Pearce, Eli M., Carl E. Wright, Binoy K. Bordoloi. Laboratory Experiments in Polymer Synthesis and Characterization. Educational Modules for Materials Science and Engineering Project, 1982 pp 83-106.
Cationic Vinyl Polymerization
The cationic polymerization of styrene, initiated by SnCl4/H2O is used to illustrate the effect of temperature on reaction rate and molecular weight and also the effect of Friedel-Crafts alkylation reactions on the molecular weight. Cationic polymerizations are initiated by electrophilic agents such as the halohydric acids (HCl, HBr, etc.), H2SO4, HClO4, etc. Lewis acids, which are electron acceptors by definition, and compounds capable of generating carbonium ions can also initiate polymerization. Examples of Lewis acids are AlCl3, SnCl4, BF3, TiCl4, AgClO4, and I2. Lewis acid initiators required a co-initiator such as H2O or an organic halogen compound. Initiation by Lewis acids either requires or proceeds faster in the presence of either a proton donor (protogen) such as water, alcohol, and organic acids or a cation donor (cationogen) such as t-butyl chloride or triphenylmethyl fluoride. Actually, the protogen or cationogen is referred to as the initiator while the Lewis acid is the co-initiator, since the protogen or cationogen ultimately supplies the proton or cation which adds to monomer that initiates polymerization. The propagation reaction in the styrene/BF3/H2O system is through repeated addition of monomer. Termination processes in these systems occur by transfer. For example, transfer of a proton to counterion, monomer, polymer, solvent, or some other added reagent can terminate the reaction. Transfer to counterion in the styrene/ BF3/H2O system is illustrated by This regenerates the original initiating species, which can then reinitiate polymerization as an active catalyst. An interesting phenomenon in the cationic system is the overall rate of polymerization and its relationship to temperature. Cationic polymerizations are normally conducted at low temperatures, and tend to go explosively fast in some cases. In most reactions, it is common for the rate of the reaction to increase with an increase in temperature. However, this is not the case in all cationic polymerizations. Considering a kinetic treatment of the polymerization of styrene/ BF3/H2O the overall rate of polymerization is given by
Thus, the rate is directly proportional to a combination of rate and equilibrium constants. The rate constant of a reaction is related to the activation energy for the process by the Arrhenius equation, where A is a constant called the pre-exponential factor, Ea, is the activation energy, R is the gas constant, and T is the absolute temperature. Using Ea(i), Ea(p), and Ea(t) to denote the energies of activation for initiation, propagation and termination respectively. For a overall rate constant for the polymerization, Since the Ea's and k's have a logarithmic relationship, the activation energy correspondingly would be If Ea(t) is greater than Ea(i) + Ea(p), the overall activation energy for polymerization would be negative and the overall rate would increase with decreasing temperature. This relationship of the energies of activation is encountered in many cases, since in initiation and propagation a dipolar species approaches a neutral molecule, a low energy process, while for termination an ion a pair is required to break apart and
rearrange, a high energy process. Furthermore, since the number average degree of polymerization, Xn, can be related to the rate constants as the Xn and consequently the molecular weight of the sample would increase with decreasing temperature if
Ea(t) > Ea(p).
Solvent effects on the counterion can also affect the reaction. Large increase in the rate and degree of polymerization are generally observed as the solvating power of the reaction medium increases. The effect of solvent is generally considered to manifest itself in two ways. The free ion concentration increases with increased solvating power, and Rp increases since the free ion propagates faster than the ion pair. Also, increasing the solvating power of the reaction medium should change the nature of the ion pair from the intimate ion pair to the solvent-separated ion pair. Objective
Introduction
Experimental
First Week
Second week
Calculations
Post Test
References
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