Effect of Room-Temperature Ionic Liquids as Replacements for

Chapter 10

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Effect of Room-Temperature Ionic Liquids as Replacements for Volatile Organic Solvents in Free-Radical Polymerization M i c h a e l G . B e n t o n and C h r i s t o p h e r S. B r a z e l * Department of Chemical Engineering, The University of Alabama, Tuscaloosa, AL 35487

Room temperature ionic liquids (RTILs) are a class of solvents that can be used in polymer synthesis to replace volatile organic compounds. However, to make the transition of ionic liquids from laboratory scale to industrial scale, the reactions must be feasible, they must produce polymers that have similar or better mechanical properties to traditional methods, and the ionic liquids must be recyclable. Polystyrene, poly(methyl methacrylate), and poly(2-hydroxyethyl methacrylate) were made by bulk and solution polymerization using both traditional organic solvents and ionic liquids. The monomer and polymer solubilities were tested in the ionic liquids, and physical properties, including the tensile moduli and glass transition temperatures, were measured. Results are encouraging, in that the tensile moduli are not statistically different whether the polymer was formed in organic solvent or in ionic liquid solvent.

Background Our society today is inundated with plastics. Products ranging from drink bottles, to computer casings, and even medical supplies are made from plastic. In fact, it has been estimated that over 80 billion pounds of plastics are produced annually in the United States alone (1). The majority of these plastics are made

© 2002 American Chemical Society

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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126 via free radical addition polymerizations and over half of these are solution polymerizations (2). Most solution polymerization reactors utilize volatile organic compounds (VOCs) as solvents, which are well known to be environmental and health hazards. Environmental hazards include smog and air pollution, while health hazards include eye, skin, and central nervous system irritation. V O C usage in chemical manufacturing industries is rampant, with over five billion dollars worth used annually worldwide (3). A n example of this widespread usage is in polystyrene production, one of the most widely made commodity polymers. In 1999, the domestic production of polystyrene was 6.5 billion pounds (1)· Almost all of the polystyrene produced commercially uses ethylbenzene, a V O C , as a solvent. It has been suggested by one source that for every ton of polystyrene produced, over 2.5 pounds of V O C emission occurs, suggesting that over 4,000 tons of volatile organic compounds are released into the atmosphere yearly from the US production of polystyrene alone (5). Clearly, major environmental problems could be on the horizon i f these large quantities of VOCs continue to be used and released. For this reason, new, environmentally sound manufacturing processes are being investigated. One of the possibilities is solvent-free reaction conditions, but this is not always feasible, especially in polymerizations, where the judicial selection of a solvent can improve reaction conversion significantly. This leads to the investigation of new solvents that can be used successfully in reactions, but have a smaller impact on the environment. Some of the solvents now being studied as replacements to VOCs include water, supercritical fluids such as carbon dioxide, and ionic liquids (6-8). We believe ionic liquids are one of the most promising of these new alternate solvents and have focused our research efforts on determining the feasibility of using ionic liquids as solvents for free radical addition polymerizations. To effectively replace VOCs with RTILs, there are several issues that must be addressed: solubility, solvent recovery and recycling, polymerization kinetics, and the structural and mechanical properties of the polymers produced. Due to the unique nature of ionic liquids and the relatively recent revitalization of interest in these substances, there has been little research reported on polymerization in ionic liquids. O f the articles published, the research on polymerizations using ionic liquids has been more concerned with catalyst development rather than studying mechanisms of polymer kinetics or the structures formed. In 1990, Carlin and Wilkes studied the polymerization of ethylene in a chloroaluminate ionic liquid, AlCl -l-ethyl-3-methylimidazolium chloride using Ziegler-Natta catalysts (9). In 1992, Kobryanskii and Arnautov 3



127 observed the electrochemical polymerization of benzene in an aluminum chloride and iV-butylpyridine chloride based ionic liquid (10) and in 1993 they used the same solvent for an oxidative polymerization (11). In 1994, Goldenberg and Osteryoung (12) also synthesized polyphenylene by electropolymerization in l-ethyl-3-methylimidazolium ionic liquid. Since these investigations, there have been no reports of subsequent research, or any record of polymerizations in non-chloroaluminate ionic liquids outside of the recent work of the groups of Mays and Haddleton, included in this monograph. Ionic liquids have several characteristics that make them excellent candidates for use as solvents in polymerizations. They are highly solvating and have been shown to dissolve both organic and inorganic compounds. They are highly thermally stable, which is extremely important due to the exothermic nature of most free radical polymerization reactions. Ionic liquids also are believed to support long free radical lifetimes(13), suggesting higher conversions would be obtained in polymerizations carried out in ionic liquids versus those carried out in VOCs. From an environmental point of view, there are also many reasons ionic liquids are preferred over VOCs. First and foremost is that they are nonvolatile and nonflammable, eliminating two of the primary environmental concerns with VOCs. Secondly, by eliminating the use of VOCs, the costly and potentially hazardous transportation and disposal of VOCs trapped by effluent scrubbers would become unnecessary. A third advantage of ionic liquids is that they are recyclable, which results in significantly less waste generated by polymer production. Free radical polymerizations follow a three-step mechanism (initiation, propagation, and termination) to rapidly produce high molecular weight polymers. Solution polymerization provides several advantages for these reactions, as the solvent can help to: - overcome diffusion limitations by creating a more fluent environment which leads to higher conversion and higher polymer molecular weight, - control viscosity allowing easier mixing, and - dissipate heat in the reactor by absorbing heat released during the polymerization reaction. There are disadvantages to consider with solution polymerizations as well, including: - contamination of the polymer by residual solvent, and - reduction in conversion due to chain transfer.


128


When V O C s are used, residual solvent can be removed by evaporation, but this is not possible with RTILs since they are nonvolatile. A n ideal way to remove polymer from solvent would be simple décantation if the polymer and solvent were insoluble. Other possible means of separation include using supercritical carbon dioxide, diethyl ether, or using a water soluble R T E . and extracting it (with water) after the reaction is complete. Chain transfer may also impact polymerization when using a solvent, as the solvent itself may absorb free radicals, resulting in polymers with lower average molecular weights and inferior physical properties. Chain transfer is dependent on the solvent, monomer, and initiator being used (2):

X,

2R„

·

'[M]

'[M]

where X is the number average degree of polymerization, Rj is the rate of initiation, R is the rate of propagation, C values represent chain transfer coefficients for monomer, solvent and initiator, and [M], [S] and [I] are concentrations of each species in the polymerization solution. As can be seen in equation (1), chain transfer affects the chain length, with the solvent (due to its relatively high concentration) often being the main barrier to reaching high molecular weights. n

p

Experimental Three polymer systems were selected, as shown in Table I. Polystyrene, PS, and

Table I. Polymerization Systems Studied Monomer Cross Linking Agent Solvent styrene divinyl benzene bmim PF " styrene divinyl benzene ethyl benzene MMA EGDMA bmim PF MMA EGDMA methanol HEMA EGDMA bmim Cr HEMA EGDMA ethanol l-butyl-3-methylimidazolium hexafluorophosphate ethylene glycol dimethacrylate l-butyl-3-methylimidazolium chloride a

+

6

b

a

+

6

b

b

C

+

b

a

b

c


129


poly(methyl methacrylate), P M M A , were chosen because they are two of the most widely-used commodity polymers, and as such their synthesis and properties have been well documented. Poly(2-hydroxyethyl methacrylate), P H E M A , was also selected because it is a material used in manufacturing biomaterials such as contact lenses, and being hydrophilic has markedly different solubility characteristics from PS and P M M A . The solubilities of monomers and polymers were tested with the solvents at 55 °C at 50 vol % monomer, except for styrene, which was tested at 40 vol % monomer. Table Π shows the solubilities of the PS, P M M A and P H E M A in the RTILs tested.

Table IL Monomer and Polymer Solubilities in Ionic Liquids Species bmirn PF ~ bmim* CT styrene + MMA + HEMA + + AIBN + + PS PMMA PHEMA + Note: (-) indicates that two separate liquid phases were present at the conditions tested. (+) indicates that the material was completely soluble at the experimental conditions, although upper solubility limits have not been established. 6

Crosslinked P H E M A and P M M A were formed by free radical solution polymerization using 50 vol % of either a V O C or an ionic liquid and one mole percent crosslinking agent. The solution was purged with nitrogen to displace any dissolved oxygen, a free radical scavenger. One weight percent 2,2*azobisisobutyronitrile (AIBN), a thermal initiator, was added to the solution, which was then injected between two glass plates to ensure uniform sample thickness. The reactions were carried out at 55 °C for twenty hours to ensure the reaction had gone to completion. Polymer samples were removed from glass plates and cut into samples of uniform size. Because the ionic liquids were trapped in the polymer mesh, a separation technique was needed to recover the polymer. Solvent extraction was accomplished through rinsing the P M M A samples in an 80% ethanol/ 20% water solution and the P H E M A samples in water, each for four days. After rinsing, the samples were dried in a vacuum oven at 40 °C to constant weight. Upon drying, various physical properties of the samples were tested.



130 To ensure that polymers formed in ionic liquids have mechanical properties that are either comparable or superior to those formed in traditional solvents, the tensile moduli and the glass transition temperatures of the polymers were determined experimentally. The tensile modulus, G , is a measure of the ability of a polymer to resist deformation when subjected to stress. A high tensile modulus is desirable in polymers that will be used as structural materials. This is important when considering the commercial applicability of polymers formed with ionic liquids, which must be as resilient as those formed with V O C s . The tensile modulus was measured with an Instron automated materials testing apparatus (Model 4465, Canton, M A ) . The samples were stretched by applying a strain in the axial direction while measuring the force required to maintain a constant 10 mm/min strain rate. The stress-strain data were essentially the same for P H E M A samples formed in bmim CI" and ethanol, as long as the solvent was extracted after polymerization (note the identical curves in Figure 1). If the RTIL remained in the polymeric structure, the mechanical behavior of the polymer was compromised significantly, as the stresses decreased by three orders of magnitude for P H E M A with 50 vol % bmim CI" remaining inside the network. +

+

6

0 I 0

.

.

1

0.5

1

1.5

Strain, ε

Figure 1. Stress-strain diagram for PHEMA samples with a constant strain rate of 10mm/min. (m) represents PHEMA containing unextracted bmim* CY (A) represents PHEMA made in bmim Cl~, after the solvent is extracted, and ( ·) represents PHEMA made in ethanol, after extraction of the ethanol +


131 Another important measure of a polymer's integrity is the glass transition temperature (T ). At temperatures above T the amorphous regions have translational and rotational energy, allowing the chains to rotate, creating a polymer that is pliable and soft. For ionic liquids to have viable industrial applications the T of polymers formed should be the same or higher as those produced using VOCs. The glass transition temperatures of the samples were measured with a differential scanning calorimeter, D S C (TA Instruments, Model 2960, New Castle, DE). The samples were heated from ambient temperature across the T at a ramp rate of 10 °C/min. The glass transitions were determined by the change in heat capacity as the temperature is ramped, denoted by the inflection point in Figure 2. g

g

g


g

Figure 2. Differential scanning calorimetry thermogram of a bulk PMMA sample. The glass transition temperature is marked at the center of the slope change.


132

Results/Discussion


The tensile moduli and glass transition temperatures were measured for various samples of P M M A and P H E M A (the testing of polystyrene is ongoing). In addition to using a V O C solvent and an ionic liquid solvent, samples made without a solvent (bulk polymerizations) were also tested (Table III).

Table III. Glass Transitions and Tensile Moduli of Polymers formed by Bulk and Solution Polymerizations Tensile Glass Modulus, Transition, T Polymer G (MPa) Solvent Extracted? (°Q PMMA none (bulk) 65 525 N/A PMMA 0.630 bmim PF "

Effect of Room-Temperature Ionic Liquids as Replacements for

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