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-Free radical polymerization is a type of chain growth polymerization. In a free radical polymerization reaction, a polymer is formed from successive addition of monomers to a free radical at the end of the polymer chain. It is a key synthesis route in industry for making various types of polymers and material composites such as polystyrene and poly (methyl methacrylate). The main drawback of the free radical polymerization technique is the poor control of both molecular weight and molecular weight distribution. The molecular weight distribution of a polymer sample is also known as the poly-dispersity index (PDI). [ref] It indicates the distribution of individual molecular masses in a batch of polymers. The PDI from polymerization is denoted as:
PDI = w/n (1)
where Mw is the weight average molecular weight and Mn is the number average molecular weight. The PDI has a value equal to or greater than 1.0. A value of 1.0 would be found if all of the polymer chains approach uniform chain length. This case is found only in natural polymers such as proteins. Such molecules are synthesized via complex biological machinery. In contrast, free radical polymerization is a far less controlled process. This process is explained in some detail below.
Free radical polymerization entails three main steps: initiation, propagation and termination. The rate constant for each step is on the order of magnitude of 10-4-10-6 s-1, 102-104 L·mol-1·s-1 and 106-108 L·mol-1·s-1 respectively. [ref] Among these three steps, initiation has the slowest rate making it the rate determining step. To maximize the speed of the first step, the choice of free radical initiators is important.
Free radical initiators are molecules with one or more bonds characterized by small dissociation energies ranging from 100 to 200 kJ/mol. [ref] These relatively weak bonds are broken by homolysis during heating or other application of energy to form free radical fragments that can consequently initiate polymerization. Therefore, from a mechanistic point of view, the initiation step actually consists of two consecutive steps: the production of primary radicals created from initiators followed by the transfer of radicals from the initiator molecules to the monomer unit present. This process can be described as follows [ref]:
where kd is on the order of magnitude of 10-4-10-6 s-1 . The value of ki is one-eight orders of magnitude larger than the rate constant of propagation of a long active chain. [ref] This enhancement soon vanishes after more than three units of monomer are attached to the radical fragment of the initiator. Therefore, the very first step of initiation (2) is the actual rate determining step.
Widely utilized initiators include organic peroxides, azo compounds and redox systems. The most commonly utilized initiator for each category is benzoyl peroxide (BPO), azoisobutylnitrile (AIBN) and hydrogen peroxide in the presence of iron (II). Among these initiators, AIBN is organic soluble and can be used over a wide range of temperature from 27 °C to 177 °C. [ref]
An important factor that determines the activity of an initiator is half life (3). The activity of initiator decreases as the half life of initiator increases. The relationship between dissociation rate constant of initiator and temperature is described in equation (4), where Ad is the frequency factor, Ed is the activation energy and R is the universal gas constant. According to this equation, dissociation rate constant of initiator increases as temperature increases which leads to an increase in the activity of initiator. On the contrary, the half life of initiator decreases as temperature increases. Figure 1 is an example that demonstrates these trends among the three factors, half life, dissociation rate constant and the temperature of an Azo initiator. [ref]
Figure 1.Half life (h) and Dissociation rate constant (/s) of an azo intiator vs. Temperature (â„ƒ)
During polymerization, a polymer spends most of its time increasing its chain length, or propagating. Once a chain is initiated, it will propagate until there is no more monomer or until termination occurs. The propagation might contain from a few to thousands of steps depending on several factors such as the reactivity of the radicals, temperature and solvent.
There are several different mechanisms for termination. These include recombination and disproportionation. The theoretical PDI values for these two types of termination are 1.5 and 2.0 respectively. Recombination is a mode of termination when two radical chains end up coupling together to form one long chain. Disproportionation is a kind of termination when a hydrogen atom on one chain is abstracted by the radical on another one, producing a polymer with a terminal unsaturated group and another one with a terminal saturated group. Termination can also happen when radical chains ends react with impurities or inhibitors. For example, oxygen is one of the most common inhibitors. The growing chain will react with molecular oxygen and produce an oxygen radical which is much less reactive compared to the growing chain.
Another important process governing molecular weight and molecular weight distribution during free radical polymerization is chain transfer. Chain transfer results in the destruction of one radical, but also the creation of another radical. Usually, the newly created radical is not capable of further propagation due to low reactivity. Similar to disproportionation, all chain transfer mechanisms also involve the abstraction of a hydrogen atom. [ref] Chain transfer can happen between the growing polymer chain and solvent, monomer, initiator and other polymer chains. It is one of the main reasons for the increase of PDI values.
A key factor that affects the reaction is the method of the activation of the initiator. Activation is the energetic influence outside that causes polymerization to begin. Different ways of activation can be applied based on the properties of the initiator and monomer system. The most commonly used and studied means of activation are conventional heating, light and irradiation activation.
Conventional heating process
In a conventional heating method, chemical synthesis can be achieved through conductive heating with an external heat source. During this process, heat is driven into the substance by first passing through the walls of the vessel and then reaching the solvent and reactants.
The conventional heating process is a relatively slow and inefficient method for transferring energy into the system because it depends on the thermal conductivity of the various materials that must be penetrated. Due to the way that heat is transferred, it results in the temperature of the vessel being higher than that of the reaction mixture inside. It requires longer time for the contents inside the vessel to finally attain a thermal equilibrium with the vessel and reach the temperature expected. In order to cool down or stop the reaction, the heat source must be physically removed and sometimes, cooling methods have to be supplied to reduce the internal bulk temperature.
Microwave heating process: Instant on and Instant off
Different from conventional heating, microwave irradiation provides noncontact, instantaneous and rapid heating. It is widely used in both private households and industrial applications for this purpose. One special property of microwave irradiation is a highly specific heating effect with materials that interact with the specific wavelength of the microwaves. Microwave irradiation of such materials inherently excites dipolar oscillation and induces ionic conduction.
Microwave ovens operate with electromagnetic non-ionizing radiation with frequencies ranging from 0.3 to 300 GHz. The corresponding wavelengths span a range from as long as one meter to as short as one millimeter. [ref] Most commercial microwave systems, including microwave ovens used in kitchens and our laboratory microwave reactors, operate at 2.45 GHz with a corresponding wavelength of 12.24 cm in order to avoid interferences with telecommunication devices. [ref] The corresponding electric fields oscillate at 4.9-109 times per second and consequently subject dipolar species and ionic particles as well as holes and electrons in semiconductors or metals to perpetual cycles of reorientation. This strong agitation leads to a fast noncontact heating that is uniform throughout the radiation chamber. Microwaves move at the speed of light (3.0-108 m/s). The energy in microwave photons (0.037 kcal/mol) is very low relative to the typical energy required to cleave molecular bonds (80-120 kcal/mol); [ref] therefore, microwaves will not affect the structure of an organic molecule.
Dipole rotation and ionic conduction are the two fundamental mechanisms for transferring energy from microwave to the substances being heated. Dipole rotation can be described as an interaction between polar molecules and the rapidly changing electric field of the microwave. The rotational motion of the polar molecules as they try to align themselves with the rapidly changing electric field, results in a transfer of energy, leading to a rapid rise of temperature. The coupling between the molecules and the electric field is dependent on the polarity of the molecules and their ability to align with the electric field. The dipole rotation coupling efficiency can be ultimately determined by many factors; however, any polar species that are present, no matter whether they are solvents or substrates will experience this energy transfer. [ref]
Ionic conduction happens, on the other hand, when free ions or ionic species are present in the substance that is being heated. The electric field generates ionic motion as the molecules try to orient themselves to the rapidly changing field. This also causes the instantaneous superheating previously described. The temperature of the substance also affects ionic conduction: for superparamagnetic materials, as temperature increases (<TB, blocking temperature), the transfer of energy becomes more efficient. [ref]
Based on the mechanisms of microwave irradiation, the heating process can be well controlled by adjusting the power of the microwave generator. Desired temperature and time can be controlled without physically adding or removing heating sources. The heating process can be instantaneously started and stopped by turning on and off the power of microwave. If there is a cooling system in the reactor, the reaction mixture can be cooled rapidly if needed. Thus, compared to the conventional heating process, microwave heating method is more efficient and much faster. Also, irradiation process provides an instant on and off heating method.
In this project, microwave irradiation is applied as an activating and heating method of free radical polymerization in order to better control the molecular weight and molecular weight distribution of polymers. A detailed mechanism is described as follows.
Based on the work of Ruhe's group [ref], a type of azo initiator that has an anchoring group at one end and a cleavable group in the middle of the molecular structure can be synthesized. Covalent bonds can be formed between the initiators and metal oxide particles. (5) If magnetic particles are used, they can be instantly heated up when microwave irradiation is applied. At the same time, the heat can be transferred to the azo initiators bonded to the particles. This would lead to an activation of the initiators and free radical polymerization. (7) After polymerization, polymers bonded to the particles can be collected by being cleaved off from the particles. (8) Due to the instant heating effect, almost all the initiators can be dissociated at the same time to form activated radicals and initiate free radical polymerization. As a result, a homogeneous propagation would then occur which leads to polymers with relatively low molecular weight distribution. Also, since almost all the initiators are activated simultaneously and instantaneously, the rate of initiation can be increased accordingly, as well as the rate of the whole reaction.
Iron (III) oxide nanoparticles (FW=159.69 g/mol, 20-40 nm, >98% gamma phase, ferrimagnetic, S.A. =30-60 m2/g) and Silica nanoparticles (FW=60.09 g/mol, 20-40 nm, S.A. =180 m2/g) were dried overnight under 100 millitorr. Toluene was distilled under nitrogen atmosphere from sodium using benzophenone as an indicator. Methylene chloride was distilled under nitrogen atmosphere from P2O5. Styrene was passed through a neutral alumina column and distilled under vacuum and stored under nitrogen at -20 °C. All the other solvents and chemicals were used as received. A discover focusedTM microwave system (single mode, self-tuning, magnetron frequency =2450 MHz, maximum power output =300 W, temperature control range =10-250 °C, in situ magnetic variable speed of stirring) of CEM corp. was used as the activating and heating source.
NMR spectra were recorded on a 300 Hz Gemini 2300 spectrometer using CDCl3 as the solvent. DRIFT spectra were recorded on FTS 3000 MX spectrometer at a resolution of 2 cm-1. The molar mass and PDI values of polymers were determined by GPC on a Waters Lambda-Max Model 481 using THF as the solvent. The column used for GPC was Jordi-gel DVB 1000 A.
Synthesis of Asymmetric Azo Initiators
62.50 g levulic acid was added to 65 mL deionized (DI) water. Then, 45.41 g NaHCO3 was added to the levulic acid solution gradually. The solution was stirred overnight to let all of the released carbon dioxide escape and the pH value reach 7 which was tested with pH paper. In a 3 L three-necked round bottom flask, 70.05 g hydrazine sulfate and 70.07 g KCN in 950 mL DI water were added and heated to 50 °C. After approximately 40 minutes, all of the solids dissolved. Then a mixture of neutralized levulic acid (filtered if solid NaHCO3 remained) and 40 mL of acetone was added dropwise to the three-necked flask. This process takes more than 30 min. The solution was stirred at 50 °C for 3 h and then cooled in ice bath. Then the solution was acidified with HCl (aq, 2N) to pH=4 (from pH=8) which was tested with pH meter. At this temperature (0 °C), 50 mL Br2 was added dropwise until the solution remained dark red. The solution needed to be stirred strongly during this process. The solution was stirred for 30 min. Sodium bisulfite was then added to destroy the excess of bromine until the red color disappeared. The solution was then stirred overnight to let the entire released HCN escape and kept at room temperature. After this, the solution was filtered. The precipitate was washed twice with DI water and suspended in about 35 mL NaOH (aq, 1N), stirred for 30 min and then filtered. The insoluble part consisted of AIBN (which was kept for future use). After filtration, the solution was acidified with (conc. HCl) and a white precipitate was formed (Product 1). The solid was filtered and dried; 8.25 g of product 1 was obtained. The filtrate was extracted twice with CH2Cl2, washed with water and dried over anhydrous sodium sulfate. The solvent was evaporated and the product was dried under high vacuum. 29.45 g of yellow oil was obtained. This yellow oil was recrystallized as follows. A solution of methanol/water (1/5, v/v) and a water bath (50-55 °C) were prepared. The solution was gradually added to the oil and at the same time the oil was warmed in the water bath. The solution was slowly added to make the oil totally dissolve until the resulting solution was clear. The solution was cooled down quickly by putting the flask into a dry ice/acetone bath. The oil layer remained at the bottom of the flask. The flask was left at room temperature for three days, and 4.20 g product 1 was obtained. The recrystallization can take longer if there are more impurities (when the oil is dark yellow). Combined yield of product 1 was 12.45 g, 10.41%.
A 250 mL flask containing 12.11 g of PCl5 in 15 mL CH2Cl2 was placed into an ice bath under nitrogen atmosphere. 3.00 g of Product 1 in 15 mL CH2Cl2 was added dropwise. The mixture was allowed to warm to room temperature and stirred overnight. The excess of PCl5 was filtered off (under N2). The remaining solution was concentrated using a rotatory evaporator and more PCl5 precipitated. This was removed by filtration and the solution was concentrated further until no more PCl5 precipitated. A few drops of CH2Cl2 were added to the concentrated solution and this solution was added dropwise to hexane cooled in dry ice/acetone bath. A white precipitate was obtained, filtered and dried to give 2.78 g of Product 2 (85% yield).
A 250 mL flask containing 0.948 mL of allyl alcohol and 2.26 mL of pyridine in 17.39 mL CH2Cl2 was placed into an ice bath and placed under nitrogen atmosphere. A solution of 2.78 g of Product 2 in 17 mL CH2Cl2 was added dropwise. The mixture was allowed to warm to room temperature and stirred overnight. The solution was washed twice with H2SO4 (aq, 2N), NaHCO3 (aq) and water respectively. The organic layer was dried over Na2SO4 and the solvent was removed using a rotatory evaporator. The resulting pale yellow oil was dissolved in a small amount of CH2Cl2 and poured into 100 mL hexane cooled in dry ice/acetone bath. 1.72 g of the ester was separated as an impure, white to yellowish solid (57% yield).
A solution of 1.72 mg of hexachloroplatinic acid in 0.29 mL of dimethyl ether/ethanol (1/1 v/v) was added to a suspension of 1.72 g of the ester and 17.2 mL of monochlorosilane or trichlorosilane. The mixture was heated to reflux for 3 h. All the solid was dissolved indicating the completion of the reaction. The excess of silane was removed under high vacuum with a liquid nitrogen cooled trap. The product was dried under high vacuum. A small amount of CH2Cl2 was added to the product and the solution was filtered over anhydrous sodium sulfate under flowing N2 to remove the residual platinum catalyst. The product was dried under high vacuum to obtain 1.85 g of pale green oil which gave a 79% yield.
Deposition of the Azo initiators on nanoparticles
Under a nitrogen atmosphere, a solution of 1.5 g of the mono- or tri-azochlorosilanes in 50 mL of toluene was added to a suspension of 3 g silica or 12 g iron (III) oxide nanoparticless in 100 mL of toluene. A total of 3 mL of pyridine was added and the mixture was stirred for 12 h. Then the modified nanoparticles were centrifuged and washed with toluene, ethanol, acidified (HCl) ehtanal/water (1/1 v/v, pH 3), ethanol/water (1/1 v/v), ethanol, and diethyl ether respectively. The remaining solids were dried overnight at 100 millitorr.
Free radical polymerization of styrene
In a 10 mL microwave tube, 2 mL/4 mL styrene/toluene mixture was added to 720mg iron (III) oxide or 120mg silica nanoparticles. The mixture was degassed in vacuum through repeated freeze-pump-thaw cycles. And then the microwave tube was placed in the chamber of microwave. Different conditions were set up to the microwave system. Detailed explanation of conditions used to perform reactions was described later in the result section. Since the Azo initiator has similar structure as AIBN, the dissociation mechanism of it is basically the same as AIBN. (14) Heat was provided by microwave irradiation. When the temperature is high enough for the activation, all the initiators on the particles can be broken down to form two types of active radicals, radical 1, radical 2 and nitrogen gas. Radical 1 is bonded to Iron (III) oxide nanoparticles and radical 2 is randomly dissolved in the solution. These two types of radicals can then initialize free radical polymerization of styrene.
Radical 1 can react with styrene monomers to form polymer brushes bonded to the particles. (15a and 15b) At the same time, radical 2 can initialize polymerization of styrene that can randomly dissolve in the solution when the temperature of the bulk solution is high enough for propagation. (16) As a result, there would be two types of polymers after the reaction, nanoparticle bonded and nonbonded polymers. The particles and the solution can be separated by centrifuge. The supernatant contains nonbonded polymers. The particles were washed with toluene for several times and all the supernatant was collected. If a large amount of polymers were produced, nonbonded polymers can be precipitated out and collected by adding methanol to the solution. If the amount of polymers produced was really small (less than 100 mg), the solution was placed under high vacuum to evaporate all the solvent. The insoluble part contains nonbonded polymers.
After separating the nanoparticles from the nonbonded polymer solution, the polymer modified particles were suspended in 24 mL of toluene and a total of 2.4 mL methanol and 12 mg of p-TsOH were added. The mixture was heated to reflux overnight. The products were isolated by the same procedures as described above for the nonbonded polymers.