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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Kinetics of Step Polymerization
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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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Chain Polymerization
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Chain Polymerization
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Chain Polymerization
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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Chain Polymerization
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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Initiation - Free Radicals
B) Photochemical Decomposition
Applicable to metal iodides, metal alkyls and azo compoundsEg AIBN decomposed by radiation with wavelength 360 nm
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Initiation - Free Radicals
C) Redox reactions
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Initiation - Free Radicals
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: Chain Transfer
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Termination: Chain Transfer
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Termination: Chain Transfer
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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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