Polymerization reactions
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outline
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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 polymerization
(free radical initiation)
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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
C
O*
+
C
O
C*
O
O
styrene
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polymer active center
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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
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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,
or could include 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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A photopolymerization case study
POLY(METHYL VINYL ETHER)
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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).
PHOTOINITIATOR DECAY
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Absorbance change during irradiation
specific 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 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).
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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/trimethyl silyl iodide, ZnI2
activator
MVE POLYMERIZATION RATES VS. T
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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.
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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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