Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

Chapter 6. Polymerization conditions and

polymer reactions

6.1. Polymerization in homogeneous systems

6.2. Polymerization in heterogeneous systems

6.3. Polymerization reaction engineering

6.4. Chemical reactions of polymers

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.1. Polymerization in homogeneous systems

The homogeneous polymerization techniques involve pure monomeror homogeneous solutions of monomer and polymer is a solvent.

These techniques can be divided into 2 methods: the bulk and thesolution polymerizations.

6.1.1. Bulk polymerization

Advantages:• Bulk polymerizations is the simplest technique and produces thehighest-purity polymers.

• Only monomer, a monomer-soluble initiator (& chain transfer agent to control the molecular weight) are used.

• This method is practiced widely in the manufacture of condensation polymers.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

Advantages: (continued)

• Easy polymer recovery and easy for cast polymerization into finalproduct forms.

• The viscosity of the mixture is still low to allow ready mixing, heat transfer,and bubble elimination.

Disadvantages:• Free-radical polymerizations are typically highly exothermic. • An increase temperature will increase the pol’n rate, generate heatdissipation and a tendency to develop of localized “hot spots”.

• Near the end of pol’n, the viscosity is very high and difficult to controlthe rate as the heat is “trapped” inside the termination rate <<<<,the propagation rate >>>>, the overall pol’n >>>>, heat production >>>.

autoacceleration process (Trommsdroff or gel effect).

This method is seldom used in commercial manufacture.E.g. PS and PMM

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.1.2. Solution polymerization

It requires the correct selection of the solvents. Both the initiator andmonomer be soluble in each other and that the solvent are suitable forchain-transfer characteristics and melting and boiling points, regardingthe solvent-removal steps.

Advantages:• Heat is removed during pol’n via solvent.• “Cheap” materials for the reactors (stainless steel or glass lined).

Disadvantages:• Small production per reactor volume.• Not suitable for dry polymers.• Difficult of complete solvent removal.

E.g. PVA, poly (acrylic acid), polyacrylamide.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.2. Polymerization in heterogeneous systems

6.2.1. Suspension polymerization

Suspension pol’n consists of an aqueous system with monomer as adispersed phase and results in polymer as a dispersed solid phase.

Method:A reactor fitted with a mechanical agitator is charged with a water-insoluble monomer and initiator (+ a chain-transfer agent to controlmolecular weight).Droplets of monomer (containing the initiator and chain-transfer agent)

are formed (∅50 – 200 µm). These sticky droplets are prevented bythe addition of a protective colloid (PVA). Near the end of pol’n, the particles are hardened, are then recovered by filtration, and followed by washing step.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

Advantages:• Excellent heat transfer because of the presence of the solvent.• Solvent cost and recovery operation are cheap.

Disadvantages:• Contamination by the presence of suspension and other additives

low polymer purity.• Reactor cost may higher than the solution cost.

E.g. PVC, PSAN, Poly(vinylidene chloride –VC)

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.2.2. Emulsion polymerization

An emulsion pol’n consists of water (as the heat-transfer agent), monomer, water-soluble initiator, a chain-transfer agent, a surfactant(such as sodium salt of a long-chain fatty acid fatty-acid soap).

Method:

The hydrophobic monomer molecules form droplets (∅0.5 – 10 µm). The fatty-acid soap forms aggregates of 50 – 100 soap moleculeswith a layered structure. This structure is called micelles.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

The hydrophobic monomer molecules form droplets (∅0.5 – 10 µm),which are surrounded by the surfactant molecules. The surfactantmolecules arrange themselves with hydrophilic ends point outwardand hydrophobic (aliphatic) ends point inward toward the monomerdroplets. This process generates free radicals in aqueous phase.The size of monomer droplets depends on the temperature pol’n andthe agitation rate.

As the polymer particles grow much larger than the original micellesand absorbs all the soap from the aqueous phase. The monomer droplets are unstable at the beginning. If the agitationstopped, the oil contains no polymer.When the polymer contains of 50% monomers (60 – 80% pol’n),both the monomer droplets and the left micelles disappear.The suspension of polymer particles in water is called a latex.Then the rate of pol’n is constant over the reaction.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

EmulsionPolymerization(Fried, 1995)

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

The kinetic of emulsion pol’n was analyzed by Smith and Ewart.The pol’n rate:

2

N[M]kR p0 =

The degree of pol’n:

where N is the number of micelles,[M] is the concentration of the monomer.

ρ

N[M]kx

p

n =

where ρ is the rate of generationof radicals.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

The Smith – Ewart kinetic is highly idealized and simple. Some factors that cannot be applied:

1. Large particles (∅ > 0.1 – 0.15 µm),2. Monomers with higher water solubility,3. Chain transfer to emulsifier.

An example of a commonly-used water-soluble initiator is the redoxpersulfate-ferrous initiator a radical sulfate anion

S2O8-2 + Fe+2 Fe+3 + SO4

-2 + SO4-•

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.3. Polymerization reaction engineering

Engineering factors of pol’n that relate to the unique polymer propertes.design of reactors.

The important parameters for reactors:1. Flow rate,2. Temperature,3. Compositions

Types of reactors:•Batch reactors,•Tubular-flow reactors,•Stirred-tank reactors.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

Figs 6.3 & 6.4 Billmeyer

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.4. Chemical reaction of polymers

Some important reactions:• Conversion of PVA through poly (vinyl alcohol) to a poly (vinyl acetal).• Linear condensation of PE and polyacrylates.• Nitration, sulfonation and recustion of PS are used to produce ion-exchange resins.

• Acetylation, nitration, and xanthation of cellulose.• Vulcanization of natural and synthetic rubbers.• Isomerization, cyclization, addition, epoxidation, and hydrogenation ofunsaturated polymers.

• Substitution on the main chain, side chain on the saturated polymers.• Terminally reactive polymers produce block copolymers.• Branching reactions produce graft copolymers.• Coupling reactions increase molecular weight of PU.• Crosslinking produces thermosetting polymers.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.4.1. Crosslinking reactions

Monomers with double bonds can be tailored.The 1st double bond monomer reacts into a polymer chain and leavesthe 2nd double bond unreactive. Then the 2nd double bond can produce branched of crosslinked polyemrs during pol’n.

The proper relative reactivity between the 2 double bonds can beselected. Only one double bond react in the pol’n and the other will react after the 1st. This condition will lead crosslinking pol’n.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.4.1.1. Crosslinking during polymerization

Example: copolymerization of methyl methacrylate and ethylene glycoldimethacrylate.

Consider a copolymerization of vinyl monomer A with divinyl monomerB – B.The i. c. equation:

Because the A and B groups are equally reactive, the i. c. equationbecomes a simple equation:

[A]

[B]

a

b=

where: [A], [BB], and [B] are the concentrations of A, B – B, and Bgroups respectively, and[B] = 2 [BB].

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

Given that:p is the fraction of the double bonds reacted, p [A] is the reacted A groups,(1 – p)[A] is the unreacted A groups,p2[BB] is the doubly reacted BB molecules,2p (1 – p) [BB] is single reacted BB molecules, and(1 – p)2[BB] is unreacted BB molecules.

The end quantities for B groups:(1 – p)2[B] is unreacted B groups on unreacted BB molecules,p(1 – p)[B] is unreacted of reacted B groups on singly reacted BB

molecules,p2[B] is reacted B groups on doubly reacted BB molecules.

In the mixture:p2[BB] is the number of crosslinks,x is the degree of pol’n,([A] + [B])/p is the total number of reacted units

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

In the mixture:number of chains:

At the onset of gelatin, the number of crosslinks per chain is ½.At critical reaction, pc:

x

[B])p([A]+

w

cx[B]

[B][A]p

+=

In a normal reaction, the distribution of the crosslinks is random.In a very low extent of reaction, the distribution of the crosslinks isnot random.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.4.1.2. Crosslinking after polymerization: vulcanization

The statistics of vulcanization, the distribution of molecular weights andcrosslinks are important in this step.The random crosslinking of polyfunctional monomers forms the [3]networks by step wise pol’n.

Define:q is the fraction of the monomer units on a chain that can be crosslinked,x is the degree of pol’n of the chain,p is the fraction of the total available crosslinks that have been formed,qx is the total number of vulcanizable groups (the functionality of thechain).

At the gelatin state, pc, the average one crosslink for every 2 chains is: pc q x = 1

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

After gelatin, the finite species constitute only part of the material.The rest is in the form of gel networks.As p increases from pc to 1, the weight fraction of the finite speciesdrops from unity to 0.

The gel point is given by: 1xqp wc =

where xw is the weight-average degree of plo’n.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.4.2. Degradation

Degradation means the reduction of molecular weight.There are 2 types of polymer degradation processes:1. Random degradation

It is a degradation by stepwise pol’n. Chain rupture of scission occursat random points, leaving fragments that are usually > a monomer unit.

2. Chain depolymerizationIt is a degradation by chain reaction. The successive releases monomerunits from a chain end in a depropagation or unizippering reaction

reverse chain pol’n.

They may occur separately or in combination and may be initiated bythermal or uv light, oxygen, ozone, or contaminated agents.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.4.2.1. Random degradation

In random degradation, the number of bonds broken per unit time isconstant as long as the total number of bonds present is largecompared to the number broken the number of chain endsincreases linearly with time (1/xn increases linearly with time).

Define:p is the number of bonds,(1 – p) is the number of broken bonds/total number of bonds.

Examples:Cellulose with an acid-catalyzed homogeneous reaction,Isobutylene-isoprene copolymer butyl rubber with an ozonolysis.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.4.2.2. Chain depolymerization

Chain depolymerization is a free-radical process that is essentiallythe reverse of chain polymerization. “Weak-line” may arise from achain defect.

Examples:Poly (methyl methacrylate), PS

6.4.2.3. Kinetics of degradation

Based on the inverse chain pol’n which includes the steps of initiation,depropagation, termination and chain transfer.

Two factors that determine the degradation, are:1. the reactivity of the depropagating radical,2. the availability of reactive hydrogen atoms for transfer.

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

Examples degradation of:

polyarcrylates and polyacrylonitrile because of α-hydrogens.poly (vinyl acetate) and poly (vinyl chloride) because of removalof side groups.

Example of degradation product:PS with average molecular weight of 264, degraded to 40% styrene,2.4% toluene, and other products (temp 360 – 420 ºC).

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

6.4.3. Radiation chemistry of polymers

Radiation involves high-energy interaction and some steps:excited and ionized then secondary electrons emitted with relatively lowspeeds and produce more ions along their tracks.

The mechanism is chain scission in 1,1-disubstituted polymers.The weight-average molecular weight decreases as the amount of radiation increases.

Examples:poly (methyl methacrylate) and its derivatives,

polyisobutylene, poly (α-methylstyrene),polymers containing halogen [ PVC, poly (vinylidene chloride),polytetrafluoroethylene].

Sem 1 2006/2007

Fisika Polimer

Ariadne L. Juwono

Crosslinking occurs on the irradiation of PS, PE, olefin polymers,polyacrylates and their derivatives, natural and synthetic rubbers.

The formation of trans-vinylene because of radiation crosslinking.