Bulk Polymerization 2015

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Polymerization techniques

Bulk polymerizationSolution polymerizationSuspension polymerization

Emulsion polymerization


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Introduction

The processes for manufacturing macromoleculescan be divided into three different categories as

shown in the scheme below.

polymerization technique 2

Process for manufacturingmacromolecular compounds

Process in solution:

-Aqueous solutions

-Organic solutions

Process in

dispersion:-Suspension

-Emulsion

Process in bulk:

-Liquid state

-Solid state

-Gas phase


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The method of synthesis and the manufacturing

processhas a large influence on size andstructureof the molecules and the physical properties of

polymers.

Changes in the reaction conditions will change

properties of the products of the same

manufacturing process.

For example, PVC produced by radical

polymerization in suspensiondiffers in MWDfrom PVC obtained by radical polymerization in

bulk.



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Bulk polymerization

The simplestand gives the highest purity polymer.

Polymerizations are carried out without solvents or

diluents.

It uses monomer, amonomer soluble initiator and chain

transfer agent to control molecular weight.

This method is economical and eco-friendly due to

recyclingandpurificationof solvents or dispersants, as

well as the disposal of liquid waste arenot necessary (as in

the case of aqueous suspension or emulsion

polymerizations). On the other hand, an increase in temperature will

increase the polymerization rate, thus generating

additional heatto be dissipated.



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Heat removal becomes particularly difficult near the end

of the polymerization when viscosity is high.

This is because high viscosity limits the diffusionof long-chain radicals as required for termination i.e. radical

concentration will increase, thus increasing the rate of

polymerization (Rp> Rt)

By comparison, the diffusion of small monomermolecules to the propagation sites is less restricted, thus

Rtdecreases more rapidly than Rp.

R0will increase, accompanying additional heat

production and is called autoaccelaration process.

In practice, heat dissipation during bulk polymerization

can be improved by providing special bafflesfor improve

heat transfer or by performing in separate stepsof low to

moderate conversion.polymerization technique 5


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utoaccelaration

Radical chain polymerizations are characterized by the

presence of an autoacceleration in the polymerization

rate as the reaction proceeds .

Normally, a reaction rate falls with time (i.e. the extent

of conversion), since the monomer and initiator

concentrations decrease with time.

However, an opposite behavior is observed in many

polymerizations, where the reaction rate increases with

conversion.

A typical example is shown in Fig. 1 for the

polymerization of MMA in benzene solution.

The plot for the 10% MMA solution shows the expected

behavior of conversion vs time.

polymeriza

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Figure 1: Autoaccelaration in benzoyl peroxide-initiated polymerization of

MMA in benzene at 500C. The different plots represent various

concentrations of monomer in solvent.


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The plot for 100% monomer (pure) shows a dramatic

autoacceleration in the polymerization rate. Such

behavior is referred to as the gel effect or Trommsdorffeffect and NorrishSmith effect.

Similar behavior has been observed for a variety of

monomers including styrene and vinyl acetate.

It turns out that the gel effect is the normalbehaviorfor most polymerizations and is observed under

isothermal reaction conditions.



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Homogenous bulk polymerization

In homogeneousbulk polymerization, the polymer

remains dissolvedin the monomer. In some cases, this canonly be achieved by carrying out the reaction above the

softening temperature+of the polymer.

The number of chain polymerizations that can be

performed in a homogeneous phase is very limited, only afew monomers are able to dissolve their own polymers

(e.g. styrene and methyl methacrylate).

However, temperatures above the softening pointoften

cannot be used for kinetic and thermodynamic reasons

(e.g. ceiling temperature*, low molecular weights, increase

of side reactions).+ For a substance which does not have a definite melting point, the temperature at which viscous flow changes to plastic flow.

*The temperature at which the propagation and de-propagation rates are equal, that is, the net rate of polymer formation is

zero. Above the ceiling temperature, de-polymerization (i.e. an unzipping reaction to reform monomer) occurs.

polymerizationtechniqu

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In contrast, the stepwise polymerizations in the

homogeneous phase are the most frequently performedprocesses on an industrial scale.

This is due to the resulting polymers usually have high

softening points (above 250 C are necessary) and high

molecular weight, thus the melt viscosity increases with

progressing reaction.

However, the removalof the highly volatile reaction

products (e.g. water, alcohol) to obtain high molecular

masses, is difficult.

This can be minimized by evacuation and intensive

mixing, which create larger surfaces and short diffusion

paths.



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High thermal stabilities of the starting materials and the

polymers produced are necessary for long reaction time

and high temperatures.

Many high-melting polyamides (2100C2650C) are not

accessible by this process and must be prepared in

solution or by interfacial condensation polymerization



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Heterogeneous bulk polymerization

In heterogeneous polymerizations in bulk, the formed

polymeris insoluble in its monomer and the reaction is

performed below the softening point of the polymer.

On an industrial scale, this type of process is especially

utilized for chain polymerizations e.g. radical

polymerization of liquidvinyl chloride or coordination

polymerization of liquid propylene with ZN or

metallocene catalysts.

Besides in the liquid phase, some reactions are also

performed in the solid state, for example, thepolymerization of acrylamide



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A third possible heterogeneous bulk polymerization is in

the gaseous phase i.e. the reaction is performed withgaseous monomers.

The gaseous monomer is adsorbed on the solid catalyst

particles and be polymerized.

With proceeding polymerization, the catalyst particles areencapsulated by the solid polymer layer. The monomer

must diffuse from the gaseous phase through the polymer

layer to the catalytically active centers.

The reaction medium is now a gas/solid dispersion of asolid polymer in its gaseous monomer.



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Advantages:High yield per reactor volume

Easy polymer recovery (pure polymer)

Option to casting the polymerization

mixture into final product form i.e. castpolymerization.

Disadvantages:

Difficulty of removing the last traces ofmonomer.

Poor heat dissipationexothermic reaction


Bulk Polymerization 2015

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