ch 1 free radical poly-filiz Şenkal ps

7/27/2019 CH 1 Free Radical Poly-Filiz enkal ps

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PST 522E Synthesis and Characterization of Macromolecules

CHAPTER 1

EMULSION AND SUSPENSION

POLYMERIZATION OF

STYRENE

1. THEORETICAL PART

1.1. Emulsion Polymerization

In emulsion polymerization two immiscible liquid phases are present, an aqueous continuous

phase and a non-aqueous discontinuous phase consisting of monomer and polymer. The

initiator is located in the aqueous phase. And the monomer-polymer particles are quite small,

of the order of 0.1 m in diameter.

Emulsion systems allow higher-molecular-weight polymer to be produced at higher rates

than do bulk or suspension systems. The soap plays an important role in emulsion

polymerization. At the beginning of the reaction it exists in the form of micelles, aggregates of

50-100 soap molecules.

Part of the monomer enters the micelles, but most of it exists as droplets a micrometer or

more in diameter. In the ideal case no polymer is formed in the monomer droplets.

Polymerization can take place (at a very low rate) in the homogeneous phase in the absence

of soap, but this cannot account for the bulk of the polymer formed. At the beginning of the

reaction, polymer is formed in the soap micelles; these represent a favorable environment for

the free radicals generated in the aqueous phase, because of the relative abundance of

monomer and the high surface / volume ratio of the micelles compared to the monomer

droplets. As polymer is formed, the micelles grow by the addition of monomer from the

aqueous phase (and ultimately from the monomer droplets.)

Soon (2-3% polymerization) the polymer particles much larger than the original micelles and

absorb almost all the soap from the aqueous phase. Any micelles not already activated

disappear; further polymerization takes place within the polymer particles already formed.

The monomer droplets are unstable at this stage; if agitation is stopped, they coalesce into a

continuous oil phase containing no polymer. The droplets act as reservoirs of monomer,

which is fed to the growing polymer particles by diffusion through the aqueous phase. Thepolymer particles may contain about 50% monomer up to the point at which the monomer

droplets disappear, at 60-80 % polymerization. The rate of polymerization is constant over

most of the reaction up to this point, but then falls off as monomer is depleted in the polymer

particles. Rate increases soap (and initial micelle) concentration.

Emulsion polymerization has three stages.

1.1.1. Stage I

The monomer diffuses to the empty micelle from droplets.

Polymerization initiated in micelles to become polymer particles.

New particles are generated as micelles are consumed.

This stage lasts for conversion ~ 0-15%.

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Chapter 1 - Emulsion and Suspension Polymerization of Styrene

Figure 1: Stage I

1.1.2. Stage II

No more surfactant available to generate new particles.

Monomer diffuses into now a constant number of particles to maintain some

equilibrium [M] with the particle.

The monomer reservoir drops get slowly consumed.

Figure 2: Stage II

An equilibrium between increasing interfacial tension within micelle and

monomer/polymer dilution leads to a constant volume fraction where 2 is the volume

fraction of the polymer and 1 is the volume fraction of the monomer.

[ ] [ ] ( )20

1 = MM

1.1.3. Stage III

This stage occurs when conversion ~ 40-60%.

All the monomers exist in particles.

Table 1: Common properties of emulsion polymerization

Common Emulsion Polymers Advantages Disadvantages

styrene + copolymers

vinyl chlorides ex. Pleather

butadiene

vinylidene chloride

vinyl acetate

vinyl acrylates (acrylics)methyl acrylates

- low (viscosity)

- great T control

- final product fine powder

or water form coatings

- a lot of soap as impurity

ex. In medical applications,

can be irritant

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1.1.4. Smith-Ewart kinetics

In an ideal emulsion system, free radicals are generated in the aqueous phase at a rate of

about 1013 per cubic centimeter per second. There are about 1014 polymer particles per cubic

centimeter. Simple calculation show that termination of the free radicals in the aqueous

phase is negligible and that diffusion currents are adequate for the rapid diffusion of freeradicals into the polymer particles- on the average, about one per particle every 10 sec.

It can also be calculated from the known termination rate constants that two free radicals

within the same polymer particle would mutually terminate within a few thousands of a

second. Therefore each polymer particle must contain most of the time either one or no free

radicals.

At any time half of the particles (on the average) contain one free radical, the other half none.

The rate of polymerization per cubic centimeter of emulsion is

Vp=kp[M] N / 2

[M]: Monomer concentration

kp: propagation rate constant

N: the number of polymer particles per cubic centimeter

Since the monomer concentration is approximately constant, the rate depends principally on

the number of particles present and not on the rate of generation of radicals.

The degree of polymerization also depends upon the number of particles:

Xn=kpN [M] /

: the rate of generation radicals

Unlike vp, xn is a function of the rate of free-radical formation. In bulk polymerization rate can

be increased only by increasing the rate of initiation; this, however, causes a decrease in the

degree of polymerization. In emulsion polymerization the rate may be increased by

increasing the number of polymer particles. If the rate of initiation is kept constant, the

degree of polymerization increases rather than decreases as the rate rises. Since the

number of polymer particles is determined by the number of soap micelles initially present,

both rate and molecular weight increase with increasing soap concentration.

The Smith-Ewart kinetics require that

Vp N, [I]0.4

, [E]0.6

N [I]0.4

, [E]0.6

xn N, [I]-0.6

, [E]0.6

[E] is the soap or emulsifier concentration

1.2. Suspension Polymerization

Hoffman and Delbruch first developed suspension polymerization in 1909. In suspensionpolymerization the initiator is soluble in the monomer phase, which is dispersed by

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comminuting into the dispersion medium (usually water) to form droplets. The solubility of the

dispersed monomer (droplet) phase and also the resultant polymer in the dispersion medium

are usually low. The volume fraction of the monomer phase is usually within the range 0.1-

0.5. Polymerization reactions may be performed at lower monomer volume fractions, but are

not usually economically viable. At higher volume fractions, the concentration of continuous

phase may be insufficient to fill the space between droplets. Polymerization proceeds in thedroplet phase and in most cases occur by a free radical mechanism. Suspension

polymerization usually requires the addition of small amounts of a stabilizer to hinder

coalescence and break-up of droplets during polymerization. The size distribution of the

initial emulsion droplets and, hence, also of the polymer beads that are formed, is dependent

upon the balance between droplet break-up and droplet coalescence. This is in turn

controlled by the type and speed of agitator used the volume fraction of the monomer phase,

and the type and concentration of stabilizer used. If the polymer is soluble in the monomer, a

gel is formed within the droplets at low conversion leading to harder spheres at high

conversion. If the polymer is insoluble in the monomer solution, precipitation will occur within

the droplets, which will result in the formation of opaque, often irregularly shaped particles. Ifthe polymer is partially soluble in the monomer mixture, the composition of the final product

can be difficult to predict. Polymer beads find applications in a number of technologies, such

as molding plastics. However, their largest application is in chromatographic separation

media (as ion exchange resin and as supports for enzyme immobilization). Such applications

frequently require large particle surface areas, which necessitates the formation of pores (of

the required dimensions) in the bead structure.

The polymer beads may be made porous by the inclusion of an inert diluent (or porogen) to

the monomer phase, which may be extracted after polymerization. Other additions to the

monomer phase can include UV stabilizers (aromatic ketones and esters), heat stabilizers

(ethylene oxide derivatives and inorganic metal salts), molding lubricants and foaming agents(porogens).

1.2.1. Polymeric stabilizers

Typical polymeric stabilizers used for oil-in-water suspension polymerization reactions are

poly (vinyl alcohol) -co- (vinyl acetate) (formed from the partial hydrolysis (80-90%) of

polyvinyl acetate), poly (vinyl-pyrrolidone), salts of acrylic acid polymers, cellulose ethers and

natural gums.

Polymeric stabilizers used in inverse suspension polymerization reactions include block

copolymers poly (hydroxy-stearic acid) -co-poly) ethylene oxide). Surfactants used for oil-in-

water suspensions include spans and the anionic emulsifier (sodium 12-butinoyloxy-9-

octadecenate).

Figure 3: Suspension polymerization

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Figure 4: Batch reactor within organic droplets (~1 m 1 cm)

Drop size determined by impeller speed within each droplet, have

initiator

monomer

Kinetics is identical to typical large scale free radical polymerization.

initiation

propagation

termination

steady state assumption

Figure 5: Suspension polymerization

1.2.2. Products

Glassy rigid beads often called latex beads

Very uniform Nice spherical shapes

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Figure 6: x-linked network

Figure 7: Vary surface chain

1.2.3. Polymerization conditions and kinetics

Extensive studies have shown that, in general, reaction kinetics in suspension polymerization

is found to show good agreement with bulk phase kinetics (in absence of any monomer

diluent). This observation suggests that in suspension polymerization, the emulsification

conditions (agitation conditions, emulsion droplet size and concentration / type of stabilizer)

appear to have little effect on reaction kinetics. Moreover, it can be concluded that any mass

transfer between two phases in the emulsion does not affect the overall reaction rate. The

major challenge in designing a suspension reaction is therefore the formation of a stable

emulsion, preferably having a uniform size distribution. The monomer droplets are large

enough to contain a large number of free radicals (may be as many as 10 5) and this is why

the polymerization in general proceeds with a similar mechanism to that of bulk

polymerization, particularly when the polymer is soluble in the monomer.

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2. EXPERIMENTAL

2.1. Emulsion Polymerization

2.1.1. Materials

Styrene, potassium persulphate, soap solution, potassium-aluminum sulfate distilled water.

2.1.2. Procedure

To a resin kettle equipped with a mechanical stirrer, condenser and nitrogen inlet tube, is

added 128.2 g. of distilled water, 71.2 g. of styrene, 31.4 mL of 0.68 % potassium persulfate,

and 100 mL of 3.56% soap solution (sodium stearate one can use 1g of either sodium

dodecyl benzenesulfonate or sodium lauryl sulfate). The system is purged with nitrogen to

remove dissolved air. Then the temperature is raised to 50 0C and kept there for 2 hour to

afford a 90% conversion of polymer. The polymer is isolated by freezing thawing or by

adding potassium-aluminum solution and boiling the mixture. The polystyrene is filtered,washed with water and methanol and dried in vacuum at 50 0C. The total yield, and the

limiting viscosity number (degree of polymerization) of one sample is determined by using

[] = km . M

equation. km and for polystyrene in benzene at 200C are 12.3 10-3 and 0.72 respectively.

2.2. Suspension Polymerization

2.2.1. MaterialsStyrene, 1, 4-divinylbenzene, poly (vinyl alcohol), dibenzoyl peroxide, methanol.

2.2.2. Procedure

Styrene and 1, 4-divinylbenzene (the latter as 50-60% solution in ethyl benzene) are

destabilized and distilled.

A three-necked flask, fitted with stirrer (preferably with revolution counter), thermometer,

reflux condenser and nitrogen inlet, is evacuated and filled with nitrogen three times. 250 mg

of poly (vinyl alcohol) are placed in the flask and dissolved in 150 mL of de-aerated water at

50C. A freshly prepared solution of 0.25g (1.03 mmol) of dibenzoyl peroxide in 25 mL (0.22

mmol) of styrene and 2 mL (7 mmol) of 1, 4-divinylbenzene is added with constant stirring so

as to produce an emulsion of fine droplets of monomer in water. This is heated to 90C on a

water bath while maintaining a constant rate of stirring and passing a gentle stream of

nitrogen through the reaction vessel. After about 1 h (about 5% conversion) the cross-linking

becomes noticeable (gelation). Stirring is continued for another 7 h at 90 C, the reaction

mixture then being allowed to cool to room temperature while stirring. The supernatant liquid

is decanted from the beads, which are washed several times with methanol and finally stirred

for another 2 h with 200 ml of methanol. The polymer is filtered off and dried overnight in

vacuum at 50 C. Yield: practically quantitative.

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3. REFERENCES

[1] Bil lmeyer, F.W., (1984).Textbook Polym. Sci., 3rd Edition,John Wiley&Sons.

[2] Braun, D., Cherdonron, H., Kern, W., (1984). Practical Macromolecular Organic Chem.,

Harwood Academic Publisher.[3] Dowding, P.J., Vincent, B., (2000). Colloids and Surfaces, 161-259.

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ch 1 free radical poly-filiz Şenkal ps

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