PSE - Polymer Synthesis (5)

8/3/2019 PSE - Polymer Synthesis (5)

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Polymer Synthesis

Dr. P.R. Shukla

DDU



Polymer Synthesis

• Preparation of Polymers from monomerscan be by:

• 1. Addition Polymerization

– Free radical polymerization

– Cationic polymerization

– Anionic polymerization

– Coordination polymerization – Ring Opening polymerization

• 2. Condensation Polymerization



Addition Polymerization

• Addition or chain reactive polymerizationinvolves chain reactions in which the chaincarrier may be a reactive substance with one

unpaired electron called a free radical, or maybe a cation, an anion, or a coordination complex.

• The free radical or ion is capable of reacting toopen the double bond and add to it, with an

electron or an ion remaining unpaired. In a veryshort time, many more monomers add to thegrowing chain.




• Under correct conditions of temperatureand pressure, and with the aid of catalystsand promotors, the monomers simply add

onto themselves like links in a chain.

• Examples – polyethylene, polypropylene






• Three Steps:

1. Initiation

2. Propagation

3. Termination



Free Radical Polymerization-Initiation

• The initiator generates free radicals as theactive species through its homolyticthermal decomposition.

• A typical redox system is that of ferrousion and hydrogen peroxide.

• Another widely used redox system is ofpersulfate with thiosulfate or biosulfite asreducing agents.







Termination terminology

Chain growth stops by bimolecular reaction of twogrowing radicals: termination

• A terminator: another free radicalForms a stable molecule with n mer units

• Recombination: the termination step• Combination:

*Mi + *M j → Mi+j

• Disproportionation:

*Mi + *M j → Mi + M j

Obtaining a monomer like double bond• Hydrogen Abstraction: obtaining a hydrogen atom with

unpaired electron



The Chain Transfer

• Often the growing chain terminates beforeit fully grows by interaction with a smallmolecule, say RH, in such a way that a

portion of the small molecule terminatesthe active chain and at the same timeproduces a new radical R.

• The chain transfer reaction can be shownas follows:

• ----M* + RH → ----MH + R*



The Chain Transfer

• The chain transfer can be done by the monomeritself, through solvent, from initiation, frominitiator, or from polymer.

• It can also be intentionally done by addingsubstances called chain transfer agents or chainmodifiers.

• The chain transfer during the polymerization

leads to low molecular weight polymers(sometimes desirable, at other timesundesirable)



Cationic Polymerization - Initiation

• Cationic polymerization involves initiatorswhich produce a cationic species thatinteracts with the monomer.

• Initiators used include Lewis acids such asBF3, AlCl3, etc.

• Protonic acids like sulfuric acid, perchloricacid, phosphoric acid also act as initiators.





Anionic Polymerization - Initiation

• Anionic polymerization uses initiatorswhich produce anionic species.

• Organoalkali compounds like butyl lithium,electron transfer reagents likedisodiumnaphthalene complex, andNaNH2 are common initiators for anionic

polymerization.





Coordination Polymerization

• The reactive species in a coordinationpolymerization is a coordination complex.

• Ziegler-Natta polymerization is a particular

case.• These catalysts involve a variety of

transition metal complexes. These

complexes are usually based on Ti, V, orCr type metals and organometalliccompounds like triethyl aluminium.





Propagation

• The newly generated “active species”

adds to another monomer, in the samemanner as the initiation step.

• Monomers simply add onto themselveslike links in a chain





Termination

• The termination step is one where thegrowing chain with active species reactswith other growing chain or by

spontaneous decomposition of the activesite.

• In free radical polymerization, termination

follows either coupling ordisproportionation mechanism.



Termination

• In cationic polymerization, termination occurs byanion attachment or by proton abstraction.

• In anionic polymerization, termination does not

take place by itself and the living chains aremade to terminate, e.g. by adding ammonia.

• In coordination polymerization, termination is byan active hydrogen compound; by transfer withmonomers; and by spontaneous internaltransfer.



Ring Opening Polymerization

• Some polymers can be prepared by reactions involvingring opening. Few typical examples of monomers forring-scission polymerization are cyclic ethers, cyclicanhydrides.

• The polymerization of ethylene oxide leading to a highmolecular weight polyethylene oxide (PEO) can beaccomplished by ring opening polymerization.

• The monomer is added to form a chain (chainpolymerization), the polymer growth is through step wise

addition, and thus molecular weight of the polymerincreases during the course of reaction(polycondensation).







Autoacceleration

• Trommsdorff effect

• In radical polymerization we speak about:• 1) low conversion, i.e. polymer chains are in dilute solution (no contact among chains)• 2) “intermediate” conversion, i.e. the area in between low and high conversion • 3) high conversion, i.e. chains are getting highly entangled; kp decreases.

• Somewhere in the “intermediate” conversion regime: • * polymer chains loose mobility.• * Termination rate decreases• * Radical concentration increases• * Rate of polymerization increases• * Molar mass increases

• This effect is called: gel effect, Trommsdorff effect,or auto-acceleration

• In the polymerization of MMA this occurs at relatively low conversion.



Techniques of Polymerization

• Bulk polymerization

• Solution polymerization

• Precipitation polymerization• Suspension (or pearl) polymerization

• Emulsion polymerization



Polymerization techniques

Sometimes for one monomer severaltechniques of polymerizing are available.Choice of a specific technique depends on anumber of factors:

• Kinetic / mechanistic factors related to chainlength, chain composition

• Technological factors e.g. heat removal, reactionrate, viscosity of the reaction mixture,

morphology of the product• Economic factors; production costs,

environmental aspects, purification steps etc.



Polymerization techniques

Homogeneous systems• Bulk polymerization

• Solution polymerization

Heterogeneous systems • Suspension polymerization

• Emulsion polymerization

• Precipitation polymerization• Polymerization in solid state• Polymerization in the gas phase



Bulk Polymerization

• The polymerization reaction is carried outwithin the monomer itself.

• The reaction is catalyzed by additives or

under the influence of heat or light.• Polymer is obtained as a powdery porous

solid.

• The polymer formed is usually pure• Examples – vinyl chloride, vinyl acetate,

and acrylic esters



Bulk polymerization

Polymerization of the undiluted monomer. Viscosityincreases dramatically during conversion. Heat removaland hot spots

Advantages Disadvantages * Pure products * heat control* Simple equipment * dangerous* No organic solvents * molecular weights

very disperse

Applications

Polymers through step reactions (nylon 6)PMMA-plates



Solution Polymerization

• When both the monomer and the polymerproduced are soluble in a suitable solvent,the polymerization reaction is carried out

with the monomer in solution.• Solution polymerization is mainly applied

when solutions of polymers are required(for ready made use) for technicalapplications, e.g. as lacquers, paints,adhesives.



Solution polymerization

Monomer dissolved in solvent, formed polymer staysdissolved. Depending on concentration of monomer thesolution does not increase in viscosity.

Advantages Disadvantages

* Product sometimes * Contaminationdirectly usable with solvent

* Controlled heat * Chain transfer torelease solvent

* Recycling solventApplications Acrylic coating, fibrespinning, film casting



Precipitation Polymerization

• In such a polymerization reaction, the monomeris soluble in the solvent, and the polymerprecipitates out as a result of polymerization(polymer being insoluble in solvent).

• The so precipitated polymer can be separated inthe form of a gel or powder by centrifugation orsimple filtration.

• Polyethylene, polyvinyl esters, polyacrylic estersare obtained commercially with hydrocarbons assolvents. Polyacryonitrile is prepared usingwater as solvent.



Suspension Polymerization

• The monomer containing initiator, modifier etc. isdispersed in a solvent (generally water) byvigorous stirring.

• For a stable suspension of monomer in the

solvent, some stabilizers are added.• The polymerization takes place within the

dispersed (or suspended) monomer particlesand the agglomeration of which is prevented by

stabilizer.• After the reaction is completed, the polymer isseparated by centrifugation or filtration, washed,and dried.



Suspension polymerization

• Water insoluble monomers are dispersed in water.• Initiator dissolved in monomer.• Stabilization of droplets/polymer particles with non-

micelle forming emulsifiers like polyvinylalcohol or Na-carboxymethylcellulose.

• Equivalent to bulk polymerization,small droplets dispersed in water.

• Product can easily be separated,particles 0.01-1mm.

• Pore sizes can be controlled by adding a combination ofsolvent (swelling agent) and non-solvent.

• Viscosity does not change much.



Suspension Polymerization

Advantages Disadvantages

* Heat control simple * Contamination with

* Product directly stabilizing agent

usable * Coagulation possible* Easy handling

Applications

Ion-exchange resins, polystyrene foam, PVC



Emulsion Polymerization

• The liquid monomer is dispersed in aninsoluble liquid, which in turn gives anemulsion (usually in water).

• So the monomer in water emulsion,containing catalyst and stabilized byemulsifying agents (e.g. by surface active

agents), is then used as the polymerizingmixture.




• Polymerization takes place within the surfactantmicelles (size about 10 nm) which grow in sizeat the expense of the dispersed monomer

droplets (size about 1000 nm).• In the end, a latex (or a polymer colloid) with

dispersed polymer (size about 100 nm) is left.

• The polymer is then isolated by evaporation of

water in spray dryer, or by coagulation of thepolymer latex, filtration and drying.






• A micelle forming emulsifier is used.

• Initiator is water soluble.

• The formed latex particles are much smaller

than suspension particles (0.05-2 µm).

• Kinetics differ considerable from othertechniques.

• Polymer is formed within the micellesand not in the monomer droplets.






• Advantages Disadvantages

• * Low viscosity even * Contamination of

at high solid contents products with additives

• * Independent control * More complicated

• of rate and in case of watermolecular-weight soluble monomers

• * Direct application of

• complete reactor contents





Condensation Polymerization

• In condensation, or step-reactionpolymerization, two reactive monomers

join together with the release (i.e.

condensation) of a molecule of water.• Examples:

– Polyester – PET (polyethylene terephthalate)

Terephthalic acid + ethylene glycol → PET

– Polyamide – Nylon

Adipic acid + hexamethylene diamine → PA





Ph T h i i



Phase Techniques inPolycondensation

• The polycondensation involves thereactions of the functional groups andtherefore the reactants need to be in the

fluid phase.• This can be achieved either by melting the

reacting monomers (meltpolycondensation) or by dissolving them ina suitable inert common solvent (solutionpolymerization).

Ph T h i i



Phase Techniques inPolycondensation

• A third technique is called interfacialpolycondensation.

• This technique uses the solutions of the

two monomers prepared separately in twosolvents immiscible with each other.

• When these two solutions are brought incontact, the polycondensation reactionbegins at the interface separating theliquids.



Kinetics of Polycondensation

• A simple relation, called the Carouthersequation, exists between the averagedegree of polymerization DP of the

polycondensation polymer at time t, andthe extent of reaction p.

• DP = 1/ (1-p)




• Question: What is the molecular weight ofa polymer prepared by the selfpolycondensation of OH-©-COOH if 99.50

of the functional groups react?




• Answer: Repeat unit (O-©-CO)n

• p=0.995; DP = 1/(1-p) = 200

• Molecular weight of repeat unit is

• O-©-CO = O + C6H4 + C

• =16 + 72+ 4+ 12+ 16 = 120

• Molecular weight of polymer = 120 x DP=120 x 200 = 24,000 gmol-1





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PSE - Polymer Synthesis (5)