FRRPP Polymerization Kinetics

V - dimensionless bulk fluid temperature, defined in Eq. 2.2.1) 9 - dimensionless temperature, defined in Eq. (2.2.7) 

A//p - heat of polymerization, J/mol a - activation energy of polymerization, J/mol K k - effective rate coefficient of polymerization, mol/m s ko - pre-exponential factor for effective rate coefficient of polymerization, mol/m s 

This section pertains to smdies made to delineate the kinetics of the FRRPP process involving polystyrene (PS) and poly(methacrylic acid) (PMAA) systems. 

Most of the systematic smdies done on the FRRPP kinetics for the formation of polystyrene is through the use of diethyl ether (or simply ether) as precipitant. Some of the work involving acetone is also presented. 

Using the cloudpoint method described in Section 1.1.2.3, ternary phase diagrams for PS/S/ether systems at various temperatures are presented. Note that the 

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An adequate understanding of the FRRPP process can never be achieved without probing into its underlying concepts, since it is a synergistic combination of thermodynamic, transport, and polymer chain-reaction kinetics. The overall result is an unconventional polymerization and energetic behavior that requires the conceptual and mathematical understanding to link all FRRPP features into a coherent picture. Since its necessary condition is found in a phase behavior of polymer mixtures under equilibrium conditions, it is appropriate that the start of presentation of the technical aspects of the FRRPP process is in its relevant thermodynamic concepts. 

Since the discovery of the FRRPP process, research has been directed toward the study of the demixing polymerization mechanism (reaction kinetics, phase separation, morphology, product characterization, etc.). The FRRPP process relies heavily on the incipience of the LCST of the system. The fact that typical LCSTs of most nonpolar systems are much higher than normal operating temperatures for the free-radical polymerization process can make the FRRPP process infeasible. From a table that lists the LCST of polystyrene (PS) in some common solvents (Caneba, 1992a, b), the following observations and conclusions were drawn ... 

Polymers are macromolecules which are composed of smaller molecules linked by covalent bonds. In terms of the reaction kinetics, polymerizations are traditionally classified into several categories stepwise polymerization, free-radical polymerization, ionic polymerization, ring-open polymerization, and coordination polymerization or polyinsertion. Each polymerization method has a combination of requirements for reaction conditions, and they exhibit certain types of product and process features (Caneba, 1992a, 1992b Odian, 1991). Even though in principle, the FRRPP process can be implemented with a wide variety of polymerization mechanisms, its discovery and immediate implementation has occurred in conjunction with free-radical kinetics. 

When we incorporate termination reactions into the analysis of monomer sequence formation in copolymerization kinetics, we note that in general the development of the copolymer sequence can be prematurely stopped. If one monomer sequence is being formed during propagation and the chain is terminated by disproportionation, then the result is more of a homopolymer than a copolymer. If the chain is terminated by recombination of a similar molecule, then the same type of homopolymer is formed. In fact, termination reactions usually prevent the formation of block copolymers when both monomers are present in the reactor fluid. With the radical trapping mechanism of the FRRPP process that will be discussed in the next chapter, formation of certain block copolymers becomes feasible in statistical-based radical copolymerizations. This is an apparent contradiction in terms, but the FRRPP process has been shown to break new ground in polymerization systems. 

Fig. 3.1.1 Kinetic data on the copolymerization of styrene and acrylic acid via FRRPP and solution polymerization processes (Replotted with permission from Caneba et 2003)...

The above data provide the kinetics, solubility, and emulsification results for three solvents ether, pyridine, and cyclohexane. Note that S polymerization in ether occurs via FRRPP, but AA polymerization in ether occurs via conventional precipitation polymerization (CPP). Pyridine is a solvent for both PS and PAA thus, copolymerization of AA and S in pyridine is via solution polymerization. Finally, AA polymerization in cyclohexane is via CPP and PS formation in cyclohexane is via solution polymerization. 

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