Polymerization reactions chapter 4. Fall 20111

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 20112

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 20113

classifications Chain Polymerization (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 20114

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 20115

Chain vs. step ChainStep Only species with active centers add monomer units Any two potentially reactive end groups can react Monomer concentration decreases steadily Monomer depletion occurs very rapidly High molecular weight polymer forms at once Polymer molecular weight increases slowly with time The concentration of reacting chains is usually low compared to the non-reacting monomer and polymer Any size species can react with another, and many chains are reacting at one time chapter 4. Fall 20116

Typical vinyl monomers No homopolymerizationTypical homopolymerization Rapid homopolymerization A- methyl styreneethyleneHydroxy methyl vinyl ketone Allyl alcoholpropyleneVinylidene cyanide stilbeneisobutyleneAcrylic acid dichloroethylenebutadieneAcrylic anhydride Maleic acidVinyl chlorideMethylene malonate Vinyl esters, ethersnitroethylene chapter 4. Fall See Table 4.3 on methods of manufacturing for vinyl polymers

chapter 4. Fall 20118

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 20119

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

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

Propagation The active center adds monomer, transfer the radical to the new unit, and continues. chapter 4. Fall

Termination chapter 4. Fall

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

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

chapter 4. Fall

chapter 4. Fall

chapter 4. Fall

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

chapter 4. Fall

chapter 4. Fall

chapter 4. Fall

IN-CLASS EXAMPLE chapter 4. Fall

TeamAnother reaction mechanism 7Inhibitor added Fantastic 4Chain transfer agent Mountain goats Plastics anonymousSecond initiator Poly-cats polymaniacsThermal initiation Team alpha chapter 4. Fall

POLY(METHYL VINYL ETHER) A photopolymerization case study chapter 4. Fall

chapter 4. Fall

PHOTOINITIATOR DECAY Objective: use a typical study of photoinitiator decomposition to estimate k d, 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, (2009). Copy on course webpage. System: triethylene glycol divinyl ether; diphenyl iodonium 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). chapter 4. Fall

Absorbance change during irradiation specific wavelengths linked to fct. Groups (Fig. 4) 2 systems chapter 4. Fall

PI rate constants photoinitiatorKd, s -1 AO CPTXO Polymerization conditions: 20 C; TEGDVE – triethylene glycol divinyl ether; 60 kJ/mol – heat of polymerization; chapter 4. Fall

chapter 4. Fall

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

MVE POLYMERIZATION RATES VS. T Objective: use a typical study of MVE polymerization vs. T to to estimate E a, 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/trimethyl silyl iodide, ZnI 2 activator chapter 4. Fall

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 k p. These are plotted as k p vs. 1/T, and the slope is related to the activation energy. chapter 4. Fall

chapter 4. Fall

chapter 4. Fall

Polymerization rates Polymer Handbook: kp 2 /kt, Chen et al.: 1.5 to 2 hours, 30 C, palladium complex Sakaguchi et al.: 30 min, -78 C chapter 4. Fall

chapter 4. Fall