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A. Different Polymerization processes:1. Step growth polymerization2. Chain growth polymerization

a. Free radical polymerizationb. Ionic polymerizationc. Coordination polymerization

Step-growth polymerization is a polymerization process that involves a chemical reactionbetween multifunctional monomers molecules. In a step-growth reaction, the growing

chains may react with each other to form even longer chains.

PPO is a heterochain homopolymer, which is created by a free-radical, step-growth, oxidative-coupling

polymerization of 2,6-dimethylphenol and oxygen.

Chain growth polymerization is a polymerization technique where unsaturated monomermolecules add on to a growing polymer chain one at a time. It is basically a three-stage

process, involving initiation of active molecules, their propagation and termination of the

active chain ends.

Thiophene polymerization using an external initiator

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o Free radical polymerization is a method of polymerization by which a polymer isformed from the successive addition of free radical building blocks.

o Ionic polymerization is a polymerization in which the propagating species is along chain cation or anion. Reactions are typically carried out in solvents of low

or moderated polarity and generally proceed much faster than radicalpolymerization.

Anionic polymerization

Cationic polymerization

o Coordination polymerization is a form of addition polymerization in whichmonomer adds to a growing macromolecule through an organometallic active

center.

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B. Characteristics of Step-growth and Chain growth polymerizationa. Differences:

Step-growth polymerizationo It uses multiple and very different reaction mechanism

Reactions at carbonyls Aromatic Nucleophilic Substitution; Aromatic Electrophilic

Substitution

Michael type additions Oxidative coupling reactions, etc.

o Products from step usually have heteroatoms (O,N,S) in main chaino Component reaction

Initiation, Propagation, Termination identical in rate and mechanismo Polymer growth

Coupling can occur between any 2 species (monomer, oligomer, orpolymer)all reactive.

Slow, random growth of MW, very high conversion required beforeANY polymer chains are formed.

All species reactive throughout polymerizationo Monomer concentration

Compared with chain final Xn~small ; Mn~40-60ko Heat of polymerization

Not very exothermic; some endothermic, most are heatedo Reaction mixture

At any time t, all molecular species present in calculable distribution. Chain growth polymerization

o Component reaction Initiation, Propagation, Termination are distinct steps with different

rates and mechanisms

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o Polymer growth Occurs by random addition of M to a very small number of active

chains.

Rapid increase In MW, high polymer forms immediately. Only very few chains grow at any point in time. Mostly monomer and

inactive chains.

o Monomer concentration Xn, Mn very high Mn~105106 common

o Heat of polymerization Very exothermic

o Reaction mixture Only M and high polymer, and ~ 10-8 mol/L growing chains.

b.

Similarities: All chains reactive throughout polymerization (both chain ends) Functional group reactivity does not change with MW Slow increase in MW HMW only after p > 99% Most step polymerizations proceed with production of volatile by-products Both have initiation, propagation, and termination

c. Advantages and Disadvantages:i. Step-growth polymerization Advantages

There is not heat generated during step growth polymerization soheat transfer is not a problem.

High Molecular Weight Excellent Optical Clarity and color Catalyst is not necessary Not necessarily need an initiator

Disadvantages By-product is produced Synthetic solvent is used

ii. Chain-growth polymerization Advantages

Organic solvent is used No by-products

Disadvantages Catalyst is necessary Is very exothermic so energy for heat control is high

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Initiator is necessary Low molecular weights compared to step-growth1. Free radical polymerization

a. ADVANTAGESi. Relatively insensitive to trace impurities

ii. Reactions can occur in aqueous mediaiii. Can use chain transfer to solvent to modify

polymerization process

b. DISADVANTAGESi. Structural irregularities are introduced during

initiation and termination steps

ii. Chain transfer reactions lead to reduced molecularweight and branching

iii. Limited control of tacticityiv. High pressure often required2. Anionic Polymerization

a. ADVANTAGESi. Narrow molecular weight distribution

ii. Limited chain transfer reactionsiii. Predictable molecular weight averageiv. Possibility of forming living polymersv. End groups can be tailored for further reactivity

b. DISADVANTAGESi. Solvent-sensitive due to possibility of chain transfer

to the solvent

ii. Can be slowiii. Sensitive to trace impuritiesiv. Narrow molecular weight distribution

3. Cationic polymerizationa. ADVANTAGES

i. Large number of reactive monomersii. High reactivity of active site, therefore quick

reaction times

iii. Reactions proceed rapidly even at low temperaturesb. DISADVATAGES

i. High reactivity leads to undesirable side reactionsii. Requires a very high purity reaction medium

iii. Chain transfer leading to low molecular weight andhigh branching

iv. Kinetic mechanisms are poorly understood

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v. Reaction often does not go to completion4. Coordination polymerization

a. ADVANTAGESi. Can engineer polymers with specific tacticities

based on the catalyst system

ii. Can limit branching reactionsiii. Polymerization can occur at low pressure and

modest temperatures

iv. Otherwise non-polymerizable monomers(e.g.,propylene) can be polymerized

b. DISADVANTAGESi. Mainly applicable to olefinic monomers