Chapter 6. Polymerization conditions and polymer reactions

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

6.1. Polymerization in homogeneous systems

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

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

6.1.1. Bulk polymerization Advantages:

• Bulk polymerizations is the simplest technique and produces the highest-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.

Advantages: (continued)

• Easy polymer recovery and easy for cast polymerization into final product 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 heat dissipation and a tendency to develop of localized “hot spots”.

• Near the end of pol’n, the viscosity is very high and difficult to control the 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.

6.1.2. Solution polymerization

It requires the correct selection of the solvents. Both the initiator and monomer be soluble in each other and that the solvent are suitable for chain-transfer characteristics and melting and boiling points, regarding the 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.

6.2. Polymerization in heterogeneous systems

6.2.1. Suspension polymerization

Suspension pol’n consists of an aqueous system with monomer as a dispersed 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 control molecular weight).

Droplets of monomer (containing the initiator and chain-transfer agent) are formed (∅50 – 200 µm). These sticky droplets are prevented by the 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.

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)

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 molecules with a layered structure. This structure is called micelles.

The hydrophobic monomer molecules form droplets (∅0.5 – 10 µm), which are surrounded by the surfactant molecules. The surfactant molecules arrange themselves with hydrophilic ends point outward and hydrophobic (aliphatic) ends point inward toward the monomer droplets. This process generates free radicals in aqueous phase.

The size of monomer droplets depends on the temperature pol’n and the agitation rate.

As the polymer particles grow much larger than the original micelles and absorbs all the soap from the aqueous phase.

The monomer droplets are unstable at the beginning. If the agitation stopped, 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.

Emulsion

Polymerization (Fried, 1995)

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

The pol’n rate:

2 [M] N k

R

₀

=

_p

The degree of pol’n:

where N is the number of micelles,

[M] is the concentration of the monomer.

ρ

N[M]

x

_n

= k

^p

where ρ is the rate of generation of radicals.

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 redox persulfate-ferrous initiator a radical sulfate anion

S₂O₈^-2 + Fe⁺² Fe⁺³ + SO₄^-2 + SO₄^-•

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.

Figs 6.3 & 6.4 Billmeyer

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 of unsaturated 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.

6.4.1. Crosslinking reactions

Monomers with double bonds can be tailored.

The 1^st double bond monomer reacts into a polymer chain and leaves the 2^nd double bond unreactive. Then the 2^nd double bond can

produce branched of crosslinked polyemrs during pol’n.

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

6.4.1.1. Crosslinking during polymerization

Example: copolymerization of methyl methacrylate and ethylene glycol dimethacrylate.

Consider a copolymerization of vinyl monomer A with divinyl monomer B – B.

The i. c. equation:

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

[A]

[B]

a b =

where: [A], [BB], and [B] are the concentrations of A, B – B, and B groups respectively, and

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,

In the mixture:

number of chains:

At the onset of gelatin, the number of crosslinks per chain is ½.

At critical reaction, p_c:

x

[B])p ([A] +

w c

[B] x

[B]

p [A] +

=

In a normal reaction, the distribution of the crosslinks is random.

In a very low extent of reaction, the distribution of the crosslinks is not random.

6.4.1.2. Crosslinking after polymerization: vulcanization

The statistics of vulcanization, the distribution of molecular weights and crosslinks 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 the chain).

At the gelatin state, p_c, the average one crosslink for every 2 chains is:

p_c q x = 1

After gelatin, the finite species constitute only part of the material.

The rest is in the form of gel networks.

As p increases from p_c to 1, the weight fraction of the finite species drops from unity to 0.

The gel point is given by:

p

_c

q x

_w

= 1

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

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 occurs at random points, leaving fragments that are usually > a monomer unit.

2. Chain depolymerization

It is a degradation by chain reaction. The successive releases monomer units 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 by thermal or uv light, oxygen, ozone, or contaminated agents.

6.4.2.1. Random degradation

In random degradation, the number of bonds broken per unit time is constant as long as the total number of bonds present is large

compared to the number broken the number of chain ends increases linearly with time (1/x_n 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.

6.4.2.2. Chain depolymerization

Chain depolymerization is a free-radical process that is essentially the reverse of chain polymerization. “Weak-line” may arise from a chain 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,

Examples degradation of:

polyarcrylates and polyacrylonitrile because of α-hydrogens.

poly (vinyl acetate) and poly (vinyl chloride) because of removal of 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).

6.4.3. Radiation chemistry of polymers

Radiation involves high-energy interaction and some steps:

excited and ionized then secondary electrons emitted with relatively low speeds 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].

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.