The Synthesis Of Hydrogel By Atom Transfer Radical Copolymerization And The Polymerization Kinetics

A hydrogel is a crosslinked network of hydrophilic polymer that is water insoluble but can swell in polar media such as water and aqueous electrolyte solutions. Since hydrogels have been successfully applied in many fields, particularly in medicine and pharmacy, the synthesis of hydrogels has become an intriguing research area. Among many methods used to prepare hydrogels, the free radical copolymerization of vinyl monomers, which have hydrophilic units in its backbone and / or hydrophilic side groups with a small amount of divinyl comonomer, is an effective route because of its versatility for monomer type and mild reaction conditions as well as tolerance to water. Although free radical copolymerization has many advantages over other methods, it offer little control over the network structure and yields a random copolymerization of monomer and crosslinking agent, leading to microgel formation during polymerization and a heterogeneous network structure. Since many applications of hydrogels require that the hydrogels must have a homogeneous network structure, recently many researchers have focused on the development of "living" and / or controlled free radical copolymerization. In the present work, a new type of controlled radical polymerization called atom transfer radical polymerization (ATRP), which has been developed by Matyjaszewiski et al recently, was used to synthesize the hrdrogel of poly [2-(N,N-dimethylamino) ethyl methacrylate-co-ethylene glycol dimethacrylate] by the copolymerization of 2-(N,N-dimethylamino) ethyl methacrylate (DMAEMA) with ethylene glycoldimethacrylate (EGDMA). The hydrogel obtained here has a homogeneous network structure. The mechanism of the atom transfer radical copolymerization of DMAEMA and EGDMA was proposed, and the results of kinetic modeling for the atom transfer radical copolymerization of DMAEMA and EGDMA was reported in this study. The main contents and results of this work are:Using copper bromide as catalyst, ethyl 2-bromoisobutyrate (EBIB) as initiator, 1,1,4,7,10,10-hexamethyl triethylenetetramine (HMTETA) as ligand, tetrahydrofuran (THF) as solvent, the atom transfer radical copolymerization of DMAEMA and EGDMA was studied at a temperature of 60 °C. The variations of the conversion of total monomers with time were measured at six initial EGDMA mole fractions of 0.01, 0.02, 0.05, 0.1, 0.2, and 0.5, respectively. The experimental results showed that the conversion increases smoothly with time and there is no "gel effect" appearing in the curves of conversion versus time. This phenomenon indicated that in the atom transfer radical copolymerization system of DMAEMA and EGDMA, the growing polymer radical growths slowly and every polymer chain has an enough time to be fully relaxed. Therefore in this system the formation of microgel was avoided, leading to the formation of a hydrogel with homogeneous network structure.The data of the variations of the conversion of total monomers with time were used to do kinetic study for the atom transfer radical copolymerization of DMAEMA and EGDMA, by plotting ln([M]0 /[m]) versus time. It was found that at low initial EGDMA concentration, such as f20=0.01, 0.02, 0.05, the ln([M]0/[M]) ~t curves are linear, indicating that at these conditions the apparent kinetic equation for the atom transfer radical copolymerization of DMAEMA and EGDMA is ln([M]0 /[m]) = kappt and the polymerizations follow the first-order kinetics. This phenomenon showed that at low initial EGDMA concentration condition there is neither diffusion-controlled propagation of growing polymer chain nor a significant increase in growing polymer termination. At high initial EGDMA concentration, such as these above 10% mole, the ln([M]0/[m]) -t curves depart from linear line. It indicated that at this condition there is some diffusion-controlled effect in the atom transfer radical copolymerizationof DMAEMA and EGDMA.The relation between gel point and the mole fraction of EGDMAT for the atom transfer radical copolymerization of DMAEMA and EGDMA was investigated and the result was compared with the Flory theory. It was found that the gel point in the atom transfer radical copolymerization of DMAEMA and EGDMA obeys the equation,xg = 0.035f-0.85 20The comparison of this equation with Flory's equation,Xg = 0.005f-1 20indicated that the atom transfer radical copolymerization of DMAEMA and EGDMA can produces hydrogel with homogeneous network structure.We studied the crosslinking density of DMAEMA/EGDMA atom transfer radical copolymerization. It was found that the relationship between the sol fraction and the conversion of monomer isand the relationship between the crosslinking density and the mole fraction of EGDMA isp=0.173f20Swelling experiments were conducted to characterize the hydrogels. It was found that with the increase of initial concentration of EGDMA and /or the conversion of total monomers, the swelling ratios of the hydrogels in water decrease. This indicated that increasing the initial concentration of EGDMA and /or the conversion of total monomers yields a tight polymer network and results in a lower swelling ratio. The sol polymers in the tom transfer radical copolymerization system of DMAEMA and EGDMA were characterized by the size exclusion chromatographic technique. The number-average molecular weight and molecular weight distribution of the solpolymers were determined. It was found that at the condition of lower initial EGDMA concentration, at the early stage of polymerization, the number-average molecular weight of the sol polymer agrees with the values calculated by the expression of [m]0C/[i]0 . This indicated that most polymer at this stage are linear chains. However, he number-average molecular weight of the sol polymer deviated from the predicted values at high conversions because of crosslinking.The kinetic investigation of polymerization is very useful for many purposes. The model that is developed from the kinetic study can quantitatively predict data which agree well with those obtained experimentally under well-controlled conditions. Therefore it can be used in the process simulation and design calculation. Moreover, kinetic investigation is helpful to understand the mechanism of the polymerization. Finally, rate constants, which are difficult to determine experimentally, can be obtained from the kinetic model for the polymerization process. In order to obtain the kinetic rate constants for the atom transfer radical copolymerization of DMAEMA and EGDMA, a kinetic modeling for the ATRP of DMAEMA was conducted firstly in the present study. Based on a polymerization mechanism, which included the atom transfer equilibrium for primary radical, the propagation of growing polymer radical, and the atom transfer equilibrium for the growing polymer radical, a set of kinetic equations was put forward. Polymerization experiments were carried out at 60 °C to measure the variations of the conversion of DMAEMA with time for the ATRP of DMAEMA, using copper bromide as catalyst, ethyl 2-bromoisobutyrate (EBIB) as initiator, 1,1,4,7,10,10-hexamethyl triethylenetetramine (HMTETA) as ligand, tetrahydrofuran (THF) as solvent. The experimental data were used to correlate the kinetic model, yielding the following kinetic data, which have been not reported so far. The rate constants of activation and deactivation in the atom transfer equilibrium for primary radical are 1.0× 104 and 0.04 L·mol-1·s-1, respectively. The rate constant of the propagation of growing polymer radical is 8.50 L·mol·s-1, and the rate constants of activation and deactivation in the atom transfer equilibrium for growing polymer radical are 0.005 and 1.2×105 L·mol·s-1. The atom transfer equilibriumconstant for primary radical K0eq is 2.5 × 105. This value is in the order of magnitude of 107 for the atom transfer equilibrium constant for primary radical. The atomtransfer equilibrium constant for growing polymer radical Keq is 4.2×10-7 . Thisvalue is in the range of 10-7 to 10-8 for the ATRP process. The low value of Keq means that during the polymerization all the growing polymer radical chains have an almost equal opportunity to grow, leading to polymers with narrow molecular weight distribution.In this study, a kinetic model was developed to describe the atom transfer radical copolymerization of DMAEMA and EGDMA. The model was based on a polymerization mechanism, which included the atom transfer equilibrium for primary radical, the propagation of growing polymer radical, the atom transfer equilibrium for the growing polymer radical, and the reaction of growing polymer radical with internal double bonds in the dead polymer. Applying the method of pseudo-kinetic rate constant, this copolymerization mechanism was expressed as: atom transfer equilibrium at initiation stageatom transfer equilibrium at propagation stagereaction of growing polymer radicals with the internal double bounds in the polymerA set of experimental data of the atom transfer radical copolymerization of DMAEMA and EGDMA were used to correlate the kinetic model. The reactivity ratios of DMAEMA, EGDAMA and rate constants, which had not been reportedbefore, were obtained. The reactivity ratios of DMAEMA and EGDAMA are 0.38 and 1.50, respectively. The activation rate constant of growing polymer radical with active center located on EGDMA is 6.10 L · mol-1· s-1, and the reaction rate constant of growing polymer radical with active center located on DMAEMA with internal double bonds is 1.30 L · mol-1· s-1 .The kinetic model was used to calculate the atom transfer radical copolymerization process of DMAEMA and EGDM. The results showed that at low initial concentration of EGDMA the calculations agree well with the measurements.