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Polymerization reactions

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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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• 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

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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

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• Step Polymerization (example: nylon 6,6)• · end groups of the monomers react• · monomer is depleted rapidly• · high molecular weight polymer is made

slowly•

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Chain vs. stepChain Step

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

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Typical vinyl monomersNo homopolymerization Typical

homopolymerizationRapid homopolymerization

A- methyl styrene ethylene Hydroxy methyl vinyl ketone

Allyl alcohol propylene Vinylidene cyanide

stilbene isobutylene Acrylic acid

dichloroethylene butadiene Acrylic anhydride

Maleic acid Vinyl chloride Methylene malonate

Vinyl esters, ethers nitroethylene

See Table 4.3 on methods of manufacturing for vinyl polymers

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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

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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.

• • • .

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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

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Propagation

The active center adds monomer, transfer the radical to the new unit, and continues.

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Termination

C* C*+

polymer

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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.

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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.

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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.

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IN-CLASS EXAMPLE

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Team Another reaction mechanism

7 Inhibitor added

Fantastic 4 Chain transfer agent

Mountain goats

Plastics anonymous Second initiator

Poly-cats

polymaniacs Thermal initiation

Team alpha

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POLY(METHYL VINYL ETHER)A photopolymerization case study

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PHOTOINITIATOR DECAY

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; 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).

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Absorbance change during irradiationspecific wavelengths linked to fct. Groups (Fig. 4)

2 systems

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PI rate constants

photoinitiator Kd, s-1

AO 0.0374

CPTXO 0.0209

Polymerization conditions:20 C; TEGDVE – triethylene glycol divinyl ether;60 kJ/mol – heat of polymerization;

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Goofy stuffDegradation 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).

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MVE POLYMERIZATION RATES VS. T

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 purposesApproach: 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, ZnI2 activator

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Semi-batch analysis

Batch analysis of the rate can be done at the end of the monomer feed phaseEach 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.

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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

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