Polymerization process

Process conditions:

1. Homogeneous polymerization:

a) Bulk

b) Solution

2. Heterogeneous polymerization

a) Suspension

b) Emulsion

Concerning:

Molecular Weight

Rate (kinetics)

Heat effects (thermodynamics)

Bulk polymerization

The bulk addition polymerization is the simplest of all polymerization processes

Bulk polymerization is the conversion of monomer into a polymer without the aid of a solvent

A bulk addition polymerization is a homogeneous system with an organic initiator.

For bulk polymerization, there are two possibilities:

a) polymer soluble in monomer

(example: polystyrene)

Polystyrene is soluble in styrene to infinite molecular weight.

Therefore, some physical changes occur with increasing molecular weight (e.g., viscosity, etc.)

b) polymer insoluble in monomer (example: vinylidine chloride) The above probably implies that polyvinylidine chloride precipitates

out of the solution of vinylidine chloride monomer when it reaches a certain molecular weight.

If the polymer is insoluble in the monomer, the rate of initiation is

proportional to the monomer concentration, the initiator concentration, and the inverse square of K over t

The higher the temperature, the lower the molecular weight of the

polymer produced. ( at higher temperatures, the initiator decomposes to form radicals at a faster rate, then for a given amount of monomer with more radicals present more polymer chains will be started (initiated), and the resulting polymers will have a lower molecular weight.

ADVANTAGES:

1. There is no problem with getting solvent out of the finished product (since no solvent is used.)

2. High rates of polymerization

3. High degrees of polymerization

4. High purity

Disadvantages: 1. Rapid increase of viscosity leads to inefficient stirring 2. Difficult to remove heat evolved upon polymerization.

The thermal conductivity of monomers and polymers is low, and as the viscosity builds up, the ability for heat transfer via convection is substantially diminished. If the heat energy cannot be dissipated, temperature rises, and at higher temperatures the reaction is going to go faster, so this is a positive feedback loop with disastrous consequences.

3. For bulk polymerization, removal of unreacted monomer can be a problem. This is a large concern if your safe polymer was prepared from monomers which are toxic.

Solution Polymerization

The main advantage of a diluent (either water or an organic solvent) is to take up the heat of polymerization.

For solution polymerizations, there are two possibilities:

a) monomer is soluble and the polymer is soluble in the diluent:

(example: polystyrene in toluene) b) monomer is soluble and the polymer is

insoluble in the diluent: (example: acrylonitrile in chloroform)

a) If both the monomer and the polymer system are soluble in the solution

(i.e. no polymer precipitation), then as the polymerization occurs:

a) The viscosity of the solution increases.

b) The rate of polymerization will decrease with time.

The rate of polymerization is proportional to monomer concentration, initiator concentration, and the inverse square of (K multiplied by t.)

b) the polymer is insoluble in the solution above a certain molecular weight

(i.e., the polymer precipitates out at that molecule weight) then the viscosity is more likely to remain fairly constant.

Dimerization termination is more likely, and the rate of chain transfer is faster.

Heat effects are much better

Advantage of solution polymerization

1. Solvent acts as a diluents and aids in removal of heat of polymerization.

2. Solvent reduces viscosity, making processing easier.

3. Thermal control is easier than the bulk

Disadvantage of solution polymerization

1. Difficult to remove solvent from final form, causing degradation of bulk properties.

2. Environmental pollution due to solvent release.

3. The removal of the diluents from the polymer requires a distillation, and that costs an appreciable amount of money.

4. Chain transfer to solvent occurs, leading to low molecular weights.

Addition polymerizations are usually carried out bulk and solution polymerizations.

Condensation polymerizations are carried out mostly without solvents (Bulk)

Suspension Polymerization (Pearl Polymerization)

The reaction mixture (monomer, inorganic stabilizer, oil-soluble initiator) is suspended as droplets in water (where it is insoluble).

Therefore, there are two separate phases throughout the whole process; water and organic, and the starting point may be 10 parts of the former, and 1 part of the latter.

The water phase becomes the heat transfer medium. Since it is a continuous phase, viscosity changes very little as the monomer converts to polymer, so the heat transfer is very good.

The initiator used can be water soluble or organic soluble [benzoyl peroxide, AIBN, or (NH4)2(SxO4)y.] Usually the initiator is organic soluble.

Droplets (Particle size ) may be 0.01 to 0.5 cm, or as low as 1 micron.

a) Vigorous agitation : consistent, efficient, and controlled throughout the reaction to keep the droplets apart. A suspending agent can also be used.

suspension agent is a material that gives a surface

activation that keeps droplets from become larger (droplets coming together to form larger droplets is called coalescence.)

The plant operator must control temperature, and the particle size (of the growing polymer mass in the bubble.). If the particle size gets to large, the particle will absorb too much heat. This probably relates to the idea that as the volume of a sphere increase, the ratio of surface area to volume decreases, and this ratio relates to heat transfer.

A typical suspending agent : PVA dissolved in the aqueous phase

This process needs:

Continue…

b) Dispersion stabilizers dissolved in the aqueous phase

c) In this system, the monomer must be either :

1) insoluble in water or

2) only slightly soluble in water, so that when it polymerizes it becomes insoluble in water.

The particle size is affect by the following four factors:

stirring rate

ratio of reactants

suspension agent

temperature

Suspension polymerization is similar to bulk polyerization, and it could be considered "bulk polymerization within a droplet."

Therefore, in suspension polymerization, initiation, propogation, and termination take place inside the droplet.

The speed at which the reaction takes place for a given temperature is the same, and just as for bulk polymerization, the kinetics or rates are proportional to monomer concentration.

The properties of the polymer are similar to those of the same polymer made by a bulk polymerization.

Suspension polymerization is used only in free radical type processes. The monomer is mechanically dispersed in a media, usually water. There are cases where an organic media is used in which neither the polymer nor the monomer are soluble in the organic media.

when using gases ; Example ; the polymerization of vinyl chloride (a gas at room temperature) pressure and containment are concerns.

The advantages of Suspension Polymerization

a) better heat control of the reaction

b) Low viscosity due to suspension

c) separation is much easier than in solution polymerization. Can be isolated by filtration or this stable latexes and dispersions can be used directly in coatings, paints and adhesives.

d) Polymer yields in the form of beads (0.1-2 mm)

The disadvantage of Suspension

Polymerization

a) Cannot be used for whose Tg is less than the polymerization temperature, or else aggregation will occur.

b) Must separate and purify polymer, or accept contaminate product.

c) It only applies to free radical process. Ionic catalysts don't work because they compete with water

d) Agitation is critical because as the viscosity within the bead rises, the reaction rate increases suddenly. This leads to a surge in heat generation which does not usually occur in solution or emulsion polymerization

Emulsion polymerization

Emulsion:

A stable colloidal suspension as milk, consisting of an immiscible liquid dispersed and held in another liquid by a substance called an emulsifier

Emulsifiers : Surfactants

Amphipathic (dual personalities)

At a certain concentration of surfactant in water, surfactant congregate and form micelle – above critical micelle concentration (CMC)

The hydrocarbon (10-20 C) core provide sites for polymerization. •Micelle became swollen by monomer molecules. •Micelles are in equilibrium with free surfactants

Spherical with 20-100Å Contain 50-150 molecules

Emulsion polymerization :

Polymerization that takes place in a emulsion typically incorporating water, monomer and surfactant

Therefore from the definition:

Immiscible liquid : hydrophobic monomer

Mother liquor : water

Emulsifier : surfactant /soap

Surfactant (PVA, Hydroxyl ethyl cellulose)

+water

Dissolved until CMC is reached

Monomer + water soluble initiator (MMA / styrene) (peroxide / persulfates)

EP are performed via

Free radical Anionic / cationic (rapid quenced by water)

Product

Latex

(dispersion of particulate polymer in water : 0.05-2µm

Location of the monomer:

a) In a large monomer droplets (10,000 Å) floating around in water. There are about 1010-1011 monomers droplets/cm3

b) Some of the monomer may be dissolved in water (unlikely)

c) Monomer may be found in micelles (exactly what we want)

EP Mechanism:

Can be carried out as:

1. Batch reaction

2. Starve fed reaction (most cases) to insure good distribution of monomers into the polymer backbone chain.

Initiation:

Initiator migrates into micelle and react with a monomer molecules.

(initiator attacks monomer in the micelle that has a large surface area than the larger monomer droplets)

Polymerization starts

Micelle is now referred as particle

Propagation

Monomer migrates from the large monomer droplets to the micelle to sustain polymerization.

On average, there is one radical / micelle

All monomer is consumed in EP. Therefore latex can be used without purification (important for paints and coatings)

Each micelle can be considered as mini bulk polymerization except no unreacted monomer and no thermal “hot-spots”

Termination

Monomer in the micelle quickly polymerized and the growing chain terminates

More monomer from the droplets diffuses to the growing particle where more initiators will eventually react

Molecular weight??

Rate of polymerization = rate of disappearance of monomer

Monomer disappear faster when there are more particle. Therefore, in order to have more particles, we must have more micelle.

For that reason : [initiator]???

Polymers that are made commercially via EP:

Polyvinyl acetate

Polychloroprene

PMMA

PVC

Polyacrylamide

Copolymers of PS,Polybutadiene and PAN

Advantages:

a) Continuous water phase is an excellent conductor of heat and allow heat to be removed from the system. Therefore, rate increased.

b) Viscosity remains close to that of water and is not dependent on molecular weight.

c) The final product can be used as is and does not generally need to be altered / processed.

d) Able to control particle structure

Disadvantages

a) Contamination by the surfactant. Cause water sensitivity