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Polymerization reactions. outline. Introduction Classifications Chain Polymerization (free radical initiation) Reaction Mechanism Kinetic Rate Expressions Definition of a Rate Equation Rate Expressions for Styrene Polymerization QSSA (Quasi-steady state assumption).
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Polymerization reactions chapter 4. Fall 2011
outline • Introduction • Classifications • Chain Polymerization (free radical initiation) • Reaction Mechanism • Kinetic Rate Expressions • Definition of a Rate Equation • Rate Expressions for Styrene Polymerization • QSSA (Quasi-steady state assumption) chapter 4. Fall 2011
we start an engineering discussion of polymers by addressing how they are made • beyond the selection of the monomer building blocks, the polymerization process is most important to properties: it sets the configuration • you should be able to model polymerizations and determine the effects of changing monomers, temperature, pressure and other variables chapter 4. Fall 2011
classifications ChainPolymerization (example: polystyrene) • monomer is added to the active center • high polymer is made in small quantities continuously • monomer concentration is decreased slowly • high molecular weight polymers are made when the concentration of active centers is low chapter 4. Fall 2011
Step Polymerization (example: nylon 6,6) • · end groups of the monomers react • · monomer is depleted rapidly • · high molecular weight polymer is made slowly chapter 4. Fall 2011
Chain vs. step chapter 4. Fall 2011
Typical vinyl monomers See Table 4.3 on methods of manufacturing for vinyl polymers chapter 4. Fall 2011
Chain polymerization(free radical initiation) · monomers have double bonds · typical monomers shown in Table 4.2 · bulk: only monomer present · emulsion: latex particles < 1 micron · suspension: particles between 50 to 500 microns · solution: monomer is dissolved in a second liquid · particle morphology has commercial value chapter 4. Fall 2011
Reaction mechanism • We will learn a generic reaction mechanism which can be modified to describe many chain polymerization. Each step can be described by a reaction rate expression. The overall reaction rate model gives us the change in the monomer concentration with time, which can be used for process control. • . chapter 4. Fall 2011
Initiation (formation of free radicals) [initiators, catalysts] Benzoyl peroxide The radical can react with a double bond, linking the initiator fragment with the monomer. The reactive site moves to the end of the chain chapter 4. Fall 2011
Propagation The active center adds monomer, transfer the radical to the new unit, and continues. chapter 4. Fall 2011
Termination chapter 4. Fall 2011
Reaction Kinetics • The minimum set of reactions which describe a free radical polymerization are: • initiation, • propagation, and • termination. • More complex systems could include: • multiple initiation, propagation, or termination steps; • side reactions such as: • chain branching, • monomer or polymer degradation, • chain transfer, etc. chapter 4. Fall 2011
We will write general equations for simple systems, and you should be able to add as much complexity as you want for a specific system. chapter 4. Fall 2011
We have generated four rate equations which describe a simple polymerization. The one which relates directly to monomer loss is the propagation reaction. We can solve this equation if we have an expression for M*, the free radical chain end concentration. We apply the quasi-steady state assumption in order to approximate M*. QSSA (Quasi-steady state assumption) If we want long chains, we need to have only a few of them reacting at one time. Therefore, we want M* to be small. We design most free radical polymerizations so that M* is much smaller than M. We make the approximation that the change in M* is nearly zero compared to the change in M. chapter 4. Fall 2011
In-class example chapter 4. Fall 2011
A photopolymerization case study Poly(methyl vinyl ether) chapter 4. Fall 2011
Objective: use a typical study of photoinitiator decomposition to estimate kd, the dissociation rate constant. Approach: use a system linked to vinyl ether polymerizations (solvents, monomers, etc. all affect the performance of catalysts, initiators and ionic catalysts) Reference: Cook, et al., Photopolymerization of vinyl ether networks using an iodonium initiator – the role of phototsensitizers, J. Polym. Sci., Part A: Polym. Chem., 47, 5474-87 (2009). Copy on course webpage. System: triethylene glycol divinyl ether; diphenyliodonium salt, one of three photosensitizers (CPTXO, AO, CQ – not consumed). Note: photosensitizer allows the use of the visible spectrum range rather than UV (which would require quartz windows, etc). Photoinitiator decay chapter 4. Fall 2011
Absorbance change during irradiationspecific wavelengths linked to fct. Groups (Fig. 4) 2 systems chapter 4. Fall 2011
PI rate constants Polymerization conditions: 20 C; TEGDVE – triethylene glycol divinyl ether; 60 kJ/mol – heat of polymerization; chapter 4. Fall 2011
Goofy stuff Degradation rate of CPTXO does not follow exponential decay over long times. As suggested on p. 5484, PI process is in competition with a side reaction that quenches CPTXO or with a process that consumes cations (perhaps an impurity). chapter 4. Fall 2011
Objective: use a typical study of MVE polymerization vs. T to to estimate Ea, the activation energy of the overall reaction process. This can be used to scale the polymerization rate vs. T for process design purposes Approach: use a system linked to vinyl ether polymerizations (solvents, monomers, etc. all affect the performance of catalysts, initiators and ionic catalysts) Reference: MVE in toluene; diethoxyethane/trimethylsilyl iodide, ZnI2 activator MVE polymerization rates vs. T chapter 4. Fall 2011
Semi-batch analysis Batch analysis of the rate can be done at the end of the monomer feed phase Each curve is modeled by an ionic polymerization eqn., yielding kp. These are plotted as kp vs. 1/T, and the slope is related to the activation energy. chapter 4. Fall 2011
Polymerization rates • Polymer Handbook: kp2/kt, • Chen et al.: 1.5 to 2 hours, 30 C, palladium complex • Sakaguchi et al.: 30 min, -78 C chapter 4. Fall 2011
