Controlled Structure Polymer Latex by Nitroxide-Mediated Polymerization Jason S. Ness, Arkema Inc., USA Stéphanie Magnet, Arkema, France Introduction Free radical emulsion polymerization is commonly employed industrially to produce polymer latex for a variety of applications, including paints and coatings. This technique gives emulsion chemists the ability to control a variety of latex and polymer particle parameters such as: particle size, particle size distribution (PSD), solid content, particle shape, morphology (e.g., core/shell), and functionality. Fine-tuning these parameters is often necessary for achieving certain final latex properties designed to meet end-use application requirements. Despite all this synthetic utility free radical emulsion polymerization does not provide a means to control polymer characteristics at the molecular level, i.e., polymer composition, molecular weight, polydispersity, copolymer sequencing, and chain architecture. Controlled free radical polymerization (CRP), in contrast, does facilitate manufacture of controlled architecture polymers having precise molecular and chemical structures. Arkema s BlocBuilder MA is a unique nitroxide-based technology well-suited for controlling free radical polymerization in both homogenous and dispersed media. This versatile alkoxyamine is able to control polymerization of a broad selection of monomer types and provides a practical, robust method for the design of novel materials spanning rich architectures (e.g., block, graft/comb, gradient copolymers, etc.) and tailored end-use application properties. In emulsion polymerization, Arkema s nitroxide-based technology provides chemists a powerful tool for designing the aforementioned controlled structure polymers within the latex particles themselves, while still controlling typical latex parameters (e.g., particle size, PSD, etc.). Arkema has demonstrated a variety of such controlled structure polymers in emulsion at industrially relevant solids content, surfactant level, and particle size. The presentation aims to highlight synthetic methodologies for producing commercially relevant controlled structure latices with BlocBuilder MA as well as to exemplify some distinctive properties of such latices with respect to paint and coating applications; properties not easily attainable using conventional free radical latex technology. Experimental All materials were used as is without purification. BlocBuilder MA was supplied by Arkema. Generic synthetic guidelines for preparation of a difunctional alkoxyamine, low molecular weight watersoluble macroinitiator and controlled structure latex are provided hereafter. All polymerizations were performed using a 1- or 2-liter high-pressure vessel with 2-3 bar nitrogen pressure. Target polymer number-average molecular weight was achieved by controlling the monomer-to-initiator ([M]/[I]) ratio and monomer conversion. The difunctional alkoxyamine DIAMINS was prepared by adding two molar equivalents of BlocBuilder MA to one molar equivalent of 1,4-butanediol diacrylate according to a previously reported procedure. 1 A representative synthetic methodology for preparing a symmetric poly(methyl methacrylate-block-butyl acrylate-block-methyl methacrylate) triblock copolymer latex with a 70/30 BA/MMA block ratio is given below. 2
Preparation of living oligo(butyl acrylate) seed latex: A 1-L reactor was charged with: 526.1 grams de-ionized water, 0.58 grams NaHC 3, 22.6 grams Dowfax 8390 aqueous solution (35-wt.% active), 1.2 grams (1.25x10-3 mols) of acidic DIAMINS alkoxyamine neutralized with an excess (1.6 equiv. based on acidic functions) of NaH aqueous solution and 7.3 grams butyl acrylate. The solution was sparged with nitrogen for ten minutes while stirring and heated to 120 C for 2 hours. Preparation of poly(butyl acrylate) central block by chain extension of seed latex: 168 grams of butyl acrylate were added to the seed latex over a period of 3 hours. The reactor temperature was maintained at 120 C until the desired conversion was obtained, which in this case was 70%. The reactor was then cooled to 80 C. Chasing the residual butyl acrylate via conventional free radical means (optional): To the reactor, held at 80 C, was charged: 1.14 grams n-dodecyl mercaptan, 0.875g sodium formaldehyde sulfoxylate (in water), and 0.868g potassium persulfate (in water). The reactor was maintained at 80 C for one hour before increasing the temperature to 105 C. Preparation of poly(methyl methacrylate) outer blocks: 75.1 grams methyl methacrylate was added to the poly(butyl acrylate) latex over a period of 2 hours. The reactor temperature was maintained at 105 C until the desired conversion was obtained, which in this case was 86%. After desired conversion was reached, the reactor is cooled to 80 C. Chasing the residual methyl methacrylate via conventional free radical means: To the reactor, held at 80 C, was charged: 0.21g n-dodecyl mercaptan, 0.15 grams sodium formaldehyde sulfoxylate (in water) and 0.15 grams potassium persulfate (in water). The reactor was maintained at 80 C for one hour. This multi-step emulsion procedure was the same regardless of the desired controlled structure polymer (e.g. homo or copolymers of controlled molecular weight and low polydispersity, or gradient, diblock or triblock copolymers) the only differences being the choice of alkoxyamine (mono- or multifunctional) and the number of sequential monomer addition (and chasing) steps. Monomers may be charged as either batch or semi-continuous. Preparation of macroinitiator Preparation of a low molecular weight poly(methacrylic acid-co-styrene) water-soluble macroinitiator (8.8 mol% styrene) was performed according to the procedure described by Charleux et al. 3 The numberaverage molecular weight of the resultant macroinitiator was 3.0kg/mol. A representative synthetic methodology for preparing a surfactant-free poly[butyl acrylate-block-(butyl acrylate-co-methyl methacrylate)] block copolymer latex with a 65/35 BA/MMA block ratio, using a watersoluble macroinitiator under batch conditions, is given below. Preparation of a surfactant-free poly(butyl acrylate) first block: To a 2-L reactor was charged: 506.7 grams de-ionized water, 1.17 grams Na 2 C 3, 12.03 grams (3.95x10-3 mols) of 3.0kg/mol p(methacrylic acid-co-styrene) macroinitiator neutralized with an excess of NaH aqueous solution (until mixture ph = 9), and 325.0 grams butyl acrylate. The mixture was degassed and heated to 118 C until the desired conversion was obtained, which in this case was 82.0%. After desired conversion is reached, the reactor was cooled to 105 C. Preparation of a surfactant-free poly(methyl methacrylate) second block: 175 grams methyl methacrylate was added to the poly(butyl acrylate) latex as batch. The reactor temperature was maintained at 105 C until the desired methyl methacrylate conversion was obtained, which in this case was 38%. The residual monomers were chased as previously described. Final latex solid content was 46%.
This surfactant-free controlled structure latex preparation procedure is the same regardless of the type of macroinitiator used or the desired final controlled structure (e.g., block copolymer). Results and Discussion BlocBuilder MA is an alkoxyamine combining a methacrylic acid radical initiating species with the nitroxide-based reaction controller, known as, in one molecule (Fig.1). 4 This attribute eliminates the need for an external initiation source while providing perfect initiator-to-controller stoichiometry, which facilitates synthesis of polymers with reduced polydispersity. An advantageous result of the methacrylic acid initiating fragment is each polymer chain contains a covalently bound carboxylic acid function, which embodies a site for further chemistry through routine transformation reactions. BlocBuilder MA H N P Heat H + N P Alkoxyamine Initiator Figure 1. Structure of BlocBuilder MA alkoxyamine molecule and dissociation products. BlocBuilder MA s carboxylic acid moiety is also the key feature enabling its use in emulsion polymerization, as it is easily ionized forming a water-soluble alkoxyamine. 5 Additionally, this monoalkoxyamine is readily converted to difunctional and multifunctional alkoxyamines through judicious control of stoichiometry and reaction conditions in a one-step process (Fig. 2). 1 These multifunctional alkoxyamines are also easily rendered water-soluble and provide means to a rich variety of polymer architectures, including triblock and star/hyperbranched copolymers in both emulsion and homogenous media. 5 + R H R H H + H H H Figure 2. General illustration of possible chemistries producing difunctional and multifunctional alkoxyamines leading to diverse polymer architectures. BlocBuilder MA controls the polymerization of a broad selection of monomers including acrylics, methacrylics, and styrenics. Highly tolerant of functional groups, this alkoxyamine also controls polymerization of a wide variety of functional monomers, for example: (meth)acrylic acid, maleic anhydride, 2-hydroxyethyl (meth)acrylate, glycidyl methacrylate, PEGylated (meth)acrylates, N,N-dialkyl (meth)acrylamides and 2-acrylamido-2-methylpropanesulfonic acid, to name but a few. Polymerization temperature is dictated by the monomer type being polymerized, e.g., styrenic acrylic and methacrylic. Table 1 shows typical polymerization temperature ranges providing a reasonable balance between polymerization kinetics and maintaining an acceptable level of polymerization control.
Table 1. Typical range of polymerization temperatures with BlocBuilder MA. Monomer Type Temperature ( o C) Styrene 90-120 Acrylate 110-120 Methacrylate 40-110 As shown in Table 1, most polymerization temperatures are above the boiling point of water, which for emulsion polymerization means reactors capable of handling pressure are required. Typically, pressures of ~2-3 bar are used, most of which is due to inerting nitrogen over pressure. Another requirement for emulsion polymerization at these temperatures is use of sulfonate-based surfactants, which have excellent thermal stability. Controlled structure latex A multi-step nitroxide-mediated controlled radical emulsion polymerization process was used to prepare a variety of controlled structure latices. The first step is formation of highly dilute living oligomer seed particles via addition of the water-soluble form of either BlocBuilder MA alkoxyamine or a difunctional alkoxyamine to a mixture of water, surfactant and monomer followed by heating. This critical first step effectively sequesters the -capped oligomer chain ends within the dispersed particle phase. Living polymer chain extension occurs through subsequent semi-continuous or batch addition steps of monomer or monomer mixtures. The latter produces copolymers having compositional uniformity or gradient-like structures (both with low polydispersity), while multiple chain extensions using different monomer feeds forms block copolymer structures. When the desired monomer conversion is attained for a given chain extension step, the residual monomer(s) can either be converted to polymer through conventional free radical means (i.e., chased ) or be allowed to remain in the latex and copolymerize with subsequent monomer(s) during the next polymerization step. Converting the residual monomers through free radical means will not impact the living polymer chains, provided this is performed at a temperature below the equilibrium dissociation temperature of the -capped polymer chain ends (typically < 90 C). At these temperatures, the living polymer chains are dormant and unreactive toward free radical polymerization. BlocBuilder MA was used to synthesize a series of ABA-type triblock copolymer latices based on poly(methyl methacrylate-block-butyl acrylate-block-methyl methacrylate) denoted as MAM, at soft/hard block ratios ranging from 40%/60% to 70%/30%. 2 Typical MAM latex characteristics were: 42% solids, 1.5-wt.% surfactant, and particle sizes ranging from 150 300nm. Charleux et al. showed that similar MAM triblock latex films nanostructure within the polymer particles themselves - just as block copolymers do in bulk or in films cast from solvent. 6 This nanostructuration is clearly evident in the image below (Fig. 3, from Charleux s work 6 ) showing an AFM micrograph of a similar p(mma/ba-b-ba-b-mma/ba) latex film cross-section. The block copolymer morphology appears onion like or layers of lamellae, which is consistent with this sample s 58%/42% mid to end block ratio.
Figure 3. AFM image (phase contrast) of a p(mma/ba-b-ba-b-mma/ba) latex film allowed to dry at room temperature for 4 days. 6 Dark regions are low modulus (or soft) phase, i.e., poly(butyl acrylate). Reprinted with permission from Elsevier. The ability of these latices to form nanostructured films allows the inherent properties of each block to be maintained, i.e., there is no property trade-off in the final film. In the case of MAM latices, the soft poly(butyl acrylate) phase allows for low temperature film formation while the high T g poly(methyl methacrylate) phase provides for excellent film block resistance properties. Both minimum film forming temperature (MFFT) and blocking temperature (T Block ) were measured for the series of controlled structure MAM latices mentioned previously. As shown in Table 2, all MAM latex films had a MFFT < 0 C and T Block > 50 C, without any coalescing agent. 2 This property balance is significantly better than the conventional free radically produced hard/soft core/shell latex films (based on similar compositions) having a MFFT of 0 at best, and a maximum T Block of only 40 C. 2 This improved balance of properties paves the way for VC-free coatings, which film form on their own (i.e., without coalescing agents), yet are not tacky. Table 2. Comparison of minimum film forming temperature (MFFT) and blocking temperature (T Block ) for a series of MAM latex and hard/soft core/shell latex films. 2 Latex Soft Hard MFFT, C T Block, C MAM 70% 30% < 0 > 50 MAM 60% 40% < 0 > 50 MAM 40% 60% < 0 > 50 hard/soft (core/shell) 75% 25% 0 40 hard/soft (core/shell) 75% 25% 12 40 Controlled structure Surfactant-free latex using a macroinitiator BlocBuilder MA was also used to prepare a low molecular weight water-soluble macroinitiator used in ab initio batch emulsion polymerization producing controlled structure surfactant-free latex. 7 This one-step process involves simply mixing together monomer, water, and the macroinitiator followed by heating. The macroinitiator functions as initiator and control agent as well as surfactant no other additives are needed, e.g., initiators, surfactants/stabilizers, etc. Mechanistically, the macroinitiator initiates polymerization forming amphiphilic diblock copolymers, which self-assemble in situ to form micelles that, as polymerization continues, lead to controlled structure polymer particles.
A poly(methacrylic acid-co-styrene) macroinitiator with 8.8 mol% styrene was first synthesized following Charleux et al. s procedure 3 using BlocBuilder MA. This macroinitiator had a number-average molecular weight of 3.0 kg/mol and was water-soluble under alkaline conditions. This macroinitiator was used to prepare an ionic ABC triblock copolymer surfactant-free latex based on an ionic poly(methacrylic acid-costyrene) A block, a low T g poly(butyl acrylate) B block and a poly(methyl methacrylate-co-butyl acrylate) C block. Block ratios were A = 3.5%, B = 73.4%, and C = 23.1% with an overall latex solid content of 46%. This surfactant-free latex formed a highly elastic film with a MFFT < 0 C and excellent block resistance. From a synthetic viewpoint, this ab initio batch surfactant-free emulsion polymerization process is highly flexible. Latex properties like particle size can be tailored by varying a given macroinitiator s molecular weight and concentration, and more interestingly the composition of the macroinitiator itself can be customized, providing access to specific end-use properties. Finally, virtually any controlled structure polymer can be produced via this surfactant-free macroinitiator technique, from simple low polydispersity homo or copolymers to more complex multiblock copolymers and more. Indeed such areas are subjects of ongoing research at Arkema. Conclusions This presentation highlighted the synthetic utility of nitroxide-mediated controlled radical emulsion polymerization using Arkema s BlocBuilder MA for producing controlled structure polymer latex. A wide range of controlled structure polymer particles are possible using this technology, from a simple copolymer having controlled molecular weight, low polydispersity, homogeneous composition, and controlled functional group placement, to more complex structures such as gradient, block, or graft copolymers. Poly(methyl methacrylate-block-butyl acrylate-block-methyl methacrylate) triblock copolymer latices were introduced that showed an improved balance of film properties exemplified by a MFFT < 0 C and T Block > 50 C in the absence any coalescent agent. Such controlled structure latices represent a potential route to VC-free coatings. Also introduced was a simple one-step ab initio batch emulsion polymerization process for preparation of high solids content surfactant-free controlled structure polymer latex produced via low-molecular weight water-soluble macroinitiator. This macroinitiator acts as initiator, control agent and stabilizer and can be tailored to meet specific latex application needs. An ionic ABC triblock copolymer latex was synthesized at 46% solid content with latex particles comprised of poly[(methacrylic acid-co-styrene)-block-butyl acrylate-block-(methyl methacrylate-co-butyl acrylate)]. Films of this latex were highly elastic with a MFFT < 0 C and excellent block properties. Arkema s BlocBuilder MA technology provides emulsion chemists with a powerful tool to design unique controlled structure polymer latices that realize heretofore unmet technical challenges in the paint and coatings industry. Acknowledgements This work performed in collaboration with Prof. Bernadette Charleux (Université de Lyon) and Stephanie Magnet and Dr. Laurence Couvreur of Arkema France. Dr. References (1) Couturier, J-L., Guerret,., Magnet, S. US Patent 7199214 B2; priority 2003. (2) Magnet, S., Russel, T. W 2009071699 A2; priority 2007. (3) Dire, C.; Charleux, B. ; Magnet, S.; Couvreur, L. Macromolecules 2007, 40, 1897 1903. (4) Couterier, J.L.; Guerret,.; Bertin, D.; Gigmès, D.; Marque, S.; Tordo, P.; Chauvin, F.; Dufils, P.E. W 2004014926; priority 2002. (5) Nicolas, J.; Charleux, B.; Guerret,.; Magnet S. Angew. Chem., Int. Ed. 2004, 43, 6186 6189, Charleux, B., Guerret,., Magnet, S., Nicolas, J. W 2005082945 A1; priority 2004. (6) Nicolas, J. ; Ruzette, A-V. ; Farcet, C. ; Gérard, P. ; Magnet, S.; Charleux, B. Polymer 2007, 48, 7029 7040. (7) Delaittre, G. ; Nicolas, J. ; Lefay, C., Save, M. ; Charleux, B. Chem. Commun. 2005, 614 616, Delaittre, G. ; Nicolas, J. ; Lefay, C., Save, M. ; Charleux, B. Soft Matter 2006, 2, 223 231, Dire, C.; Magnet, S.; Couvreur, L.; Charleux, B. Macromolecules 2009, 42, 95 103. BlocBuilder is a registered trademark of Arkema, Inc. Dowfax is a registered trademark of Dow Chemical Co.