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Polymerization – chain & condensation
Dr. B. R. Thorat
Department of Chemistry
Government of Maharashtra
Government College of Arts and Science, Aurangabad
Unit 3:
Condensation polymerization:
Introduction, Monomers; Condensation polymerization, Mechanism, interfacial polycondensation, ladder polymers, Spiro
polymers.
Unit 4:
Chain Polymerization: Introduction
 Free radical Polymerization: Initiators – AIBN, peroxides- BPO, TBP, tert-Butyl hydroperoxide, Cumyl hydroperoxide,
peroxy acetic acid and per sulphate. Mechanism-Initiation, Propagation, modes of termination, Living Polymer,
inhibitors and retarders, ring opening polymerization of vinylcyclopropane derivatives.
 Cationic Polymerization: Initiators: Protic acids, Lewis acids; Mechanism: Initiation, propagation, modes of termination,
Living Polymer, ring opening polymerization cyclic ethers, acetals.
 Anionic Polymerization: Initiators- Hydroxides, Alkoxide, Alkali metals, Metal Hydrides, Organometallic compounds
and metal amides. Mechanism- Initiation, Propagation, Modes of termination, living polymer, ring opening
polymerization of lactam, lactone.
Polymerization is the synthetic process by means of which polymers are synthesized from low molecular weight units called
as monomer.
Chemistry of Polymerisations
There are many kinds of polymerization such as vinyl, diene, emulsion, bulk, cationic, anionic, etc. But we are interested here
only on the mechanism and kinetics of the polymerization. There are two types of polymerization reactions are addition
and condensation reactions based on functionality of the monomer.
Thus the condensation of glycerol
HOCH2CH(OH)CH2OH with adipic acid
HOOC(CH2)4COOH will give a branched structure.
Monomers such as adipic acid and hexamethylenediamine are described as bifunctional because they have two reactive
groups. As such they can only form linear polymers.
Similarly, the simple vinyl monomers such as ethylene
CH2=CH2 and vinyl acetate CH2=CHOOCCH3 are
considered to be bifunctional.
In both types of polymerization, branched chains and
cross net-works can be obtained by increasing the
functionality of the monomer i.e. by using divinyl
compounds and tri-functional or tetra-functional
alcohols or acids.
Condensation polymerization
Step Polymerization
• The polymer build-up proceeds through a reaction between functional groups of the bifunctional
monomers.
• The reaction takes place in a step-wise manner (i.e., one after another), and the polymer build-
up is, therefore, slow (unlike in chain polymerisation where the chain growth is very rapid).
• Although many known reactions with organic functional groups can be made use of in step
polymerisation, condensation, addition, ring opening, amidation and ester-interchange
reactions are the most commonly used ones.
• In step polymerisation reactions are accompanied by the elimination of small molecules.
Condensation polymerization
This type of polymerisation generally involves a repetitive condensation reaction (two molecules join together, resulting loss
of small molecules) between two bi-functional monomers. These polycondensation reactions may result in the loss of some
simple molecules as water, alcohol, etc., and lead to the formation of high molecular mass condensation polymers.
In these reactions, the product of each step is again a bi-functional species and the sequence of condensation goes on. Since,
each step produces a distinct functionalised species and is independent of each other; this process is also called as step growth
polymerisation.
Monomers with only one reactive group terminate a growing chain,
and thus give end products with a lower molecular weight.
Polyester is created through ester linkages between monomers
The formation of polyester like terylene or dacron by the interaction of ethylene glycol and terephthalic acid.
Polyamide is created through amide linkages between monomers
Nylon-6 is an example which can be manufactured by the condensation polymerisation of hexamethylenediamine with
adipic acid under high pressure and at high temperature.
1. Those monomers should have two reactive
functional groups for polymerisation to proceed.
2. That polymerisation proceeds by step-wise
reaction between reactive functional groups.
3. That only one type of reaction (i.e.,
condensation reaction, in this case) between two
functional groups is involved in polymer formation.
4. That the polymer formed still contains both the
reactive functional groups at its chain ends (as
end group) and, hence, is 'active' and not 'dead',
as in chain polymerization.
For polycondensation the monomer should have:
A diacid can be allowed to react with a diol in the
presence of an acid catalyst to afford polyester.
In this case, chain growth is initiated by the reaction of one of the
diacid's carboxyl groups with one of the diol's hydroxyl groups.
The free carboxyl or hydroxyl group of the
resulting dimer can then react with an
appropriate functional group in another
monomer or dimer. This process is repeated
throughout the polymerization mixture
until all of the monomers are converted to
low molecular weight species, such as
dimers, trimers, tetramers, etc.
These molecules, which are called oligomers, can then further
react with each other through their free functional groups.
Polymer chains that have moderate molecular weight can he built
in this manner.
The following are several general characteristics of this type of polymerization:
1) The polymer chain forms slowly, sometimes requiring several hours to several days,
(2) All of the monomers are quickly converted to oligomers, thus, the concentration of growing chains is high,
(3) Since most of the chemical reactions employed have relatively high energies of activation, the
polymerization mixture is usually heated to high temperatures,
(4) Step-reaction polymerizations normally afford polymers with moderate molecular weights, i.e., < 100,000
(5) Branching or crosslinking does not occur unless a monomer with three or more functional groups is used.
Some cross linked polymers are also synthesized. Anhydride was condensed with triols.
Alkyds were synthesized by Kienle in
the 1920s from trifunctional alcohols
and dicarboxylic acids.
Unsaturated oils called drying oils were transesterified with the phthalic anhydride in
the reaction so that an unsaturated polyester was obtained.
Other examples are – Polyurethane, Polyurea, Silkfibroin, Cellulose, phenol-aldehyde resin, urea-aldehyde resin, polysulfide,
polysiloxane, etc.
Polyamide:
Polyacetal:
Polyanhydride:
Interfacial Condensation
The reaction of an acid halide with a glycol or a diamine proceeds rapidly to high-molecular-weight polymer if carried out
at the interface between two liquid phases, each containing one of the reactants.
Very-high-molecular-weight polymer can be formed. Typically, an aqueous phase containing the diamine or glycol and an
acid acceptor is layered over an organic phase containing the acid chloride at room temperature .
The polymer formed at the interface can be pulled off as a continuous film or filament.
The method has been applied to the formation of polyamides, polyurethanes, polyureas, polysulfonamides, and
polyphenyl esters.
It is particularly useful for preparing polymers that are unstable at the higher temperatures usual in step reaction
polyrnerization.
Ladder polymers
A typical example is poly(imidazopyrrolone),
which is obtained by the polymerization of
aromatic dianhydrides such as pyromellitic
dianhydride or aromatic tetracarboxylic acids with
orthoaromatic tetramines like 1,2,4,5-
tetraaminobenzene.
This constitute a group of polymers with a regular sequence of cross-links. A ladder polymer consists of two parallel linear
strands of molecules with a regular sequence of crosslinks.
Ladder polymers have only condensed cyclic units in the chain; they are also commonly referred to as double-chain or
double-strand polymers.
Their thermal stability is due to the molecular structure, which requires that two bonds must be broken at a cleavage site
in order to disrupt the overall integrity of the molecule; when only one bond is broken, the second holds the entire
molecule together.
The molecular structure of ladder polymers is more rigid than that of conventional linear polymers. Many such polymers
display exceptional thermal, mechanical, and electrical behaviour.
Spiro polymers:
An example of a spiro polymer is the polyspiroketal synthesized from 1,4-cyclohexanedione and pentaerythritol. The low
solubility and intractability of spiro polymers makes it difficult to synthesize or utilize high-molecular-weight polymers.
Polymer chains based on spiro structures have been studied as another route to heat-resistant polymers.
A spiro structure is a double-strand structure in which the uninterrupted sequence of rings have one atom in common
between adjacent rings. (Adjacent rings in ladder polymers have two or more atoms in common.)
Addition polymerization:
Addition polymerization can be takes place by chain mechanism. Here polymer is formed by progressive linking of the
monomers into larger ones and these again continue to grow by further coupling of monomers.
Chain Growth Polymerisation
1. The monomers contain at least a one double bond that participates in the
polymerisation reaction.
2. The initiators for chain polymerisation may be different depending upon the
nature of initiation i.e. free radical or ions (cation and anion).
3. The carbon-carbon double bond in vinyl monomers and the carbon-oxygen
double bond in aldehydes and ketones are the two main types of linkages
that undergo chain polymerisation.
4. Once the initiation occurs, the polymer chains form very quickly i.e. in the time
scale of 10-1 to 10-6 s.
5. The catalyst concentration needed is very low i.e. during the course of
polymerisation only monomers and polymer are present,
6. The process is exothermic
7. High polymers with molecular weights of 10,000 to 10 million can be obtained,)
e.g. ethylene (CH2=CH2), vinyl chloride
(CH2=CHCl), styrene (Ph-CH=CH2),
butadiene (CH2=CH-CH=CH2), etc.
The mechanism of addition polymerization
involves in three basic steps as- initiation,
propagation and termination of chain.
C6H5CH=CH2 and the vinyl ethers
CH2=CHOR, formaldehyde, acetone,
etc.
Free radical polymerisation
A free radical may be defined as an intermediate compound formed during the polymerization containing an odd number of
electrons, but which do not carry an electric charge and are not free ions.
Initiators in free radical polymerisation:
Initiators of the type required for vinyl polymerisations are formed from compounds with relatively weak valency links which
are relatively easily broken thermally. Irradiation of various wavelengths is sometimes employed to generate the radicals
from an initiator.
Thermal polymerization initiators
azo compounds such as 2,2'-azobis(isobutyronitrile) (AIBN) and organic peroxides such as benzoyl peroxide (BPO)
are well-known thermal radical initiators, and
benzenesulfonic acid esters and alkylsulfonium salts have been developed as thermal cation initiators
The first stable free radical, triphenylmethyl (C6H5)3C•, was isolated by Gomberg in 1900, and in gaseous reactions the
existence of radicals such as methyl •CH3 was postulated at an early date.
The decomposition of azodiisobutyronitrile leads to the
formation of radicals from this initiator is accelerated by
irradiation:
It decomposes into two radicals, (CH3)2C•CN and nitrogen N2
Dibenzoyl peroxide, which decomposes in two stages:
the phenyl radical C6H5
• that adds on to the monomer
Dibenzoyl peroxide decomposes at a rate suitable for most direct polymerisations in bulk, solution and aqueous
media, whether in emulsion or bead form, since most of these reactions are performed at 60–100°C.
Dibenzoyl peroxide has a half-life of 5 h at 77°C.
o-, m- and p-bromobenzoyl peroxides, in which the bromine atoms are useful as markers to show the fate of the
radicals. Dilauroyl hydroperoxide C11H23CO.OO.OCC11H23 has been used technically.
Hydroperoxides as represented by t-butyl hydroperoxide (CH3)3COOH and cumene hydroperoxide C6H5C(CH3)OOH represent
an allied class with technical interest:
These hydroperoxides are of interest in redox initiators.
Dialkyl peroxides of the type di-t-butyl peroxide (CH3)3C.O.O.C(CH3)3 are also of considerable interest, and tend to be subject
to less side reactions except for their own further decomposition as shown below:
These peroxides are useful for polymerisations that take place at 100–120 °C.
A number of peresters are in commercial production, e.g. t-butyl perbenzoate (TBPB) (CH3)3C.O.O.OC.C6H5, which acts as a
source of t-butoxy radicals at a lower temperature than di-t -butyl hydroperoxide, and also as a source of benzoyloxy radicals
at high temperatures.
Some tertiary phosphines were reported as initiator/catalyst such as tributylphosphine (TBP) and dimethylphenylphosphine
(DMPP).
Hydrogen peroxide H2O2 is the simplest compound in this class and is available technically as a 30–40% solution.
Initiation is not caused by the simple decomposition H2O2 ⇆ 2 OH•, but the presence of a trace of ferrous ion, of the order
of a few parts per million of water present, seems to be essential, and radicals are generated according to the Haber–Weiss
mechanism:
The hydroxyl radical formed commences a polymerisation chain in the usual manner and is in competition with a second
reaction that consumes the radical:
Major class of water-soluble initiators consists of the persulfate salts, i.e. salts of persulfuric acid H2S2O8. Potassium
persulfate K2S2O8 is the least soluble salt than sodium persulfate Na2S2O8 or ammonium persulfate (NH4)2S2O8.
The decomposition of persulfate may be regarded as thermal dissociation of sulfate ion radicals:
A secondary reaction may, however, produce hydroxyl radicals by reaction with water, and these hydroxyls may be the true
initiators:
Free radical Polymerization:
Let R be a radical from any source called initiator. CH2=CHX represents a simple vinyl monomer where X is a substituent,
which may be H as in ethylene CH2=CH2, Cl as in vinyl chloride CH2=CHCl, OOCCH3 as in vinyl acetate
CH2=CHOOCCH3 or many other groups, which will be indicated in lists of monomers.
The initiation process, consists of the attack of the free radical (obtained
from initiator) on one of the doubly bonded carbon atoms of the
monomer. One electron of the double bond pairs with the odd electron of
the free radical to form a bond between the latter and one carbon atom
other electron shift next carbon forming new free radical.
The new free radical can,
however, in its turn add on extra
monomer units, and a chain
reaction occurs, representing the
propagation stage:
The final stage is termination, which may take place by one of several processes.
Combination:
Disproportionation: Disproportionation in which hydrogen transfer results in the formation of two molecules with one
saturated and one unsaturated end group.
Chain Transfer: This is not a complete termination reaction, but it ends the propagation of a growing chain and enables a
new one to commence.
The reaction involves the transfer of an atom
between the radical and the molecule.
Chain transfer takes place very often via a
fortuitous impurity or via a chain transfer agent
which is deliberately added - Alkyl mercaptans
Living Radical Polymerization:
Bimolecular termination and chain transfer are ever present in radical chain polymerization and limit the lifetime of
propagating radicals.
Chain polymerizations without chain-breaking reactions, referred to as living polymerizations, would be highly desirable
because they would allow the synthesis of block copolymers by the sequential addition of different monomers.
A reactive species I* initiates polymerization of monomer A.
𝐼∗
+ 𝐴 → 𝐼 − 𝐴𝑛
∗
+ 𝐵 → 𝐼 − 𝐴𝑛 − 𝐵𝑛
∗
When the polymerization of monomer A is complete, the reactive centers are intact because of the absence of chain-breaking
reactions. Addition of a second monomer B results in the formation of a block copolymer containing a long block of A repeat
units followed by a long block of B repeat units.
It is possible to continue the sequential propagation by adding a third batch of monomer, or one can terminate the process by
the addition of a reagent to terminate the propagating centers.
Block copolymers have commercial potential for obtaining products that can incorporate the desirable properties of two or
more homopolymers.
The situation is very different for conventional (nonliving) radical polymerizations since the lifetime of propagating radicals in
these systems is very short (typically less than a second or, at most, a few seconds) because of the ever-present occurrence of
normal bimolecular termination (coupling and/or disproportionation). The lifetime enhances by by either reversible
termination or reversible transfer..
Living radical polymerization (LRP) with reversible termination generally proceeds as follows:
The initiator RZ undergoes homolytic bond breakage,
to produce one reactive and one stable free radical.
The reactive radicals quickly initiate polymerization,
but the stable radicals are too stable to initiate
polymerization. Once polymerization over, Z terminate
reversibly the growth of chain.
The stable radical acts as a controlling or mediating agent because it is sufficiently reactive to couple rapidly with
propagating chains to convert them reversibly into dormant, nonpropagating species. The stable radical is often called the
persistent radical, and its suppression of bimolecular termination between living polymers is called the persistent radical
effect (PRE).
Retardation and inhibition
A retarder is defined as a substance that can react with a radical to form
products incapable of adding monomer.
It both reduces the concentration of radicals and shortens their average
lifetime and thus the length the polymer chain.
In the simplest case, the retarder may be a free radical, such as
triphenylmethyl or diphenylpicrylhydrazyl, which is too unreactive to initiate a
polymer chain.
The mechanism of retardation is simply the combination or
disproportionation of radicals.
free radical 2,2-diphenyl-1-
picryl hydrazyl
Some other compounds, such as phenols, or even molecular oxygen, are inhibit the polymerization; and transforming the
propagating radical to an oxygencentered radical that is unable to initiate polymerization. Tetramethylpiperidinyloxy
(TEMPO) are two examples of radicals used to inhibit the radical polymerization.
Inhibitors are useful in determining initiation rates; since their reaction with radicals is so rapid that the
decomposition of inhibitor is independent of its concentration but gives directly the rate of generation of radicals.
As a result, the length of the induction period before polymerization starts is directly proportional to the number of
inhibitor molecules initially present to one radical.
If the addition of a chain transfer agent to a polymerising system works efficiently, it will both slow the polymerisation rate
and reduce the molecular weight because the free radical formed after termination may be much less active than the
original radical (polymer chain) in starting new chains, and when these are formed, they are terminated after a relatively
short growth. e.g. termination by mercaptons.
p-benzoquinone produces radicals that are
resonance stabilised and are removed from a
system by mutual combination or
disproportionation. Only a small amount of
inhibitor is required to stop polymerisation of a
system.
Aromatic compounds such as nitrobenzene C6H5NO2 and the dinitrobenzenes (o-, m-, p-)C6H4(NO2)2 are retarders for most
monomers, e.g. styrene, but tend to inhibit vinyl acetate polymerisation, since the monomer produces very active radicals
which are not resonance stabilized
Ring opening polymerization of vinylcyclopropane derivatives
Ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer chain acts as
a reactive center where further cyclic monomers can react by opening its ring system and form a longer polymer chain.
Some cyclic monomers such as norbornene or cyclooctadiene can be polymerized to high molecular weight polymers by
using metal catalysts.
The driving force for the ring-opening of cyclic monomers is via the relief of bond-angle strain or steric repulsions between
atoms at the center of a ring.
Mechanisms of ring opening polymerization:
di-functional monomers
anionic ring-opening polymerization (AROP):
Two examples are caprolactam and caprolactone
A typical example of anionic ROP is that of ε-caprolactone, initiated by an alkoxide functional group.
Radical ring opening polymerization
Ring-opening polymerization can also proceed via free radical polymerization.
The introduction of an oxygen into the ring will usually promote free radical ring-opening polymerization, because the
resulting carbon–oxygen double bond is much more stable than a carbon-carbon double bond.
Thus, cyclic hetero monomers that carry a vinyl side group like cyclic ketene acetals, cyclic ketene aminals, cyclic vinyl
ethers, and unsaturated spiro ortho esters will readily undergo free radical ring-opening polymerization.
The terminal vinyl group accepts a
radical. The radical will be transformed
into a carbon radical stabilized by
functional groups (i.e. halogen,
aromatic, or ester groups). This will lead
to the generation of an internal olefin.
Cationic ring-opening polymerization (CROP)
A small amount of an electrophilic reagent (Lewis acid) is added to the monomer to initiate polymerization.
A cyclic monomer undergoes CROP depends on the ring size, to be more specific, on the ring strain.
Cyclic monomers with small or no ring strain will not polymerize whereas small rings with greater ring strain like 4, 6, and
7- membered rings of cyclic esters, ethers, lactones, lactams, and epoxides polymerize readily through CROP.
Cationic polymerization
Use of cationic initiators which include reagents capable of providing positive ions or H+ ions. Typical examples are
aluminium chloride with water (AlCl3+H2O) or boron trifluoride with water (BF3+H2O).
They are effective with monomers containing electron releasing groups like methyl (-CH3) or phenyl (-C6H5) etc. They include
propylene (CH3CH=CH2) and the styrene (C6H5CH=CH2).
i) Chain Initiation
Initiators: Protonic acids such as HBr, HI, sulfuric acid, phosphoric acid, perchloric acid, trifluoromethyl sulfonic acid, etc.
and Lewis acid such as AlCl3, AlBr3, BF3, SnCl4, TiCl4, with a co-initiator (water, protonic acids, alkyl halides) is needed to
activate the Lewis acid.
𝐵𝐹3 + 𝐻2𝑂 → 𝐵𝐹3 − 𝐻2𝑂
𝐵𝐹3 − 𝐻2𝑂 + 𝐶𝐻3 2𝐶 = 𝐶𝐻2 → 𝐶𝐻3 3𝐶+ 𝐵𝐹3𝑂𝐻 −
Decomposition of the initiator is shown as BF3 + H2O → H+ + BF3(OH–). The proton (H+) adds to C – C double bond of alkene
to form stable carbocation. Where, G is electron donating group.
ii) Chain Propagation
Carbocation add to the C – C double bond of another monomer molecule to from new carbocation.
iii) Chain Termination
Reaction is terminated by combination of carbocation with negative ion (or) by loss of proton.
Anionic polymerization
Use of anionic initiators which include reagents capable of providing negative ions.
Typical catalysts include sodium in liquid ammonia, alkali metal alkyls, Grignard reagents and triphenylmethyl sodium
[(C6H5)3C-Na].
They are effective with monomers containing electron withdrawing groups like nitrile (–CN) or chloride (-Cl), etc. They
include acrylonitrile [CH2=C(CN)], vinyl chloride [CH2=C(Cl)], methyl methacrylate [CH2=C(CH3)COOCH3], etc.
Ability of substituents to stabilize carbanions decreases as: -NO2 > -C=O > -SO2 > -CO2 ~ -CN > -SO > -Ph ~ -CH=CH2 >>> -CH3.
i) Chain Initiation
Base Initiators are often organometallic compounds or salt of a strong base, such as an alkali metal alkoxide or alacoholates
(tert-BuOLi, tert-BuOK), Sodium metal in tetrahydrofuran, sodium or Potassium with liquid ammonia, Stable alkali metal
complexes may be formed with aromatic compounds (e.g. Na/naphthalene) in ether, alkyl amides such as NaNH2 or KNH2, .
Organolithium salts (n-BuLi, sec-BuLi, tert-BuLi), Grignard reagent
Potassium amide (K+NH2
-) adds to C – C double bond of alkene to form stable carbanion. Where, W is electron withdrawing
group.
ii) Chain Propagation
Carbanion adds to the C – C double bond of another monomer molecule to from new carbanion.
Anionic polymerization has no chain termination reaction. So it is called living polymerization.
The first living polymerizations were achieved in anionic polymerizations of styrenes and 1,3-dienes in the mid-1950s
A general mechanism for living anionic polymerization of a vinyl monomer is shown below, encompassing only initiation and
propagation steps; chains are terminated only by the deliberate addition of a Bronsted acid or an electrophile.
Initiation:
Propagation:
Deliberate Termination (no spontaneous termination):
For any living polymerization, one
initiator generates one polymer
chain and that the product after all
of the monomer has been
consumed is a polymer with an
active anionic chain end.
The number average molecular weight
(Mn) in living anionic polymerization is a
simple function of the stoichiometry and
the degree of monomer conversion, since
one polymer is formed for each initiator
molecule.
In many cases, the impurities react
quickly with initiators, especially
alkyllithium initiators, so that
molecular weight control can still be
achieved by determining the
impurity level
Anionic polymerization of styrene using sodium naphthalene as initiator.
Initiation
Propagation:
Monomers used for different polymerization:
Monomer Radical Anionic Cationic  Almost all substituents allow
resonance delocalization.
 Electron-withdrawing
substituents lead to anionic
mechanism.
 Electron-donating substituents
lead to cationic mechanism.
Ethylene + - +
Propylene - - -
1-butene - - -
Isobutene - - +
1,3-Butadiene + + -
Isoprene + + -
Styrene + + +
Vinyl chloride + - -
Acrylonitrile + + -
Methacrylate ester + + -

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Polymerisation reactions and synthesis of important polymers

  • 1. Polymerization – chain & condensation Dr. B. R. Thorat Department of Chemistry Government of Maharashtra Government College of Arts and Science, Aurangabad
  • 2. Unit 3: Condensation polymerization: Introduction, Monomers; Condensation polymerization, Mechanism, interfacial polycondensation, ladder polymers, Spiro polymers. Unit 4: Chain Polymerization: Introduction  Free radical Polymerization: Initiators – AIBN, peroxides- BPO, TBP, tert-Butyl hydroperoxide, Cumyl hydroperoxide, peroxy acetic acid and per sulphate. Mechanism-Initiation, Propagation, modes of termination, Living Polymer, inhibitors and retarders, ring opening polymerization of vinylcyclopropane derivatives.  Cationic Polymerization: Initiators: Protic acids, Lewis acids; Mechanism: Initiation, propagation, modes of termination, Living Polymer, ring opening polymerization cyclic ethers, acetals.  Anionic Polymerization: Initiators- Hydroxides, Alkoxide, Alkali metals, Metal Hydrides, Organometallic compounds and metal amides. Mechanism- Initiation, Propagation, Modes of termination, living polymer, ring opening polymerization of lactam, lactone.
  • 3. Polymerization is the synthetic process by means of which polymers are synthesized from low molecular weight units called as monomer. Chemistry of Polymerisations
  • 4. There are many kinds of polymerization such as vinyl, diene, emulsion, bulk, cationic, anionic, etc. But we are interested here only on the mechanism and kinetics of the polymerization. There are two types of polymerization reactions are addition and condensation reactions based on functionality of the monomer.
  • 5. Thus the condensation of glycerol HOCH2CH(OH)CH2OH with adipic acid HOOC(CH2)4COOH will give a branched structure. Monomers such as adipic acid and hexamethylenediamine are described as bifunctional because they have two reactive groups. As such they can only form linear polymers. Similarly, the simple vinyl monomers such as ethylene CH2=CH2 and vinyl acetate CH2=CHOOCCH3 are considered to be bifunctional. In both types of polymerization, branched chains and cross net-works can be obtained by increasing the functionality of the monomer i.e. by using divinyl compounds and tri-functional or tetra-functional alcohols or acids.
  • 6. Condensation polymerization Step Polymerization • The polymer build-up proceeds through a reaction between functional groups of the bifunctional monomers. • The reaction takes place in a step-wise manner (i.e., one after another), and the polymer build- up is, therefore, slow (unlike in chain polymerisation where the chain growth is very rapid). • Although many known reactions with organic functional groups can be made use of in step polymerisation, condensation, addition, ring opening, amidation and ester-interchange reactions are the most commonly used ones. • In step polymerisation reactions are accompanied by the elimination of small molecules.
  • 7.
  • 8. Condensation polymerization This type of polymerisation generally involves a repetitive condensation reaction (two molecules join together, resulting loss of small molecules) between two bi-functional monomers. These polycondensation reactions may result in the loss of some simple molecules as water, alcohol, etc., and lead to the formation of high molecular mass condensation polymers. In these reactions, the product of each step is again a bi-functional species and the sequence of condensation goes on. Since, each step produces a distinct functionalised species and is independent of each other; this process is also called as step growth polymerisation. Monomers with only one reactive group terminate a growing chain, and thus give end products with a lower molecular weight. Polyester is created through ester linkages between monomers The formation of polyester like terylene or dacron by the interaction of ethylene glycol and terephthalic acid. Polyamide is created through amide linkages between monomers Nylon-6 is an example which can be manufactured by the condensation polymerisation of hexamethylenediamine with adipic acid under high pressure and at high temperature.
  • 9. 1. Those monomers should have two reactive functional groups for polymerisation to proceed. 2. That polymerisation proceeds by step-wise reaction between reactive functional groups. 3. That only one type of reaction (i.e., condensation reaction, in this case) between two functional groups is involved in polymer formation. 4. That the polymer formed still contains both the reactive functional groups at its chain ends (as end group) and, hence, is 'active' and not 'dead', as in chain polymerization. For polycondensation the monomer should have:
  • 10. A diacid can be allowed to react with a diol in the presence of an acid catalyst to afford polyester. In this case, chain growth is initiated by the reaction of one of the diacid's carboxyl groups with one of the diol's hydroxyl groups. The free carboxyl or hydroxyl group of the resulting dimer can then react with an appropriate functional group in another monomer or dimer. This process is repeated throughout the polymerization mixture until all of the monomers are converted to low molecular weight species, such as dimers, trimers, tetramers, etc. These molecules, which are called oligomers, can then further react with each other through their free functional groups. Polymer chains that have moderate molecular weight can he built in this manner.
  • 11. The following are several general characteristics of this type of polymerization: 1) The polymer chain forms slowly, sometimes requiring several hours to several days, (2) All of the monomers are quickly converted to oligomers, thus, the concentration of growing chains is high, (3) Since most of the chemical reactions employed have relatively high energies of activation, the polymerization mixture is usually heated to high temperatures, (4) Step-reaction polymerizations normally afford polymers with moderate molecular weights, i.e., < 100,000 (5) Branching or crosslinking does not occur unless a monomer with three or more functional groups is used.
  • 12. Some cross linked polymers are also synthesized. Anhydride was condensed with triols. Alkyds were synthesized by Kienle in the 1920s from trifunctional alcohols and dicarboxylic acids. Unsaturated oils called drying oils were transesterified with the phthalic anhydride in the reaction so that an unsaturated polyester was obtained. Other examples are – Polyurethane, Polyurea, Silkfibroin, Cellulose, phenol-aldehyde resin, urea-aldehyde resin, polysulfide, polysiloxane, etc.
  • 14. Interfacial Condensation The reaction of an acid halide with a glycol or a diamine proceeds rapidly to high-molecular-weight polymer if carried out at the interface between two liquid phases, each containing one of the reactants. Very-high-molecular-weight polymer can be formed. Typically, an aqueous phase containing the diamine or glycol and an acid acceptor is layered over an organic phase containing the acid chloride at room temperature . The polymer formed at the interface can be pulled off as a continuous film or filament. The method has been applied to the formation of polyamides, polyurethanes, polyureas, polysulfonamides, and polyphenyl esters. It is particularly useful for preparing polymers that are unstable at the higher temperatures usual in step reaction polyrnerization.
  • 15. Ladder polymers A typical example is poly(imidazopyrrolone), which is obtained by the polymerization of aromatic dianhydrides such as pyromellitic dianhydride or aromatic tetracarboxylic acids with orthoaromatic tetramines like 1,2,4,5- tetraaminobenzene. This constitute a group of polymers with a regular sequence of cross-links. A ladder polymer consists of two parallel linear strands of molecules with a regular sequence of crosslinks. Ladder polymers have only condensed cyclic units in the chain; they are also commonly referred to as double-chain or double-strand polymers. Their thermal stability is due to the molecular structure, which requires that two bonds must be broken at a cleavage site in order to disrupt the overall integrity of the molecule; when only one bond is broken, the second holds the entire molecule together. The molecular structure of ladder polymers is more rigid than that of conventional linear polymers. Many such polymers display exceptional thermal, mechanical, and electrical behaviour.
  • 16. Spiro polymers: An example of a spiro polymer is the polyspiroketal synthesized from 1,4-cyclohexanedione and pentaerythritol. The low solubility and intractability of spiro polymers makes it difficult to synthesize or utilize high-molecular-weight polymers. Polymer chains based on spiro structures have been studied as another route to heat-resistant polymers. A spiro structure is a double-strand structure in which the uninterrupted sequence of rings have one atom in common between adjacent rings. (Adjacent rings in ladder polymers have two or more atoms in common.)
  • 17. Addition polymerization: Addition polymerization can be takes place by chain mechanism. Here polymer is formed by progressive linking of the monomers into larger ones and these again continue to grow by further coupling of monomers. Chain Growth Polymerisation 1. The monomers contain at least a one double bond that participates in the polymerisation reaction. 2. The initiators for chain polymerisation may be different depending upon the nature of initiation i.e. free radical or ions (cation and anion). 3. The carbon-carbon double bond in vinyl monomers and the carbon-oxygen double bond in aldehydes and ketones are the two main types of linkages that undergo chain polymerisation. 4. Once the initiation occurs, the polymer chains form very quickly i.e. in the time scale of 10-1 to 10-6 s. 5. The catalyst concentration needed is very low i.e. during the course of polymerisation only monomers and polymer are present, 6. The process is exothermic 7. High polymers with molecular weights of 10,000 to 10 million can be obtained,) e.g. ethylene (CH2=CH2), vinyl chloride (CH2=CHCl), styrene (Ph-CH=CH2), butadiene (CH2=CH-CH=CH2), etc. The mechanism of addition polymerization involves in three basic steps as- initiation, propagation and termination of chain. C6H5CH=CH2 and the vinyl ethers CH2=CHOR, formaldehyde, acetone, etc.
  • 18. Free radical polymerisation A free radical may be defined as an intermediate compound formed during the polymerization containing an odd number of electrons, but which do not carry an electric charge and are not free ions. Initiators in free radical polymerisation: Initiators of the type required for vinyl polymerisations are formed from compounds with relatively weak valency links which are relatively easily broken thermally. Irradiation of various wavelengths is sometimes employed to generate the radicals from an initiator. Thermal polymerization initiators azo compounds such as 2,2'-azobis(isobutyronitrile) (AIBN) and organic peroxides such as benzoyl peroxide (BPO) are well-known thermal radical initiators, and benzenesulfonic acid esters and alkylsulfonium salts have been developed as thermal cation initiators
  • 19. The first stable free radical, triphenylmethyl (C6H5)3C•, was isolated by Gomberg in 1900, and in gaseous reactions the existence of radicals such as methyl •CH3 was postulated at an early date. The decomposition of azodiisobutyronitrile leads to the formation of radicals from this initiator is accelerated by irradiation: It decomposes into two radicals, (CH3)2C•CN and nitrogen N2 Dibenzoyl peroxide, which decomposes in two stages: the phenyl radical C6H5 • that adds on to the monomer Dibenzoyl peroxide decomposes at a rate suitable for most direct polymerisations in bulk, solution and aqueous media, whether in emulsion or bead form, since most of these reactions are performed at 60–100°C. Dibenzoyl peroxide has a half-life of 5 h at 77°C. o-, m- and p-bromobenzoyl peroxides, in which the bromine atoms are useful as markers to show the fate of the radicals. Dilauroyl hydroperoxide C11H23CO.OO.OCC11H23 has been used technically.
  • 20. Hydroperoxides as represented by t-butyl hydroperoxide (CH3)3COOH and cumene hydroperoxide C6H5C(CH3)OOH represent an allied class with technical interest: These hydroperoxides are of interest in redox initiators. Dialkyl peroxides of the type di-t-butyl peroxide (CH3)3C.O.O.C(CH3)3 are also of considerable interest, and tend to be subject to less side reactions except for their own further decomposition as shown below: These peroxides are useful for polymerisations that take place at 100–120 °C.
  • 21. A number of peresters are in commercial production, e.g. t-butyl perbenzoate (TBPB) (CH3)3C.O.O.OC.C6H5, which acts as a source of t-butoxy radicals at a lower temperature than di-t -butyl hydroperoxide, and also as a source of benzoyloxy radicals at high temperatures. Some tertiary phosphines were reported as initiator/catalyst such as tributylphosphine (TBP) and dimethylphenylphosphine (DMPP).
  • 22. Hydrogen peroxide H2O2 is the simplest compound in this class and is available technically as a 30–40% solution. Initiation is not caused by the simple decomposition H2O2 ⇆ 2 OH•, but the presence of a trace of ferrous ion, of the order of a few parts per million of water present, seems to be essential, and radicals are generated according to the Haber–Weiss mechanism: The hydroxyl radical formed commences a polymerisation chain in the usual manner and is in competition with a second reaction that consumes the radical: Major class of water-soluble initiators consists of the persulfate salts, i.e. salts of persulfuric acid H2S2O8. Potassium persulfate K2S2O8 is the least soluble salt than sodium persulfate Na2S2O8 or ammonium persulfate (NH4)2S2O8. The decomposition of persulfate may be regarded as thermal dissociation of sulfate ion radicals: A secondary reaction may, however, produce hydroxyl radicals by reaction with water, and these hydroxyls may be the true initiators:
  • 23. Free radical Polymerization: Let R be a radical from any source called initiator. CH2=CHX represents a simple vinyl monomer where X is a substituent, which may be H as in ethylene CH2=CH2, Cl as in vinyl chloride CH2=CHCl, OOCCH3 as in vinyl acetate CH2=CHOOCCH3 or many other groups, which will be indicated in lists of monomers. The initiation process, consists of the attack of the free radical (obtained from initiator) on one of the doubly bonded carbon atoms of the monomer. One electron of the double bond pairs with the odd electron of the free radical to form a bond between the latter and one carbon atom other electron shift next carbon forming new free radical. The new free radical can, however, in its turn add on extra monomer units, and a chain reaction occurs, representing the propagation stage:
  • 24. The final stage is termination, which may take place by one of several processes. Combination: Disproportionation: Disproportionation in which hydrogen transfer results in the formation of two molecules with one saturated and one unsaturated end group. Chain Transfer: This is not a complete termination reaction, but it ends the propagation of a growing chain and enables a new one to commence. The reaction involves the transfer of an atom between the radical and the molecule. Chain transfer takes place very often via a fortuitous impurity or via a chain transfer agent which is deliberately added - Alkyl mercaptans
  • 25. Living Radical Polymerization: Bimolecular termination and chain transfer are ever present in radical chain polymerization and limit the lifetime of propagating radicals. Chain polymerizations without chain-breaking reactions, referred to as living polymerizations, would be highly desirable because they would allow the synthesis of block copolymers by the sequential addition of different monomers. A reactive species I* initiates polymerization of monomer A. 𝐼∗ + 𝐴 → 𝐼 − 𝐴𝑛 ∗ + 𝐵 → 𝐼 − 𝐴𝑛 − 𝐵𝑛 ∗ When the polymerization of monomer A is complete, the reactive centers are intact because of the absence of chain-breaking reactions. Addition of a second monomer B results in the formation of a block copolymer containing a long block of A repeat units followed by a long block of B repeat units. It is possible to continue the sequential propagation by adding a third batch of monomer, or one can terminate the process by the addition of a reagent to terminate the propagating centers. Block copolymers have commercial potential for obtaining products that can incorporate the desirable properties of two or more homopolymers. The situation is very different for conventional (nonliving) radical polymerizations since the lifetime of propagating radicals in these systems is very short (typically less than a second or, at most, a few seconds) because of the ever-present occurrence of normal bimolecular termination (coupling and/or disproportionation). The lifetime enhances by by either reversible termination or reversible transfer..
  • 26. Living radical polymerization (LRP) with reversible termination generally proceeds as follows: The initiator RZ undergoes homolytic bond breakage, to produce one reactive and one stable free radical. The reactive radicals quickly initiate polymerization, but the stable radicals are too stable to initiate polymerization. Once polymerization over, Z terminate reversibly the growth of chain. The stable radical acts as a controlling or mediating agent because it is sufficiently reactive to couple rapidly with propagating chains to convert them reversibly into dormant, nonpropagating species. The stable radical is often called the persistent radical, and its suppression of bimolecular termination between living polymers is called the persistent radical effect (PRE).
  • 27. Retardation and inhibition A retarder is defined as a substance that can react with a radical to form products incapable of adding monomer. It both reduces the concentration of radicals and shortens their average lifetime and thus the length the polymer chain. In the simplest case, the retarder may be a free radical, such as triphenylmethyl or diphenylpicrylhydrazyl, which is too unreactive to initiate a polymer chain. The mechanism of retardation is simply the combination or disproportionation of radicals. free radical 2,2-diphenyl-1- picryl hydrazyl Some other compounds, such as phenols, or even molecular oxygen, are inhibit the polymerization; and transforming the propagating radical to an oxygencentered radical that is unable to initiate polymerization. Tetramethylpiperidinyloxy (TEMPO) are two examples of radicals used to inhibit the radical polymerization.
  • 28. Inhibitors are useful in determining initiation rates; since their reaction with radicals is so rapid that the decomposition of inhibitor is independent of its concentration but gives directly the rate of generation of radicals. As a result, the length of the induction period before polymerization starts is directly proportional to the number of inhibitor molecules initially present to one radical. If the addition of a chain transfer agent to a polymerising system works efficiently, it will both slow the polymerisation rate and reduce the molecular weight because the free radical formed after termination may be much less active than the original radical (polymer chain) in starting new chains, and when these are formed, they are terminated after a relatively short growth. e.g. termination by mercaptons. p-benzoquinone produces radicals that are resonance stabilised and are removed from a system by mutual combination or disproportionation. Only a small amount of inhibitor is required to stop polymerisation of a system. Aromatic compounds such as nitrobenzene C6H5NO2 and the dinitrobenzenes (o-, m-, p-)C6H4(NO2)2 are retarders for most monomers, e.g. styrene, but tend to inhibit vinyl acetate polymerisation, since the monomer produces very active radicals which are not resonance stabilized
  • 29. Ring opening polymerization of vinylcyclopropane derivatives Ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer chain acts as a reactive center where further cyclic monomers can react by opening its ring system and form a longer polymer chain. Some cyclic monomers such as norbornene or cyclooctadiene can be polymerized to high molecular weight polymers by using metal catalysts. The driving force for the ring-opening of cyclic monomers is via the relief of bond-angle strain or steric repulsions between atoms at the center of a ring. Mechanisms of ring opening polymerization: di-functional monomers anionic ring-opening polymerization (AROP):
  • 30. Two examples are caprolactam and caprolactone A typical example of anionic ROP is that of ε-caprolactone, initiated by an alkoxide functional group.
  • 31. Radical ring opening polymerization Ring-opening polymerization can also proceed via free radical polymerization. The introduction of an oxygen into the ring will usually promote free radical ring-opening polymerization, because the resulting carbon–oxygen double bond is much more stable than a carbon-carbon double bond. Thus, cyclic hetero monomers that carry a vinyl side group like cyclic ketene acetals, cyclic ketene aminals, cyclic vinyl ethers, and unsaturated spiro ortho esters will readily undergo free radical ring-opening polymerization. The terminal vinyl group accepts a radical. The radical will be transformed into a carbon radical stabilized by functional groups (i.e. halogen, aromatic, or ester groups). This will lead to the generation of an internal olefin.
  • 32. Cationic ring-opening polymerization (CROP) A small amount of an electrophilic reagent (Lewis acid) is added to the monomer to initiate polymerization. A cyclic monomer undergoes CROP depends on the ring size, to be more specific, on the ring strain. Cyclic monomers with small or no ring strain will not polymerize whereas small rings with greater ring strain like 4, 6, and 7- membered rings of cyclic esters, ethers, lactones, lactams, and epoxides polymerize readily through CROP.
  • 33. Cationic polymerization Use of cationic initiators which include reagents capable of providing positive ions or H+ ions. Typical examples are aluminium chloride with water (AlCl3+H2O) or boron trifluoride with water (BF3+H2O). They are effective with monomers containing electron releasing groups like methyl (-CH3) or phenyl (-C6H5) etc. They include propylene (CH3CH=CH2) and the styrene (C6H5CH=CH2). i) Chain Initiation Initiators: Protonic acids such as HBr, HI, sulfuric acid, phosphoric acid, perchloric acid, trifluoromethyl sulfonic acid, etc. and Lewis acid such as AlCl3, AlBr3, BF3, SnCl4, TiCl4, with a co-initiator (water, protonic acids, alkyl halides) is needed to activate the Lewis acid. 𝐵𝐹3 + 𝐻2𝑂 → 𝐵𝐹3 − 𝐻2𝑂 𝐵𝐹3 − 𝐻2𝑂 + 𝐶𝐻3 2𝐶 = 𝐶𝐻2 → 𝐶𝐻3 3𝐶+ 𝐵𝐹3𝑂𝐻 − Decomposition of the initiator is shown as BF3 + H2O → H+ + BF3(OH–). The proton (H+) adds to C – C double bond of alkene to form stable carbocation. Where, G is electron donating group.
  • 34. ii) Chain Propagation Carbocation add to the C – C double bond of another monomer molecule to from new carbocation. iii) Chain Termination Reaction is terminated by combination of carbocation with negative ion (or) by loss of proton.
  • 35. Anionic polymerization Use of anionic initiators which include reagents capable of providing negative ions. Typical catalysts include sodium in liquid ammonia, alkali metal alkyls, Grignard reagents and triphenylmethyl sodium [(C6H5)3C-Na]. They are effective with monomers containing electron withdrawing groups like nitrile (–CN) or chloride (-Cl), etc. They include acrylonitrile [CH2=C(CN)], vinyl chloride [CH2=C(Cl)], methyl methacrylate [CH2=C(CH3)COOCH3], etc. Ability of substituents to stabilize carbanions decreases as: -NO2 > -C=O > -SO2 > -CO2 ~ -CN > -SO > -Ph ~ -CH=CH2 >>> -CH3. i) Chain Initiation Base Initiators are often organometallic compounds or salt of a strong base, such as an alkali metal alkoxide or alacoholates (tert-BuOLi, tert-BuOK), Sodium metal in tetrahydrofuran, sodium or Potassium with liquid ammonia, Stable alkali metal complexes may be formed with aromatic compounds (e.g. Na/naphthalene) in ether, alkyl amides such as NaNH2 or KNH2, . Organolithium salts (n-BuLi, sec-BuLi, tert-BuLi), Grignard reagent Potassium amide (K+NH2 -) adds to C – C double bond of alkene to form stable carbanion. Where, W is electron withdrawing group.
  • 36. ii) Chain Propagation Carbanion adds to the C – C double bond of another monomer molecule to from new carbanion. Anionic polymerization has no chain termination reaction. So it is called living polymerization. The first living polymerizations were achieved in anionic polymerizations of styrenes and 1,3-dienes in the mid-1950s
  • 37. A general mechanism for living anionic polymerization of a vinyl monomer is shown below, encompassing only initiation and propagation steps; chains are terminated only by the deliberate addition of a Bronsted acid or an electrophile. Initiation: Propagation: Deliberate Termination (no spontaneous termination):
  • 38. For any living polymerization, one initiator generates one polymer chain and that the product after all of the monomer has been consumed is a polymer with an active anionic chain end. The number average molecular weight (Mn) in living anionic polymerization is a simple function of the stoichiometry and the degree of monomer conversion, since one polymer is formed for each initiator molecule. In many cases, the impurities react quickly with initiators, especially alkyllithium initiators, so that molecular weight control can still be achieved by determining the impurity level Anionic polymerization of styrene using sodium naphthalene as initiator. Initiation Propagation:
  • 39. Monomers used for different polymerization: Monomer Radical Anionic Cationic  Almost all substituents allow resonance delocalization.  Electron-withdrawing substituents lead to anionic mechanism.  Electron-donating substituents lead to cationic mechanism. Ethylene + - + Propylene - - - 1-butene - - - Isobutene - - + 1,3-Butadiene + + - Isoprene + + - Styrene + + + Vinyl chloride + - - Acrylonitrile + + - Methacrylate ester + + -