Polymerization Polymerization techniquestechniques

Process conditions:Process conditions:

1.1. Homogeneous polymerization:Homogeneous polymerization:

a)a) Bulk Bulk

b)b) Solution Solution

2.2. Heterogeneous polymerizationHeterogeneous polymerization

a)a) Suspension Suspension

b)b) EmulsionEmulsion

Concerning: Concerning: Molecular Weight Molecular Weight Rate (kinetics) Rate (kinetics) Heat effects (thermodynamics) Heat effects (thermodynamics)

Bulk polymerization Bulk polymerization

The The bulk addition polymerizationbulk addition polymerization is the is the simplest of all polymerization processessimplest of all polymerization processes

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

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

For bulk polymerization, there are two For bulk polymerization, there are two possibilities: possibilities:

a)a) polymer soluble in monomerpolymer soluble in monomer

(example: polystyrene) (example: polystyrene)

Polystyrene is soluble in styrene to Polystyrene is soluble in styrene to infinite infinite molecular molecular weight.weight.

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

b)b) polymer insoluble in monomerpolymer insoluble in monomer (example: vinylidine chloride) (example: vinylidine chloride)

The above probably implies that polyvinylidine chloride The above probably implies that polyvinylidine chloride precipitates out of the solution of vinylidine chloride precipitates out of the solution of vinylidine chloride monomer when it reaches a certain molecular weight.monomer when it reaches a certain molecular weight.

If the polymer is insoluble in the monomer, the rate of If the polymer is insoluble in the monomer, the rate of initiation is proportional to the monomer concentration,initiation is proportional to the monomer concentration,the initiator concentration, and the inverse square of K over the initiator concentration, and the inverse square of K over tt

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

ADVANTAGES:ADVANTAGES:

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

2.2. High rates of polymerizationHigh rates of polymerization

3.3. High degrees of polymerizationHigh degrees of polymerization

4.4. High purityHigh purity

Disadvantages:Disadvantages:1.1. Rapid increase of viscosity leads to inefficient stirringRapid increase of viscosity leads to inefficient stirring

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

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

Solution PolymerizationSolution Polymerization

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

For solution polymerizations, there are two For solution polymerizations, there are two possibilities: possibilities: a)a) monomer is soluble and the polymer monomer is soluble and the polymer is soluble in the diluent: is soluble in the diluent:

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

(example: acrylonitrile in chloroform)(example: acrylonitrile in chloroform)

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

(i.e. no polymer precipitation), then as the (i.e. no polymer precipitation), then as the polymerization occurs:polymerization occurs:a)a) The viscosity of the solution increases. The viscosity of the solution increases. b)b) The rate of polymerization will decrease The rate of polymerization will decrease with with time. time. The rate of polymerization is proportional to The rate of polymerization is proportional to monomer concentration, initiator monomer concentration, initiator concentration, and the inverse square of (K concentration, and the inverse square of (K multiplied by t.) multiplied by t.)

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

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

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

Heat effects are much better Heat effects are much better

Advantage of solution Advantage of solution polymerizationpolymerization

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

2.2. Solvent reduces viscosity, making Solvent reduces viscosity, making processing easier.processing easier.

3.3. Thermal control is easier than the Thermal control is easier than the bulk bulk

Disadvantage of solution Disadvantage of solution polymerizationpolymerization

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

2.2. Environmental pollution due to solvent Environmental pollution due to solvent release.release.

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

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

Addition polymerizations are usually Addition polymerizations are usually carried out bulk and solution carried out bulk and solution polymerizations.polymerizations.

Condensation polymerizations are Condensation polymerizations are carried out mostly without solvents carried out mostly without solvents (Bulk)(Bulk)

Suspension PolymerizationSuspension Polymerization (Pearl Polymerization) (Pearl Polymerization)

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

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

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

The initiator used can be The initiator used can be water soluble or organic water soluble or organic soluble [benzoyl peroxide, soluble [benzoyl peroxide, AIBN, or (NHAIBN, or (NH44))22(S(SxxOO44))yy.] .] Usually the initiator is organic Usually the initiator is organic soluble. soluble.

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

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

suspension agentsuspension agent is a material that gives a is a material that gives a surface activation that keeps droplets from surface activation that keeps droplets from become larger (droplets coming together to form become larger (droplets coming together to form larger droplets is called larger droplets is called coalescencecoalescence.) .)

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

A typical suspending agent : PVA dissolved in the A typical suspending agent : PVA dissolved in the aqueous phaseaqueous phase

This process needs:This process needs:

ContinueContinue……

b)b) Dispersion stabilizers dissolved in the Dispersion stabilizers dissolved in the aqueous phaseaqueous phase

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

1) insoluble in water or 1) insoluble in water or

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

in water.in water.

The particle size is affect by the The particle size is affect by the following four factors: following four factors: stirring rate stirring rate ratio of reactants ratio of reactants suspension agent suspension agent temperaturetemperature

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

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

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

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

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

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

The advantages of Suspension The advantages of Suspension Polymerization Polymerization

a)a) better heat control of the reactionbetter heat control of the reactionb)b) Low viscosity due to suspension Low viscosity due to suspension c)c) separation is much easier than in separation is much easier than in

solution polymerization. Can be solution polymerization. Can be isolated by filtration or this stable isolated by filtration or this stable latexes and dispersions can be used latexes and dispersions can be used directly in coatings, paints and directly in coatings, paints and adhesives.adhesives.

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

The disadvantage of The disadvantage of Suspension Polymerization Suspension Polymerization

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

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

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

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

Emulsion polymerizationEmulsion polymerization

Emulsion:Emulsion:

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

Emulsifiers : Surfactants Emulsifiers : Surfactants

Amphipathic (dual personalities)

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

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

Spherical with 20-100ÅContain 50-150 molecules

Emulsion polymerization :Emulsion polymerization :

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

Therefore from the definition:Therefore from the definition:Immiscible liquid : hydrophobic Immiscible liquid : hydrophobic monomermonomer

Mother liquor : waterMother liquor : water Emulsifier : surfactant /soapEmulsifier : 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:Location of the monomer:

a)a) In a large monomer droplets In a large monomer droplets (10,000 (10,000 Å) Å) floating around in water. floating around in water. There are about 10There are about 101010-10-101111 monomers droplets/cmmonomers droplets/cm33

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

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

EP Mechanism:EP Mechanism:

Can be carried out as:Can be carried out as:

1.1. Batch reaction Batch reaction

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

Initiation:Initiation:

Initiator migrates into micelle and Initiator migrates into micelle and react with a monomer molecules.react with a monomer molecules.

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

Polymerization starts

Micelle is now referred as particle

PropagationPropagation

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

On average, there is one radical / micelleOn average, there is one radical / micelle All monomer is consumed in EP. Therefore All monomer is consumed in EP. Therefore

latex can be used without purification latex can be used without purification (important for paints and coatings)(important for paints and coatings)

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

Termination Termination

Monomer in the micelle quickly Monomer in the micelle quickly polymerized and the growing chain polymerized and the growing chain terminatesterminates

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

Molecular weight??Molecular weight??Rate of polymerization = rate of Rate of polymerization = rate of

disappearance of monomerdisappearance of monomerMonomer disappear faster when Monomer disappear faster when

there are more particle. Therefore, in there are more particle. Therefore, in order to have more particles, we order to have more particles, we must have more micelle.must have more micelle.

For that reason : [initiator]???For that reason : [initiator]???

Polymers that are made Polymers that are made commercially via EP:commercially via EP:

Polyvinyl acetatePolyvinyl acetatePolychloroprenePolychloroprenePMMAPMMAPVCPVCPolyacrylamidePolyacrylamideCopolymers of PS,Polybutadiene and Copolymers of PS,Polybutadiene and

PANPAN

Advantages:Advantages:

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

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

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

d)d) Able to control particle structureAble to control particle structure

DisadvantagesDisadvantages

a)a) Contamination by the surfactant. Contamination by the surfactant. Cause water sensitivityCause water sensitivity