4 Polymerization Process 1 Methods for radical polymerization

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4. Polymerization Process 1

4. Polymerization Process 1

Methods for radical polymerization Bulk polymerization 本体聚合 Solution polymerization 溶液聚合 Suspension polymerization 悬浮聚合 Emulsion

Methods for radical polymerization Bulk polymerization 本体聚合 Solution polymerization 溶液聚合 Suspension polymerization 悬浮聚合 Emulsion polymerization 乳液聚合 Gas phase polymerization 气相聚合 Slurry polymerization 淤浆聚合 2

Problems in Engineering • • Diffusion, mixing, mass transfer Heat transfer Removal of unreacted

Problems in Engineering • • Diffusion, mixing, mass transfer Heat transfer Removal of unreacted monomer (devolatilization脱挥) Stability of dispersion system Immobilization of homogenoues catalyst Polymer morphology Reactor fouling Continuous process 3

4. 1 Bulk polymerization The simplest of all polymerization processes. If the polymer is

4. 1 Bulk polymerization The simplest of all polymerization processes. If the polymer is insoluble in monomer, then initiation, propagation, and termination might happen in the monomer phase. e. g. VC、VDC、AN. If the polymer is soluble in the monomer, then the concentration of monomer decreases continuously and the viscosity changes. e. g. St、MMA、VAc. Polymerization formulation: Initiator, monomer, additives (colorant, plasticizer, and UV absorbers etc). 4

Advantages: • high purity and high MW Disadvantages: • Difficult stirring due to high

Advantages: • high purity and high MW Disadvantages: • Difficult stirring due to high viscosity • 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. • Solution: Stepwise heating/polymerization • Removal of unreacted monomer is difficult. • limitation on how much monomer can be present in a polymer system used for food containment. 5

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Bulk Polymerization of Poly(methylmethacrylate) • Stir in the monomer with 5% BPO at 90

Bulk Polymerization of Poly(methylmethacrylate) • Stir in the monomer with 5% BPO at 90 C for 10 min, which results in a syrup which can then be cooled to room temperature. • Add colorant, plasticizer, and UV absorbers if necessary • Pour the syrup into casting cells • Pass the filled cell through a heating tunnel, with a temperature maintained at 40 C for 15 hours, then finish the polymerization at 95 C for 1 hour. • Cool and remove from the cell • Bubbles may form because of the exothermic reaction if the casting has a thickness greater than 2 cm. If bubbles are a problem, the polymerization may be carried out under pressure to increase the temperature at which the monomer boils (so that the monomer won't boil during the polymerization. ) • Molecular weights on the order of millions (106) may be obtained. 7

4. 2 Solution Polymerization If both the monomer and the polymer system are soluble

4. 2 Solution Polymerization If both the monomer and the polymer system are soluble in the solution, then as the polymerization occurs, the viscosity of the solution increases. The rate will decrease with time. The rate is proportional to monomer concentration, initiator concentration. e. g. polystyrene in toluene. If the polymer is insoluble in the solution above a certain molecular weight, then the viscosity is more likely to remain fairly constant. The rate of chain transfer is faster. Heat effects are much better. e. g. acrylonitrile in chloroform. Polymerization formulation: Initiator, monomer, solvent, additives (colorant, plasticizer, and UV absorbers etc). 8

Influence of solvent Advantages: • A diluent (either water or an organic solvent) take

Influence of solvent Advantages: • A diluent (either water or an organic solvent) take up the heat of polymerization. • Prevent auto-acceleration and gel effect. • No chain transfer to macromolecules Disadvantages: • That costs for the removal of the diluent from the polymer. • Low initiation efficiency due to enhanced cage effect. • Decreased polymerization rate. • Low MW due to chain transfer to solvent. 9

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Application Monomer Initiator Solvent Mechanism Application acrylonitrile AIBN Na. SCN/water radical Solution for spining

Application Monomer Initiator Solvent Mechanism Application acrylonitrile AIBN Na. SCN/water radical Solution for spining Vinyl acetate AIBN Methanol radical polyvinyl alcohol Acrylates BPO Aromatic radical Coating/ adhensive Butadiene Bu. Li hexane anionic Rubber Iso-butene BF 3 Iso-butane cationic adhensive 11

4. 3 Suspension Polymerization • Use mechanical agitation to mix the monomer or mixture

4. 3 Suspension Polymerization • Use mechanical agitation to mix the monomer or mixture of monomers in a liquid phase such as water, polymerizing the monomer droplets while they are dispersed by continuous agitation. • also known as pearl polymerization, bead polymerization and granular polymerization, e. g. polymerization of methyl methacrylate, and vinyl chloride. Polymerization formulation: Initiator, monomer, media(water), suspension agent, additives (colorant, plasticizer, and UV absorbers etc). 12

Suspension polymerization organic initiator suspension agent polymer monomer Bulk polymerization in droplets 13

Suspension polymerization organic initiator suspension agent polymer monomer Bulk polymerization in droplets 13

Characteristics • There are two separate phases throughout the whole process. • The initiator

Characteristics • There are two separate phases throughout the whole process. • The initiator used can be water soluble or organic soluble. Usually the initiator is organic soluble. • The droplets must be kept far apart: agitation. • A suspending agent can be used: water soluable polymer or inorganic powder. • The rate of suspension polymerization is similar to bulk polymerization, but the heat transfer is much better. Initiation, propogation and termination take place inside the droplet. • Particle size may be 0. 01 to 0. 5 cm, or as low as 1 micron, which is affect by the following four factors: stirring rate, ratio of reactants, suspension agent, temperature. 14

Advantages: • Better heat control of the reaction, and separation is much easier than

Advantages: • Better heat control of the reaction, and separation is much easier than in solution polymerization. Disadvantages: • It only applies to free radical process, ionic catalysts don't work because they compete with water. • Few monomers are water soluble. • Suspension agent as impurity 15

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4. 4 Emulsion polymerization usually starts with an emulsion incorporating water, monomer, and surfactant.

4. 4 Emulsion polymerization usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex particles. These latex particles are typically 100 nm in size, and comprise many individual polymer chains. Polymerization formulation: monomer(s), emulsifier, dispersants, water-soluble initiators, additives 17

Type of emulsifier/surfactant CMC Critical Micelle Concentration anionic cationic 40 - 50Å( L )

Type of emulsifier/surfactant CMC Critical Micelle Concentration anionic cationic 40 - 50Å( L ) 100 - 300 nm( H ) amphoteric/zwitterionic 18

Critical Micelle Concentration (CMC) conductivity osmotic pressure surface tension Variation of solution propertites with

Critical Micelle Concentration (CMC) conductivity osmotic pressure surface tension Variation of solution propertites with surfactant concentration 19

4. 4. 1 Ingredients • Monomers Typical monomers are those that undergo radical polymerization,

4. 4. 1 Ingredients • Monomers Typical monomers are those that undergo radical polymerization, are liquid or gaseous at reaction conditions, and are poorly soluble in water. If monomer solubility is too high, particle formation may not occur and the reaction kinetics reduce to that of solution polymerization. • Comonomers Copolymerization is common in emulsion polymerization. The same rules and comonomer pairs that exist in radical polymerization operate in emulsion polymerization. 20

 • Initiators Both thermal and redox generation of free radicals have been used

• Initiators Both thermal and redox generation of free radicals have been used in emulsion polymerization. e. g. Persulfate salts and Redox initiation (below 50 o. C). Although organic peroxides and hydroperoxides are used in emulsion polymerization, initiators are usually water soluble and partition into the water phase. 21

 • Emulsifier/Surfactants – The surfactant must enable a fast rate of polymerization, minimize

• Emulsifier/Surfactants – The surfactant must enable a fast rate of polymerization, minimize coagulum or fouling in the reactor and other process equipment. – Anionic, nonionic, and cationic surfactants have been used, although anionic surfactants are by far most prevalent. – Surfactants with a low critical micelle concentration (CMC) are favored; the polymerization rate shows a dramatic increase when the surfactant level is above the CMC. – Minimization of the surfactant is preferred for economic reasons and the (usually) adverse effect of surfactant on the physical properties of the resulting polymer. – Mixtures of surfactants are often used. – Examples of surfactants commonly used in emulsion polymerization include fatty acids, sodium lauryl sulfate, and alpha olefin sulfonate. 22

Roles of surfactants • Low surface tension • Formation of micelle/Emulsification – CMC –

Roles of surfactants • Low surface tension • Formation of micelle/Emulsification – CMC – HBL • Maintain stabilility of latex particle • Micelle solubilization 23

 • Non-surfactant stabilizers Some grades of poly(vinyl alcohol) and other water soluble polymers

• Non-surfactant stabilizers Some grades of poly(vinyl alcohol) and other water soluble polymers can promote emulsion polymerization even though they do not typically form micelles and do not act as surfactants. Dispersions prepared with such stabilizers typically exhibit excellent colloidal stability (for example, dry powders may be mixed into the dispersion without causing coagulation). However, they often result in products that are very water sensitive due to the presence of the water soluble polymer. • Other ingredients chain transfer agents, buffering agents, and inert salts. Preservatives are added to products sold as liquid dispersions to retard bacterial growth. These are usually added after polymerization. 24

4. 4. 2 The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion polymerization A

4. 4. 2 The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion polymerization A monomer is emulsified in a solution of surfactant and water forming relatively large droplets of monomer in water. emulsified monomer droplets D=1 -10 um, N=1012– 1014 dm-3 Excess surfactant creates micelles in the water. submicron latex particles D=0. 05 -1 mm, N=1016 -1018 dm-3 Monomer swollen Micelles D=5 -10 nm; N=1019 -1021 dm-3 25

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Interval 1. Waterborne free radicals first polymerize with monomers dissolved in the continuous aqueous

Interval 1. Waterborne free radicals first polymerize with monomers dissolved in the continuous aqueous phase. This would result in the increased hydrophobicity of oligomeric radicals. When a critical chain length is achieved, these oligomeric radicals become so hydrophobic that they show a strong tendency to enter the monomer-swollen micelles and then continue to propagate by reacting with those monomer molecules therein. differs from suspension polymerization where an oil-soluble initiator dissolves in the monomer, followed by polymer formation in the monomer droplets themselves. 27

Interval 2. The total surface area of the micelles is much greater than the

Interval 2. The total surface area of the micelles is much greater than the total surface area of the fewer, larger monomer droplets; therefore the initiator typically reacts in the micelle and not the monomer droplet. Monomer in the micelle quickly polymerizes and the growing chain terminates. The monomer-swollen micelle has turned into a polymer particle. When both monomer droplets and polymer particles are present in the system. 28

Interval 3. More monomer from the droplets diffuses to the growing particle, where more

Interval 3. More monomer from the droplets diffuses to the growing particle, where more initiators will eventually react. Eventually the free monomer droplets disappear and all remaining monomer is located in the particles. The final product is a dispersion of polymer particles in water. It can also be known as a polymer colloid, a latex, or commonly and inaccurately as an 'emulsion'. 29

4. 4. 3 Polymerization kinetics I: particle nucleation stage From initiation to micelle disappear,

4. 4. 3 Polymerization kinetics I: particle nucleation stage From initiation to micelle disappear, Rp ↑ II: particle growth stage monomer droplets disappear, particle growth stage, Rp → III: Decline stage from droplets dissapear to end, Rp ↓ 30

Polymerization rate In the steady stage (interval II): [Radical] in micelle [latex particles] Average

Polymerization rate In the steady stage (interval II): [Radical] in micelle [latex particles] Average radical number particle, 0. 5 (1) Nucleation and coagulation of particles do not occur and the number of particles per unit volume of water remains constant during polymerization. (2) The particle size distribution is relatively monodisperse. (3) Desorption of free radicals out of the particles does not take place. (4) Bimolecular termination of the polymeric radical inside the particle upon the entry of an oligomeric radical from the aqueous phase is instantaneous. 31

N↑ [M]↓ N, [M] → [M]~5. 0 M; N~1012– 1014 dm-3; [M • ]~10

N↑ [M]↓ N, [M] → [M]~5. 0 M; N~1012– 1014 dm-3; [M • ]~10 -7 M 32

Variation of particles in emulsion polymerization I II III micelle 1017 -18↓ to none

Variation of particles in emulsion polymerization I II III micelle 1017 -18↓ to none - - latex 0↑ 1013 -15 → → droplet N →, V ↓ ↓ to none - Rp ↑ → ↓ 33

Polymerization degree In a single latex particle N /Ri, life of radical, 10~ 100

Polymerization degree In a single latex particle N /Ri, life of radical, 10~ 100 s Rate of generation of radical 34

Features of emulsion polymerization • Both of polymerization rate and molecular weight are determinded

Features of emulsion polymerization • Both of polymerization rate and molecular weight are determinded by number of latex particles per unit volune 35

Number of latex particle, N 36

Number of latex particle, N 36

Comments on emulsion polymerization • High molecular weights are developed in emulsion polymerization because

Comments on emulsion polymerization • High molecular weights are developed in emulsion polymerization because the concentration of growing chains within each polymer particle is very low. • In conventional radical polymerization, the concentration of growing chains is higher, which leads to termination by coupling, which ultimately results in shorter polymer chains. • The original Smith-Ewart-Hawkins mechanism required each particle to contain either zero or one growing chain. • Improved understanding of emulsion polymerization has relaxed that criterion to include more than one growing chain per particle, however, the growing chains per particle is still considered to be very low. 37

The Smith-Ewart-Harkins theory • Smith-Ewart theory does not predict the specific polymerization behavior when

The Smith-Ewart-Harkins theory • Smith-Ewart theory does not predict the specific polymerization behavior when the monomer is somewhat water-soluble, like methyl methacrylate or vinyl acetate. • In these cases homogeneous nucleation occurs: particles are formed without the presence or need for surfactant micelles. • Because of the complex chemistry that occurs during an emulsion polymerization, including polymerization kinetics and particle formation kinetics, quantitative understanding of the mechanism of emulsion polymerization has required extensive computer simulation. 38

Advantages: • High molecular weight polymers can be made at fast polymerization rates. •

Advantages: • High molecular weight polymers can be made at fast polymerization rates. • The continuous water phase is an excellent conductor of heat, allowing many reaction methods to increase their rate. • Viscosity remains close to that of water and is not dependent on molecular weight. • The final product can be used as is. Disadvantages: • Surfactants and other adjuvants remain in the polymer or are difficult to remove. • For dry polymers, water removal is an energy-intensive process. • at high conversion of monomer to polymer. This can result in significant chain transfer to polymer. 39

Applications Plastics PVC Polystyrene PMMA Acrylonitrile-butadiene-styrene (ABS) Polyvinylidene fluoride PTFE Dispersions Polyvinyl acetate copolymers

Applications Plastics PVC Polystyrene PMMA Acrylonitrile-butadiene-styrene (ABS) Polyvinylidene fluoride PTFE Dispersions Polyvinyl acetate copolymers Latex acrylic paint Styrene-butadiene VAE (vinyl acetate - ethylene copolymers) Synthetic rubber Styrene-butadiene (SBR) Polybutadiene Polychloroprene (Neoprene) Acrylic rubber Fluoroelastomer (FKM) 40

Emulsion vs Suspension Polymerization • The suspension polymerization is a mechanical process, and must

Emulsion vs Suspension Polymerization • The suspension polymerization is a mechanical process, and must have a stabilizing agent until the droplets are far apart. • The emulsion polymerization is a chemical process which requires a surfactant to make the monomer "emulsify“. 41

Development of emulsion polymerization • Soap-free emulsion polymerization • Micro emulsion polymerization (5~ 80

Development of emulsion polymerization • Soap-free emulsion polymerization • Micro emulsion polymerization (5~ 80 nm) • core-shell latex polymer 42

Illustration of emulsion polymerization 43

Illustration of emulsion polymerization 43