Concentrations in Water at 25 °C under Either Air or CO2 at 1 bar of Absolute Pressure
While reactions 1 and 3 are typically readily reversible, carbamate formation (eq 2) is the most difficult to reverse, requiring higher temperatures and longer switching times. Using amines, amidines, or guanidines with bulky substituents or containing no N−H bonds precludes carbamate formation. While carbamate formation is generally not desirable, it may be advantageous when the salt form needs to be used at higher temperatures (∼60 to 100 °C) without prematurely converting back to the neutral form. A critical decision in designing CO2-switchable polymers is the basicity of the polymer, as this determines whether switching will be effective (i.e., achieving nearly complete conversion to each of the neutral and charged states respectively) under the conditions where the switching is to occur. The ideal basicity (here measured as the pKaH, meaning the pKa of the protonated state) for a chosen application depends on both the temperature and the concentration of the switchable species. To maximize the change in properties, the switchable moieties should ideally have a very low degree of protonation in the absence of CO2 (e.g., under air or inert atmosphere) and conversely a very high degree of protonation in the presence of CO2 (Scheme 1). If the switchable species are entirely dissolved in an aqueous solution both under air and under CO2, then the pKaH is equal to the system midpoint (pH at which half of the switchable groups are protonated). However, if a two-phase oil/water system exists or if the neutral form of the polymer is incompletely soluble, then the system midpoint will be at a pH lower than the pKaH. In the absence of CO2, the pH should be significantly higher than the system midpoint, while under CO2 the pH should be significantly lower than the system midpoint.
[B] (mM)
pH under air
% protonation under air
pH under CO2
% protonation under CO2
0.01 0.1 1 10 100 1000
8.4 9.0 9.5 10.0 10.5 11.0
26.6 9.5 3.1 1.0 0.3 0.1
3.9 4.1 4.8 5.8 6.8 7.7
99.99 99.99 99.9 99.4 94 69
addition of CO2, the pH would decrease to 5.8, resulting in near complete protonation of the switchable groups and a significant change in the material’s properties. However, if the concentration of switchable species was much lower, such as 0.01 mM, then the initial pH under air would be only 8.4, and about 27% of the switchable species would already be protonated, dramatically limiting the change in material properties that can be achieved upon subsequent addition of CO2. Thus, a switchable polymer with a pKaH of 8.0 would only exhibit large changes in properties upon CO2-switching if the polymer were at a concentration of about 0.1−100 mM. Polymers intended for use at lower concentrations would need to have a lower pKaH. Alshamrani et al. have presented plots to assist in determining suitable pKaH ranges for different concentrations of switchable species based on consideration of temperature and CO2 pressure.5 Plots indicating the range of acceptable basicity have been prepared for a variety of conditions based on the chemical equilibria in the system. Figure 1 shows such a plot for 25 °C, assuming that the neutral B
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Since our first publications demonstrating CO2-switchable particles, which utilized a switchable surfactant to stabilize micrometer-size polymer particles7 and then submicrometer latex particles,8,9 a rich variety of particle morphologies and processes have been developed, ranging from simple spherical particles whose surface charge can be switched to di- and triblock copolymers that can reversibly self-assemble to spheres, worms, micelles, and vesicles.
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POLYMER PARTICLES Microsuspension polymerization7 and emulsion polymerization8,9 were used in conjunction with CO2-switchable amidine or tertiary amine surfactants to prepare aqueous dispersions of spherical particles. Emulsion polymerization is a common commercial process, most notably used in the production of water-based paints, coatings, sealants, and adhesives which contain ∼50 wt % water. Energy and cost savings could be significant if latexes could be aggregated, shipped as concentrated wet cakes or dry powder, and then redispersed at the point of application. In some applications, polymer resins and not particles are the desired product, necessitating particle coagulation which is usually accomplished by adding large amounts of salts, acids, or bases. Particles whose colloidal stability can be switched from a stable dispersion to particles that can settle upon standing could offer environmentally benign alternate solutions to these types of challenges. The first surfactants we used in emulsion polymerization were based on alkyl amidines as the switchable moiety.9 Styrene and MMA were as monomers in these studies, although any free radical monomer that is used in free radical polymerization would also be suitable. The only differences with conventional emulsion polymerization are (1) they are run with a CO2 atmosphere (ambient pressure) instead of an inert atmosphere such as nitrogen and (2) the maximum temperature that can be used is ∼65 °C (at higher temperatures the dissolved CO2 concentration in the water is too low to maintain a high degree of protonation of the surfactants). The latexes remained colloidally stable under CO2 but could be destabilized by sparging the latex with air (Figure 2). Mild heat can be used if desired to accelerate the deprotonation. Caution must be exercised, with not only emulsion polymerizations but also any radical polymerization, to use an appropriate initiator and more specifically to avoid using hydrochloride salts since the presence of the HCl interferes with switching process. Aggregated latexes can be redispersed by addition of CO2 and sonication, leading to near recovery of the original particle size distribution and zeta potential.10 Redispersion cannot be achieved by using conventional surfactants. Because the radical derived from initiator decomposition forms the polymer chain end, it is possible to use a switchable initiator (e.g., VA-061) without additional surfactant to prepare polystyrene latexes via surfactant-free emulsion polymerization.11 Alkyl amidine surfactants are quite basic and therefore difficult to deprotonate although they readily become protonated. If faster “switching off” times are desired, less basic surfactants based on tertiary amine and aryl amidines can be used although these are not protonated as easily and thus will have a lower degree of protonation under CO2 than alkyl amidines, resulting in larger particle sizes.8 An alternative CO2-switchable surfactant, N,N-dimethyldodecylamine, was effective in miniemulsion polymerization as well as in the preparation of
Figure 1. Percent protonation of an organic base such as an amine or amidine in water under air (dashed lines) or 0.1 MPa of CO2 (solid lines) at 25 °C as a function of the nominal concentration of the switchable groups (moles of amine or amidine groups per liter of solution) and the pKaH. The greatest change in properties upon CO2 addition or removal will occur if the pKaH of the switchable groups falls between the red dashed and blue solid lines. Reproduced with permission from ref 5. Copyright 2016 Royal Society of Chemistry.
and charged forms are fully dissolved. Dashed lines show the percent protonation of organic bases in aqueous solution under air. Solid lines show the percent protonation of organic bases in aqueous solution under 1 bar (absolute pressure) of CO2. Ideally, a switchable group would have a very low percent protonation under air (pKaH below the red dashed line) and a very high percent protonation under CO2 (above the blue dashed line). At these conditions, if a switchable compound is at high concentrations (e.g., 1 M), then a pKaH of 9−11 would be best, but if the required concentration is much lower (e.g., 1 mM), then a pKaH of 6−8.5 would be best. At higher temperatures the solid lines do not move much, but the dashed lines come down significantly, so the ideal range of pKaH values is narrower. (Note: Figure 1 was developed for small molecules and not polymers, so that caution needs to be used in their interpretation for polymeric systems. Factors such as incomplete solubility of the neutral polymer at some concentrations, and pKaH being a function of concentration and molecular weight for polymers, will affect the calculations.) The ease of reversibility depends on the basicity and enthalpy of protonation of the base. Amines, imidazoles, and aryl amidines have moderate basicity in water and consequently readily switch to the bicarbonate salt upon CO2 exposure and revert back to neutral when CO2 is removed, even at room temperature. Higher temperatures favor the conversion of bases from the bicarbonate salt to the neutral form and therefore can be used to accelerate the deprotonation step. Compounds having higher basicity (e.g., alkyl amidines) form quite stable bicarbonate salts that take some time and elevated temperatures to be converted back to their neutral state. Aryl amidines and tertiary amines are less basic, switching “off” more easily than alkyl amidines, but they are more difficult to completely protonate. Compounds with insufficient basicity may not effectively respond to CO2 (e.g., pyridines and anilines). Yin et al. empirically investigated the relationship between CO2 switchability and basicity using melamine derivatives. They derived guidelines for understanding switching behavior (the derived equations are valid only for the concentration tested).6 C
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restabilized, probably due to the switchable moieties being covalently bound to the particle surface such that they could not migrate, unlike small molecule surfactants. Additional functionality (e.g., fluorescence and UV-responsiveness) can be introduced by incorporating appropriate monomers such as spiropyranethyl acrylate with MMA gave UV−vis-sensitive P(MMA-co-SPEA) latexes, which are also CO2-responsive because they were made by using P(DMAEMA-b-MMA) stabilizer.18 Interestingly, the latex color depends on both light exposure and whether the CO2 were protonated (Scheme 2). Scheme 2. Schematic Representation of Spiropyran Isomerization in Accordance with Acidochromism/ Basochromism before and after Polymerization (PMSP-2 Latex Nanoparticles)18
Figure 2. PMMA latexes prepared using amidine switchable surfactants after 17 days with (at left) bubbling of CO2 for 30 min each day for the first 7 days and (at right) no CO2 bubbling (vials were inverted for the photograph to show the increased viscosity of the sample that received no CO2 treatment).9
particles with polyvinylformal (PVF) shell and N,N-dimethyldodecylamine core, where the hydrophobic core could be switched to hydrophilic by CO2 addition.12 Amphiphilic block copolymers are an alternative form of stabilizer in emulsion polymerizations, often providing more effective stabilization than their small molecule surfactant counterparts. This approach has been used to make CO2switchable latexes.13,14 Poly(2-(dimethylamino)ethyl methacrylate)-block-poly(methyl methacrylate) (PDMAEMA-bPMMA) block copolymers made by RAFT were reported first.13 Emulsion polymerizations were performed at pH ∼ 3 (achieved by HCl addition) since conducting the polymerization under CO2 gave unstable latexes, perhaps due to incomplete protonation. Shirin-Abadi et al. also examined PDMAEMA-b-PMMA stabilizers and compared their effectiveness when the stabilizing moieties originated from DMAEMA copolymerized in situ.14 Differences in both aggregation and redispersion behavior were noted depending on whether the tertiary amine moieties originate from monomer polymerized in situ versus added block copolymer. An alternate approach to making switchable surfactants prior to polymerization is to make surfactants in situ at the beginning of the polymerization with a small amount of switchable monomer, for example (N-amidino)dodecyl acrylamide,15 diethylaminoethyl methacrylate (DEAEMA),16 or N[3-(dimethylamino)propyl] methacrylamide (DMAPMAm).17 At the beginning of an emulsion polymerization, protonated switchable monomer (water-soluble) copolymerizes with the hydrophobic monomer(s) in the aqueous phase to give CO2switchable oligomeric chains that act as surfactants. Zhang et al. found that stable latexes could be made under CO2 and then aggregated with N2 using the switchable monomer (Namidino)dodecyl acrylamide.15 Pinaud et al. reported similar results with DEAEMA and noted that only 0.5 wt % DEAEMA (with respect to total monomer) was sufficient to yield monodisperse PMMA latexes.16 The polymer particles could be dried under vacuum and redispersed by using sonicationa notable improvement compared to previous results with added surfactant where only destabilized wet cakes could be
Similarly, the fluorescent aminobromomaleimide methacrylate can be copolymerized with DEAEMA (core) in particles with an oligoethylene glycol methacrylate shell.19 CO2 addition protonates the DEAEMA units and quenches the fluorescence emission, thereby yielding an effective probe particle. More sophisticated morphologies can also be constructed, including hollow nanospheres20 and particles made from an interpenetrating network.21 CO2-switchable particles have also been prepared using reverse ATRP in emulsion and AGET ATRP in miniemulsion.22 The emulsion ATRP process used premade CO2switchable surfactants. Much better control over the polymerization was achieved with AGET ATRP miniemulsion polymerization which featured use of a CO2-switchable ATRP inisurf 1,1-(diethylamino)undecyl 2-bromo-2-methylpropanoate (BrC11N) that acted as both initiator and surfactant (Scheme 3). When protonated, the amine group acts as a surfactant. This process demonstrates the ability to D
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Macromolecules Scheme 3. Structures of the CO2-Switchable ATRP Inisurf BrC11N in Its Neutral and Protonated States22
make particles with both CO2 switchability and controlled microstructure.
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SELF-ASSEMBLY OF CO2-SWITCHABLE POLYMERS The most intriguing CO2-switchable microstructures and morphologies have been prepared by using self-assembly. A variety of self-assembled structures (usually reversibly selfassembled) have been constructed from diblock, triblock, or random copolymers and some have incorporated more than one stimuli-responsive trigger. Assembly and disassembly are often triggered by addition or removal of CO2 although polymerization-induced self-assembly (PISA) has also been used to initially prepare the structures, which then retain CO2responsiveness due to the presence of CO2-switchable moieties in the polymer. Achieving reversible complete self-assembly typically requires a balance of amphiphilicity in the polymers such that in one state the chains are fully water-soluble (or soluble in the solvent of interest), while in the alternate state the chains become amphiphilic. The hydrophobic segments then associate with each other, yielding particles, micelles, vesicles, or wormlike structures. CO2 addition or removal may result in complete dissolution of a self-assembled structure or, if desired, in the reversible transformation to a different type and/or size of self-assembled structure (e.g., micellar to vesicular).3
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Figure 3. Gas-switchable structural change of amidine-containing diblock copolymer PEO-b-PAD (top) and schematic representation of its self-assembly into vesicles and their reversible gas-responsive “breathing” in aqueous media (bottom). Reproduced with permission from ref 23. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.
SELF-ASSEMBLY OF DIBLOCK COPOLYMERS CO2-switchable diblock copolymers can form the basis for “breathable” vesicles.23 Starting from a poly(ethylene oxide)based macroinitiator, by using ATRP, PAD was added to a poly(ethylene oxide) block giving PEO-b-PAD. When the PAD block is neutral (hydrophobic), vesicles of ∼110 nm diameter form in water, stabilized by the PEO block. CO2 addition leads to reversible expansion of the vesicles to ∼200 nm (Figure 3). Because the pKaH of the switchable groups in PAD is low (5.4), the PAD was only partially protonated upon CO2 addition (pH = 5.7), which prevents complete dissolution of the PEO-bPAD. To illustrate the importance of the type of polymer used, including its pKaH, consider results from a similar study of breathing vesicles using a PDEAEMA switchable block with the hydrophilic block being poly(N,N-dimethylacrylamide) (PDMA) or poly(ethylene oxide) (PEO).24 PDEAEMA has a higher pKaH (∼7.5) than PAD (5.4) and does fully dissolve upon CO2 addition. Therefore, the vesicles fully dissolve under CO2. With light cross-linking, the vesicle remains intact and displays a large volume expansion upon CO2 addition.
provides steric stabilization in water, the middle block is often hydrophobic and is required for self-assembly, and the third block is commonly the CO2-switchable block that provides the unique responsiveness. In most studies, the three blocks are chosen to be mutually incompatible such that they will undergo distinct phase separation. The relative and absolute lengths of each individual blocks are critical in determining whether self-assembly occurs, what types of different structures are produced in the presence and absence of CO2, and the magnitude of changes upon addition or removal of CO2. Starting from a simple PEO-b-PS polymer, very different materials can be prepared by varying the third block. Consider two polymers we just discussed: PAD and PDEAEMA. With PDEAEMA as the third block, vesicles formed with a PEO corona, PS core, and PDEAEMA inner core. With CO2 addition the PDEAEMA expansion is constrained by the PS layer. Volume expansion of the spheres, stretching of nanofibers, and compartmentalization of vesicles may then occur depending on the rigidity of the PS layer. With PAD as the third block, microtubules were formed under neutral conditions. Slow addition of CO2 yielded a transformation first into vesicles and then spheres as the degree of protonation
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SELF-ASSEMBLY OF TRIBLOCK COPOLYMERS Triblock polymers have, not surprisingly, a richer and more diverse range of achievable structures and stimuli-responsive behavior than diblocks, particularly when three different polymers are used. One end block is usually hydrophilic and E
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Macromolecules increased (Figure 4).25 Using PEO and PDEAEMA as the hydrophilic and switchable blocks, respectively, and a poly-
MCMs offer a potential route to storing different guest molecules in different compartments within the micelle.
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SELF-ASSEMBLED STRUCTURES FROM BLOCK−RANDOM COPOLYMERS “Block−random” copolymers can also display switching behavior and allow fine-tuning of the morphological or structural changes in response to the addition or removal of CO2. A “block−random” copolymer is a block copolymer, but instead of having two homopolymer blocks, one block is a random copolymer, i.e., P(polymer A)-b-P(polymer B-rpolymer C). Liu et al. compared the behavior of PEO45-b(DEAEMA90-r-St66) and PEO45-b-DEAEMA93-b-St66.28 The incorporation of styrene in the second block increases hydrophobicity, having a pronounced effect when DEAEMA is protonated but unsurprisingly much less effect when DEAEMA is neutral. Morphologies for the triblock and the block−random polymer differed with addition of CO2. The presence of styrene units impacts the pKaH, making ionization more difficult. Liu et al. observed 97% protonation of the tertiary amines in the triblock polymer but only 35% protonation in the block−random polymer. When 4-vinylpyridine (4VP), which undergoes hydrogen bonding in water, was used as the hydrophobic comonomer, even more diverse differences were observed between the triblock and the block− random.29 4-Vinylpyridine units are also pH-sensitive (pKaH ∼ 4.7).30 The concept of CO2 induced coassembly was used by Lin et al. using block−random copolymers.31 In a mixed solvent they coassembled poly(styrene)-block-poly[(4-vinyl-
Figure 4. Gas-switchable amidine-containing triblock copolymer EAS (top) and representation of its CO2-driven controlled self-assembly and shape transformation behavior (bottom). Reproduced with permission from ref 25. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.
(2,2,3,4,4,4-hexafluorobutyl methacrylate) hydrophobic block, CO2-responsive MCMs (multicompartment micelles) were produced.26,27 Incompatibility of the fluoropolymer with the PDEAEMA and PEO gave segregation of the core or corona.
Scheme 4. Schematic of the “Schizophrenic” Aggregation Behavior for the Diblock Polymer PDEAEMA-b-PNIPAM Controlled by CO2 and Temperature (Reproduced with Permission from Ref 32. Copyright 2014 Royal Society of Chemistry)
F
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process.37 In the one-pot process, protonated DEAEMA was polymerized in water to make the stabilizing block. Methyl methacrylate (with styrene comonomer) was then added to form the hydrophobic block, giving spherical nanoparticles of ∼50−65 nm in diameter. An important consideration when polymerizing many of the ester monomers is hydrolysis. The monomers hydrolyze under acidic conditions (pH < ∼5) as occurs under CO2 at atmospheric pressure. Reducing the pH to only ∼6 by using HCl alleviates this issue and minimizes hydrolysis during formation of the first block. Nonspherical CO2-switchable morphologies can also be prepared via PISA.38 Photoinitiated RAFT dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA) and DMAEMA in water allowed preparation of a range of nano-objects (wormlike micelles, short worms, and spheres) depending on reaction conditions and especially the respective monomer concentrations.
pyridine)-random-((2-(diethylamino)ethyl methacrylate)] with either poly(ethylene oxide)-block-poly[(4-vinylpyridine)random-((2-(diethylamino)ethyl methacrylate)] or poly(ethylene oxide)-block-poly(acrylic acid), producing Janus micelles and wormlike micelles respectively in the absence of CO2. Upon CO2 addition, these two morphologies switched to core−shell and spherical micelles, respectively.
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DUALLY RESPONSIVE SELF-ASSEMBLED STRUCTURES Combining CO2-responsiveness with sensitivity to an additional trigger expands the versatility and potential applications of stimuli-responsive materials. Most publications featuring dually sensitive systems have used temperature as the second trigger, leaving a myriad of opportunities to explore the combination of CO2 with other triggers. PDEAEMA-bPNIPAM diblock copolymers (NIPAM: N-isopropylacrylamide) made by RAFT spontaneously formed vesicles at room temperature under neutral conditions, stabilized by PNIPAM32 (Scheme 4). Protonation of the PDEAEMA block induced complete dissolution of the vesicle. The effects of increasing temperature above the LCST of PNIPAM (∼37 °C) are complex with this particular structure and depend on the state of the CO2 responsive block, behavior referred to by the authors as “schizophrenic”. Either spherical micelles (hydrophobic PNIPAM core and charged PDEAEMA corona) or vesicles can result. PDMAEMA has been more widely used for its temperature sensitivity than for its CO2-switchability, but both triggers can be used in the same polymer.33 PADS-b-PDMAEMA is intriguing because both blocks are CO2-responsive but with quite different pKaH values and because the PDMAEMA block is also temperature-sensitive. This combination of features results in complex self-assembly behavior. At room temperature at neutral conditions, vesicles with a PADS interior vesicular wall and water-soluble PDMAEMA corona result. Note that the low pKaH (∼7.4) of PDMAEMA means that there is appreciable protonation at neutral pH (even in the absence of CO2). CO2 addition further protonates the PDMAEMA block and the amidine groups in PADS, giving a very large size increase (224 to >500 nm radius). Heating vesicles at room temperature (no CO2) to 45 °C (just above the PDMAEMA LCST) leads to micelles with a PADS core and PDMAEMA outer corona. Protonating these micelles with CO2 makes both blocks more hydrophilic and causes significant expansion. Dual CO2-temperature-sensitive PNIPAM-b-PCL-b-PDMAEMA triblocks where the middle block, poly(ε-caprolactone), is not stimuli-responsive also exhibit complex and not readily predictable vesicular behavior in response to application of each trigger.34 A PEG block coupled with a copolymer block of DEAEMA and O2-responsive 2,2,2trifluoroethyl methacrylate gives dually responsive vesicles.35 O2 addition resulted in an ∼8 fold volume increase of the vesicles because of fluorine’s affinity for O2. Somewhat paradoxically, CO2 addition led to volume contraction and not expansion because of the unique morphology exhibited by the vesicles.
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CO2-SWITCHABLE TECHNOLOGY IN POLYMER SYNTHESIS Previous sections have illustrated the range of stimuliresponsive polymeric structures that CO2-switching enables. There is another important role, however, for applying the principles of CO2-switching in polymer science, more focused on offering an alternative, improved process for preparing polymers. This section will highlight a few examples where applying CO2-switching technology provides a much-improved route to the preparation of precipitating polymers from solution, preparing artificial latexes, and removing/recycling both catalyst and ligand from ATRP reactions. Although there is a trend to replace solvent-borne polymerizations with waterborne systems, solution polymerizations remain common in industry. In many cases the solvent needs to be removed to isolate the polymeran energyintensive process that also carries the risks of handling large volumes of volatile solvent. Phan et al. first demonstrated the use of CO2-switchable solvents for both the polymerization and isolation steps in free radical polymerization, although the process should be considered generically applicable to a wide variety of polymerizations.39 Styrene was polymerized in a lowpolarity mixed solvent (DBU (1,8-diazabicyclo[5.4.0]undec-7ene)/1-propanol) in the absence of CO2. Addition of CO2 to the polymerized mixture switched the DBU solvent to its highpolarity form [DBUH+][C3H7CO3−], resulting in precipitation of the polystyrene. Subsequent removal of CO2 switched the DBU back to its nonpolar form suitable for reuse in the next polymerization. Artificial latexes, sometimes termed secondary latexes, are aqueous dispersions of small particles that are prepared, not directly by a process such as emulsion polymerization but rather by dissolving polymer in organic solvent, dispersing the solvent/polymer mixture as small droplets in an aqueous solution with the aid of a surfactant, and then evaporating the solvent to leave small polymer particles dispersed in the aqueous phase. This process is useful for making latexes from polymers not easily made by dispersed phase polymerization processes such as emulsion or suspension polymerization but has the undesirable feature of requiring extensive solvent handling in addition to the significant energy needed to evaporate the solvent. Waste rubber from the natural rubber industry, known as field coagula, can be used for making artificial latexes. Su et al. developed a new process for making artificial latexes that utilizes CO2-switching.40 Poly(butyl
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POLYMERIZATION-INDUCED SELF-ASSEMBLY (PISA) Spherical CO2-switchable nanoparticles were prepared by NMP using either a two-step process36 or a simpler one-pot G
DOI: 10.1021/acs.macromol.9b00914 Macromolecules XXXX, XXX, XXX−XXX
Perspective
Macromolecules
polymerizations over three cycles, without requiring makeup ligand, was also established.
methacrylate) (PBMA) and natural rubber artificial latexes were prepared using the CO2-responsive switchable hydrophilicity solvents N,N-dicyclohexylmethylamine (Cy2NMe) and 2-(diisopropylamino)ethanol. The polymers were first dissolved in the nonprotonated switchable solvent, and then water and surfactants (either anionic or cationic) were added to form an emulsion of polymer/solvent droplets in water. Subsequent addition of CO2 converted the switchable solvent to its water-soluble bicarbonate salt form, inducing its diffusion into the aqueous phase and leaving an artificial latex of dispersed polymer particles ∼1 μm in diameter. Using a different switchable solvent, 2-(diisopropylamino)ethanol, left lower residual solvent concentrations in the polymer particles than the Cy2NMe, a key consideration in the design of such a process. A miniemulsion-based process was also developed, yielding much smaller particle sizes (∼240 nm) and narrower size distributions than the emulsion-based method. Su et al. also applied CO2-switching to addressing the issue of residual copper and ligand in ATRP-made polymers.41 ATRP is well-known for its versatility in designing and manipulating polymers with controlled microstructure and morphology. However, the issue of residual copper (and to a lesser extent ligand) remains a concern for industry, where values 10 ppm is still desirable, necessitating expensive postreaction treatment to reduce the Cu levels. Efficient removal and recycling of the ligand is also desirable from both economical and toxicity perspectives. Conveniently, many ATRP ligands, including Me6TREN (a trialkylamine), are CO2-switchable. Su et al. reported three different routes for removing copper catalyst and recycling the ligands after ATRP reactions. The first approach employed the CO2-switchable solvent Cy2NMe as the solvent for a solution ATRP. After the reaction is complete, addition of water and CO2 sparging precipitates the polymer while leaving the copper salt and ligand in the nowhydrophilic solvent. In the second route, solution ATRP was conducted in a traditional solvent such as toluene. Following polymerization, addition of water and CO2 sparging causes the ligand (Me6TREN) and copper salt to diffuse to the aqueous phase, while the polymer remains in the organic phase where it can then be isolated using conventional techniques. Experiments conducted using the first two routes used moderately high Cu concentrations to provide a rigorous test of the effectiveness of each approach, but the question of their suitability for low Cu processes such as ARGET remained. Therefore, a third route was also done by using ARGET ATRP in toluene at very low initial Cu concentrations. For the first route, in all cases residual copper in the polymer was
