Wk 3 - Polymer synthesis

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Polymer synthesis


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Contents

Polymerization reactions

Step Reaction Polymerization

C

hain Polymerization Ionic Polymerization

Production of addition polymerization

Co-Polymerization


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Introduction to Polymer

Synthesis/Polymerization

Polymerization: process of bonding monomerstogether through a variety of reaction mechanisms toform longer chains

Polymers exist as a variety of 3-D shapes, each withspecific individual properties relevant to the monomersor reaction mechanisms they are formed from.

Monomers to form polymer must have functionality > 2


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Polymer Synthesis/Polymerization

Old Classification of Synthesis : based on Reaction Stoichiometric

(W.H Carothers 1929)

a) Condensation : Formation of by-product (e.g. elimination of smallmolecules such as water); weight loss (polymer repeat unit hasfewer atoms)

b) Addition Polymerization : No by-product, No loss of weight(polymer repeat unit same as monomer)

New Classification based on Mechanism (Paul Flory -1953)

a) Step-Growth (Step-Reaction): By functional group - All speciesgrow step by step

b) Chain-Growth (Chain-Reaction): By free radical or ions -Successive linking of monomers to the end of a growing chain


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Polymer synthesis - cont

E.g. Polyurethane polymerizes with addition

polymerization (because its polymerization don't

produce any small molecules, called

"condensate"), but its reaction mechanismconcern to a step-growth polymerization.

Mechanism of

chemical reaction of polyurethane polymerization catalyzed by tertiary amine


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Large number of step growth have basic structure--- -----R------ -------R------ -----

Where R can be (CH2)x or

Common functional groups (A, B)

Hydroxyl (-OH)

C

arboxyl (-C

OOH

) Amine (-NH2)

Ethylene oxide (-CH-CH2)

Isocyanate (-NCO)

can be one of the important group : ester, amide, urethane

Step Polymerization


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Step Polymerization - Characteristics


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nXDP!

No

= original no. of molecules present in monomer

N = no. of all remaining molecules after time tN

NXDP

o

n !!

p=0 forstartofreaction

p=1 completionofreaction (canneverbereached)


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SP Characteristic

8. Any two molecular species in mixture can react

9. Longer reaction time and high conversions are necessary forproduction of polymer with large xn

10. Reaction rates are slow at ambient temp but increases with risein temp (but little effect on chain length)

11. Growing chains may react with each other to form even longerchains

E.g.: Polyesterification reaction

HO-R-OH + HOOC-R-COOH HO-R-OCO-R-COOH + H2O

Reaction between diol and diacid monomers each has twoidentical functional groups (dimers)

Product has 2 different functional groups (difunctional)


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Step Polymerization -Gelation A monomer with functionality 3 will introduce branching in a polymer and

ultimately form a cross-linked macrostructure.

Gel Point is the point at which an infinite polymer 3-D network first appears.

Caused an abrupt change in viscosity of polymer, reaction medium losses allfluidity and bubbles cease to rise.

Assume we are able to measure the extent of reaction, p, defined as the fractionof monomers that appear in cross-links, we can determine the gel point.

The critical extent of reaction for the gel point to be formed is given by:

pc=1/(N-1)1/N

For example, a polymer with N200 is able to reach the gel point with only0.5% of monomers reacting.

This shows the ease at which polymers are able to form infinite networks.


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Kinetics of Step Polymerization


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MW control - importance

Very high MW of product will be difficult toprocess

Low MW polymer may not exhibit the desired

properties in the end product Thus

need to control amount of each species inmixture

High purity of reagents Need to stop reaction at required value ofp


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Stoichiometric imbalance

E.g.: to use excess of one monomersexcess of diamine over an acid chloride would produced apolyamide with two amine end groups incapable of further growthwhen the acid chloride was all consumed.


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Stoichiometric imbalance

r = ratio of number of molecules of reactants


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MW Distribution


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Polyester


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Polycarbonates:

PolyurethanesPolyurethanes are synthesized by the reaction of diols with diisocyanates:


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Chain Polymerization


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A monomer with double bond (vinyl, divinyl, 1,3diene)

Two growing chain cant join together like in steppoly.

can be very high

Monomer consumed relatively slow but high MWpolymers are achieved quickly

Initiation and propagation mechanisms are

different. Active centre always located at end of growing

chain

DP


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Two types

Mostly found with addition reactions butthere are exceptions (eg:

Chain/condensation polymerization ofdiazomethane)

1. Free radicals addition polymerization

2. Ionic polymerizations 2 types Cationic Anionic


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Vinyl polymerizations: converting a double bond into

two single bonds (exothermic).

E.g. ofChain Polymerization


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Free radical Polymerization

Initiation Step

Polymerization is initiated by formation of free radicals orinitiators

Free radicals are species containing an unpaired electron

Denoted as Ry

Free radicals can generated by

1. thermal breakdown of organic peroxides,Hyperperoxides, Azo,diazo compounds

2. photochemical decompostion of metal iodides, metal alkyls and azocompounds

3. Redox reactions

4. Persulphates decomposition in aqueous phase

5. Ionizing radiation such as E, F, K or x-rays

DEF: Initiators are molecules that break into radicals uponexposure to light (UV) or species generated from redox reaction


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Initiation - Free Radicals

A) Thermal Homolysis or decomposition

Useful for organic peroxides or azo compound like benzoyl peroxides

homolysis is chemical bond dissociation of a neutral molecule generating two free radicals.

That is, two electrons that are involved in the bond are distributed one by one to the two species.AB A + B


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Initiation Free radicals


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B) Photochemical Decomposition

Applicable to metal iodides, metal alkyls and azo compoundsEg AIBN decomposed by radiation with wavelength 360 nm


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C) Redox reactions


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D) Persulphates

Useful in emulsion polymerisation

Decomposition occurs in aqueous phase

Radical diffuses into a hydrophobic monomer containing droplets

Eg

S2O82- 2 SO4

y -

E) Ionizing Radiation

E, F, K or x-rays can be used to initiate a polymerization Cause the ejection of an electron, then dissociation and electron

capture to produce radical


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Free Radical Polymerization: Propagation

Process of adding monomer to a terminal free

radical reactive site (called active center)


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Free Radical Polymerization: Termination

Termination of chains can take place in several ways1. Interaction between two active chain ends

2. Reaction of an active chain end with an initiator radical

3. Termination by transfer of active centre to another

molecule (may be solvent, initiator or monomer )

4. Interactions with impurities (e.g. oxygen) or inhibitors

Most important termination reaction

Combination : Process ofCombining two chains, coupling

together at their ends (e.g. Polystyrene for temp > 333K)

Disproportionation: involves the abstraction of a hydrogen

atom from one end to give an unsaturated group and two

dead polymer chain (e.g. PMMA for T > 333K)


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Kinetics and Mechanism


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Kinetics and Mechanism


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Termination: Chain Transfer

Cases of premature termination.

Chain transfer is essential to continue chain reaction


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Termination : Inhibitor

Objective: To suppress polymerization


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Termination: Inhibitor


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ATRP involves the chain initiation of free radical polymerization by a halogenated

organic species in the presence of a metal halide species.

Transfer ofCl atom prevent termination


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Techniques of Free Radical Polymerizations

Industrial radical initiated

polymerizations can be carried out by

With monomer only bulk

In a solvent solution

With monomer dispersed in an aqueous

phase suspension

As emulsion See supplements notes for details

description


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Techniques of Free Radical Polymerizations


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Chain GrowthPolymerization Step GrowthPolymerization

The only growth reaction is addition of

monomer to a growing chain with a reactive

terminus

Reaction can occur independently

between any pair of molecular

species

The reaction mixture consists of high

polymer and unreacted monomers, with

very few actively growing chains

The reaction mixture consists of

oligomers of many sizes, in a

statistically calculable distribution

Monomer concentration decreases

steadily as reaction time increases

Monomers disappear early, in favor

of low oligomers

High polymer appears immediately,average molecular weight doesn't change

much as reaction proceeds

Oligomers steadily increase in size,polymer average molecular weight

increases as reaction proceeds

Increased reaction time increases overall

product yield, but doesn't affect polymer

average molecular weight

Increase T, increase reaction rate,

decrease molecular mass

Long reaction times are essential

to produce polymer with high

average molecular weight

Reaction rate slow at Tambient but

increase in T has little effect on

chain length of polymer


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Monomer Reactivity


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Education's purpose is to

replace an empty mind withan open one.

- Malcom S. Forbes

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Wk 3 - Polymer synthesis

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