COMPILED AND CIRCULATED BY DR. SK MOHAMMAD AZIZ, …

COMPILED AND CIRCULATED BY DR. SK MOHAMMAD AZIZ, ASSISTANT PROFESSOR, DEPARTMENT OF CHEMISTRY,

NARAJOLE RAJ COLLEGE

CHEMISTRY: SEM-VI, PAPER- DSE4T: POLYMER CHEMISTRY, FUNCTIONALITY & IT’S IMPORTANCE

Polymer Chemistry

DSE-4T

Narajole Raj College

Department of Chemistry

Functionality and its importance

Lecture Note-2




DEGREE OF POLYMERISATION

The number of repeating units in the polymer molecule is called the degree of

polymerisation, denoted by the letter n or P or Dp. The product of the degree of

polymerisation and the molecular mass (mo) of a monomeric unit equals the molecular mass

(M) of the polymer.

M = mo Dp.m

The degree of polymerisation may vary over a wide range, from a few units to 5000 -

10,000, and more. Polymers with high degree of polymerisation are called high polymers,

while those with a low degree of polymerisation are known as oligomers. High polymers

have very large molecular masses, of the order from 104 to 106, and hence, are high

molecular-mass compounds.

If the molecular mass of a polymer is high, the terminal or end groups of its chain

need to be taken into account; and the polymer molecule can be written without them,

showing only a few or even only one monomeric unit.

POLYMERISATION AND FUNCTIONALITY

Polymerisation is a chemical reaction in which the product molecules are able to grow

indefinitely in size as long as reactants are supplied. Polymerisation can occur if the

monomers involved in the reaction have the proper functionalities. The functionality of a

molecule is the number of sites available for bonding to other molecules under the specific

conditions of the polymerisation reaction.

A trifunctional monomer can be linked to two other molecules under appropriate

conditions. Examples are :




A polyfunctional molecule can react with more than two other molecules to form the

corresponding number of new bonds during the polymerisation reaction.

If an a- functional monomer reacts with a b- functional monomer in a nonchain reaction,

the functionality of the product molecule is a+b−2. This is because every new linkage

consumes two bonding sites. Production of a polymer in such reactions can occur only if a

and b are both greater than one. The following points about functionality should be noted :

(i) Use of the term functionality here is not the same as in organic chemistry where a

carbon-carbon double bond, for example, is classified as a single functional group. Double

bond is treated as bifunctional group in polymerisation.

(ii) Functionality refers in general to the overall reaction of monomers to yield products. It

is not used in connection with the individual steps in a reaction sequence. For example, the

free radical polymerisation of styrene is a chain reaction in which a single step involves

attack of a radical with ostensible functionality of one on a monomer with functionality

two.

(iii) Functionality is defined only for a given reaction. A glycol has a functionality of two

in esterification, but its functionality is zero in amide forming reactions. Similarly the

functionality of 1, 3-butadiene may be two or four depending on the particular double-

bond addition reaction.

(iv) The condition that monomer be difunctional or polyfunctional is a necessary, but not

sufficient, condition for polymerisation to occur in practice. Not all reactions between

polyfunctional monomers actually yield polymers. The reaction must also proceed clearly

with good yield to give high molecular-weight products. For example, propene has a




functionality of two in reactions involving the double bonds but free radical reactions do

not produce macromolecules whereas polymerisation in heptane at 70·°C with an

(C2H5)3Al/TiCl3 catalyst does yield high polymers.

(v) Functionality is a very useful concept in polymer science.

Carother's equation

According to the Carother's average functionality (fav) can be calculated by the

equation (1).

where fi is the symbol for functionality of monomeric species i and ni is the number of moles

of species i.

The definition of equation (1) holds strictly when fupctional groups of opposite kinds

are present in equal concentrations.

If there is an excess of functional groups of one kind, the monomer carrying these

groups will be able to react only until the opposite functional groups are consumed. In such

nonstoichiometric mixtures the excess reactant does not infer the polymerisation in the

absence of side reactions and should not be counted in calculating fav. Consider a

polymerisation which forms AB linkes and in which nB > nA where ni is the number of

equivalents of functional groups of type i. In this case the number of B equivalents which can

react cannot exceed nA, and therefore

The initial number of monomers is , and no fav is the total number of useful

equivalents of functional groups of all kinds that are present at the start of the reaction. We

define P as the extent of reaction equal to the fraction of functional groups in deficient

concentration which have reacted. Obviously 0 ≤ P ≤ 1, and P in stoichiometric mixtures is

the fraction of functional groups of either kind or of both kinds which have reacted. Also, n is

the total number of moles of molecules (monomers plus polymers of all sizes) when the

reaction has proceeded to an extent P.




Neglecting intramolecular linkages, every time a new linkage is formed the reaction

mixture will contain one less molecule. Therefore, when the number of molecules has been

reduced from no to n moles, the number of linkages which have been formed is equal to (no −

n) moles. It takes two functional groups to form a linkage and so 2 (no − n) moles of

functional groups will have been lost in forming these (no − n) moles of linkage.

Equation (6) is called the Carother's equation. (2) When the number average degree of

polymerisation becomes infinite, the term can be dropped and the equation (6)

reduces to equation (7)

Problem 1. Calculate the extent of reaction where phthalic anhydride and glycerol react in

the stoichiometric amounts.

For stoichiometric amounts, 2 moles of glycerol should react with 3 moles of anhydride. Thus

there is 12 functional groups per five moles of the mixture and




Problem 2 : Calculate the extent of reaction when phthalic anhydride and glycerol react in

the molar ration 1.500 : 0.980.

POLYMERIZATION MECHANISM

The presence of structural and compositional differences between polymers, Flory

[1953] stressed the very significant difference in the mechanism by which polymer molecules

are built up. Although Flory continued to use the terms condensation and addition in his

discussions of polymerization mechanism, the more recent terminology classifies

polymerizations into step and chain polymerizations. Chain and step polymerizations differ in

several features, but the most important difference is in the identities of the species that can

react with each other. Another difference is the manner in which polymer molecular size

depends on the extent of conversion. Step polymerizations proceed by the stepwise reaction

between the functional groups of reactants as in reactions. The size of the polymer molecules

increases at a relatively slow pace in such polymerizations. One proceeds from monomer to

dimer, trimer, tetramer, pentamer, and so on

until eventually large-sized polymer molecules have been formed. The characteristic of step

polymerization that distinguishes it from chain polymerization is that reaction occurs between

any of the different-sized species present in the reaction system.

Table 1. shows many of the common addition polymers and the monomers from which they

are produced.

Table 1. Typical Addition Polymers




The situation is quite different in chain polymerization where an initiator is used to

produce an initiator species R* with a reactive center. The reactive center may be either a free

radical, cation, or anion. Polymerization occurs by the propagation of the reactive center by

the successive additions of large numbers of monomer molecules in a chain reaction. The

distinguishing characteristic of chain polymerization is that polymer growth takes place by

monomer reacting only with the reactive center. Monomer does not react with monomer and

the different-sized species such as dimer, trimer, tetramer, and n-mer do not react with each

other. By far the most common example of chain polymerization is that of vinyl monomers.

The process can be depicted as

Each monomer molecule that adds to a reactive center regenerates the reactive center.

Polymer growth proceeds by the successive additions of hundreds or thousands or more




monomer molecules. The growth of the polymer chain ceases when the reactive center is

destroyed by one or more of a number of possible termination reactions.

The typical step and chain polymerizations differ significantly in the relationship

between polymer molecular weight and the percent conversion of monomer. Thus if we start

out step and chain polymerizations side by side, we may observe a variety of situations with

regard to their relative rates of polymerization. However, the molecular weights of the

polymers produced at any time after the start of the reactions will always be very

characteristically different for the two polymerizations. If the two polymerizations are

stopped at 0.1% conversion, 1% conversion, 10% conversion, 40% conversion, 90%

conversion, and so on, one will always observe the same behavior. The chain polymerization

will show the presence of high-molecular-weight polymer molecules at all percents of

conversion. There are no intermediate-sized molecules in the reaction mixture only monomer,

high-polymer, and initiator species. The only change that occurs with conversion (i.e.,

reaction time) is the continuous increase in the number of polymer molecules (Fig. 1. a). On

the other hand, highmolecular-weight polymer is obtained in step polymerizations only near

the very end of the reaction (>98% conversion) (Fig. 1. b). Thus both polymer size and the

amount of polymer are dependent on conversion in step polymerization.

The classification of polymers according to polymerization mechanism, like that by

structure and composition, is not without its ambiguities. Certain polymerizations show a

linear increase of molecular weight with conversion (Fig. 1. c) when the polymerization

mechanism departs from the normal chain pathway. This is observed in certain chain

polymerizations, which involve a fast initiation process coupled with the absence of reactions

that terminate the propagating reactive centers. Biological syntheses of proteins also show the

behavior described by Fig. 1. c because the various monomer molecules are directed to react

in a very specific manner by an enzymatically controlled process.




Fig. 1. Variation of molecular weight with conversion; (a) chain polymerization; (b) step

polymerization; (c) nonterminating chain polymerization and protein synthesis.

CHEMISTRY OF POLYMERISATION

Polymerisation is a chemical reaction in which the product molecules are able to grow

indefinitely in size as long as reactants (i.e., monomers) are supplied. Polymerisation occurs

if the monomers involved in the reaction have the proper functionalities. For the

polymerisation the monomers must have two or more reactive groups (two or more

functionality). This may, for example, be an amino group and a carboxylic group in the case

of polyamide:




Alternatively, reactive groups may be carbon-carbon double bond as in the case of vinyl

polymerisation.

The polymerisation process may be divided into three categories on the basis of the type of

reaction taking place :

(i) Condensation polymerisation

(ii) Addition polymerisation, and

(iii) Ring opening polymerisation.

CONDENSATION POLYMERISATION

Condensation polymerisation is a process of formation of polymers from

polyfunctional (generally bifunctional) monomers of organic molecules with the elimination

of some small molecules such as water, alcohol, HX, ammonia, etc. Condensation

polymerisation is also known as step-growth polymerisation. In this type of polymerisation,

the molecular weight of the polymer chain builds up slowly and there is only one reaction

mechanism for the formation of the polymer. The polymerisation process proceeds by

individual reactions of the functional groups of the monomers. Thus two monomers react to

form a dimer. The dimer may now react with another dimer to produce a tetramer, or the

dimer may react with more monomer to form a trimer. The process continues, each reaction

of the functional groups proceeding essentially at the same reaction rate. The reaction

proceeds for a relatively long period of time until a high molecular weight polymer is

obtained.

ADDITION POLYMERISATION




Addition polymerisation is the process of formation of addition polymers from

monomers without the loss of small molecules. Unlike condensation polymers, the repeating

unit of an addition polymer has the same composition as the original monomer. The

polymerisation of ethylene to give poly (ethylene) is an example of this type of reaction.

Addition polymers are prepared from olefins by a chain polymerisation reaction, which

usually leads to high molecular weight materials. Addition polymerisation may be divided

into the following categories :

(i) Vinyl polymerisation : Addition polymerisation given by ethylene and its derivatives is

known as vinyl polymerisation. Some examples are :

Vinyl polymerisation involves a three-part process.

(a) Initiation: In this step formation of active species takes place. This active species

initiate the polymerisation reaction with unreactive vinyl monomers. (b) Propagation: In

propagation steps high molecular weight polymer is formed. (c) Termination: In

termination step deactivation occurs to produce the final stable polymer. The active

species in vinyl polymerisation may be of three different types: namely free radicals,

anions and cations and these possibilities give rise to three distinct methods of

accomplishing polymerisation.

(ii) Diene polymerisation : Addition polymerisation reaction given by 1, 3-butadiene and

2-substituted 1, 3-butadienes are known as diene polymerisation. Diene polymerisation is

also of three types, free radical, anionic and cationic polymerisations.

(iii) Heteromultiple bond polymerisation : In vinyl polymerisation and diene

polymerisation, a carbon-carbon double bond is the active site. However, multiple bonds




involving carbon-heteroatom may also be utilised in the preparation of polymers, which

then contain hetero atom in the main chain. The most common monomers of this category

is the carbonyl compounds. Formaldehyde has been most widely studied in this respect

and its polymers are of commercial importance.

A further example of this category of polymerisation is the polymerisation of

monoisocyanates through the carbon-nitrogen double bond.

(iv) Ring opening poiymerisation : A fourth type of polymerisation is ring opening

polymerisation. An example of this type is the polymerisation of caprolactam.

This type of polymerisation has some of the features of both condensation and

addition polymerisation as far as kinetics and mechanisms are concerned. It resembles

addition or chain polymerisation in that it proceeds by the addition of monomer to growing

chain molecules. However, the chain-initiating and subsequent addition proceeds at similar

rates. If so, these are not chain reactions in the kinetic sense. As in stepwise polymerisation

the polymer molecules continue to increase in molecular weight throughout the reaction.

Most of the cyclic compounds like cyclic ethers, cyclic imines, cyclic sulphides, lactams and

lactones gives this type of polymerisation. Polymerisation through ring opening are

accomplished The polymerisations are often very rapid.

CONDENSATION AND ADDITION POLYMERS

By convention, polymers whose main chain consist entirely of carbon-carbon bonds

are generally classified as addition polymers while those in which hetero atoms (O, N, S, Si)

are present in the polymer backbone are considered to be condensation polymers. Exception

to this is polyformaldehyde and polyurethane which are addition polymers.




STEP GROWTH POLYMERISATON

In the case of step-growth (condensation) polymers, the mechanism is simply an

extension of the normal organic condensation reactions in which a small molecule e.g., H2O

or HCl is expelled as the link is built. For example :

In step growth polymerisation polycondensation reaction takes place.

Polycondensation reaction is brought about by monomers containing two or more reactive

functional groups hydroxyl, carboxyl and amino). When monomers have only one functional

group, following reaction will be possible:

It should, however, be noted that in this ester formation, the reactive functional groups

(e.g. hydroxyl and carboxylic) are consumed, giving an unreactive functional groups (e.g.

ester). There is no more reactive functional group left with the product and hence, it cannot

react further with any other reactant molecules in order to forn a different product.

Let us see what will happen if one monomer is bifunctional (I) and other monomer is,

mono functional (II)

First, one molecule of (I) reacts with one molecule of (II) as follows :

The resulting product however, still contains a hydroxyl group which can react with

another molecule of monobasic acid (II) to form diester.




The resulting product is diester which does not contain reactive functional group and,

hence, cannot react further either with (I) or with (II) to give any other product. Let us

experiment further.

Let us replace monobasic acid with dibasic acid and take two molecules each of

dibydroxy alcohol (III) and dibasic acid (IV) and see what happens. As expected, one

molecule of (I) reacts with one molecule of (III) to form a monoester product (V).

Apart from a non-reactive ester group, this product (V) still contains two reactive groups (OH

and COOH), and each can react with one more molecule of (III) or (IV).

The resultant molecule contains at its ends one - COOH and one - OH respectively

and hence if more molecules of (III) and (IV) are made available, it is capable of reacting

further, resulting in an even bigger molecule. In the above case four molecules of the

monomers (two molecules of III and two molecules of IV) react to give a single product (VI)

(which contains three ester groups) and three water molecules. The resultant product

molecule in this case is still capable of reacting further to form a bigger molecule, because

each of the species of the starting reactants (III, IV, V, VI etc) contains two functional groups

in its molecule.




Now, the product formed from two molecules of (III) and two molecules of (IV) can

be written as:

So the general reaction can be written as follows :

From this illustration, the following conclusions can be drawn regarding step polymerisation :

(i) The monomer should have two reactive functional groups.

(ii) Polymerisation proceeds by step-wise condensation reaction between reactive functional

groups.

(iii) Only one type of reaction is evolved between two functional groups in the

polymerisation.

(iv) The polymer formed still contains both the reactive functional groups at its chain ends (as

end groups). Such type of polymers are known as active polymers.

The following features characterise step-growth polymerisations :

(i) The group of polymer molecules proceeds by a stepwise intermolecular reaction. Only

one

reaction type is involved in the polymerisation.

(ii) Monomer units can react with each other or with polymers of any size.

(iii) The functional group on the end of a monomer is usually assumed to have the same

reactivity as that on a polymer of any size.

(iv) A high conversion of functional groups is required in order to produce high-molecular

mass product.

(v) Many step-growth polymerisation reactions are reversible.




(vi) Condensation polymers are usually produced by step-growth polymerisation but not

all step-growth syntheses are condensation reactions. Thus there is no elimination product

in polyurerthane synthesis from a diol and a diisocyanate.

Step-growth polymer reactions of the above type are essentially irreversible. This type

of polymerisation reaction is very fast.

REQUIREMENTS FOR STEP GROWTH POLYMERISATION

Step-growth polymers can be prepared from the bifunctional monomers. For example

poly (ethylene terephthalate) can be prepared theoretically by the following five

combinations of the two bifunctional monomers.

Reaction (b) is the fastest of those listed and proceeds very quickly at room

temperature because acid chloride derivative is the most reactive for condensation reactions.

Commercially this process is expensive, because the monomer diacid chloride is costly.

Process (a) is the preferred synthetic route. It involves a melt polymerisation which takes

place at high temperature (~275 °C). Since reaction is reversible, the by product water

produced should be driven out to get maximum yield. Thus the actual polymerisation of

reaction (a) is slower than that of (b) but the overall cost of production of polymer is less.




Distinguishing Features of Chain and Step Polymerization Mechanisms

CHAIN POLYMERISATION OR CHAIN-GROWTH POLYMERISATION

This class of polymerisation accounts for a large proportion of the synthetic polymer

industry and includes the large-tonnage materials such as polyethylene, polystyrene, PVC and

acrylics. The mechanism of the reaction for all these is the opening of the pi double bond in

the alkene to give an all carbon backbone of single sigma bonds. If diene is a monomer than a

double bond remains in the polymer. For examples:

However, since the empirical formula of the polymer produced is simply the sum of the

requisite number of monomers, these are often called addition polymers, particularly in older

texts.

The alkene systems involve chain reaction mechanism and hence the class of materials is

better called chain polymers. In general chain polymers can be prepared in one of five ways.

(1) Free radical polymerisation

(2) Cationic polyrnerisation




(3) Anionic polymerisation

(4) Coordination polyrnerisation

(5) Metathesis polyrnerisation

In general chain polymerisation have the following common features:

(1) An initiation step in which a reactive species is generated and attacks the first

monomer molecule. The reactive species may be free radical cation or anion.

(2) A propagation step in which a large number of further monomers are sequentially

added to give the long polymer chain, still remaining the reactive end group.

(3) A termination step in which the reactive end-group is deactivated.

(4) Each polymer molecule increases in size at a rapid rate once its growth has been

started. When the macromolecule stops growing it cannot generally react with more

monomers.

(5) Growth of polymer molecules is caused by a kinetic chain of reactions.

(6) In chain growth polymerisations the mechanisms and rates of reactions that initiate,

continue, and terminate polymer growth are different.

(7) Chain growth polymerisation is usually initiated by some external source (energy,

highly reactive compound or catalyst), and the reaction is allowed to proceed under

conditions in which monomer cannot react with each other without the intervention of

an active center.

(8) Polymers made by chain-growth reactions are often addition polymers by Carother's

definition i.e., polymers made by these processes have only carbon-carbon links in their

backbones.

Table 2. Characteristics bearing on the chemical and physical properties of

macromolecules




POLYMER STRUCTURE

In this section we will consider the micro-structure of the polymer i.e., the

arrangement of the structural units relative to one another. Several variations are possible and

these can have pronounced effects on the properties of a polymeric material. The structural

arrangements may be considered under the following headings.

FORMS OF POLYMERS

A polymer may consist of monomers of identical or of different chemical structure.

Polymer consisting of identical monomers are called homopolymers. Thus homopolymer is a

macromolecule derived from a single monomer. Polymeric compounds containing several

types of monomeric units in their chain are known as copolymers or mixed polymers. Thus a

copolymer is a macromolecule derived from two or more different monomers. Monomeric

units may combine with each other into a macromolecule to form a polymer of linear,

branched, or crosslinked (three dimensional) structures.

Linear polymers are polymers whose macromolecules are long chains with a high

degree of asymmetry. Denoting a monomeric residue by A, one would write the formula of a

linear polymer as follows:




A linear polymer is one in which each repeating unit is linked only to two others.

Polystyrene and poly (methyl methacrylate) are linear polymers.

A branched polymer is a long chain (usually called the main chain or backbone chain)

with side branches (side chains), the number and length of which may vary widely.

Crosslinked or three-dimensional polymers consist of long chains connected up into

three-dimensional network by chemical crosslinks :

TYPE OF COPOLYMERS

In copolymer molecules the monomer residues may be arranged in the chain at random,

according to the laws of chance or regularly. Copolymers of the former group are called

statistical (irregular) and those of the latter-regular. Depending on the monomers chosen and

the experimental techniques used, various distributions of structural units within the polymer

chain of copolymer may be achieved. Various possible arrangements of the two structural

units A and B are shown below :




RANDOM COPOLYMER

A random polymer is one in which the monomer residues are located randomly in the

polymer molecule. Random polymers are prepared by the polymerisation of an appropriate

mixture of monomers. Many commercial products of this type are available. Most of them are

based on vinyl and/or conjugated diene monomers, e.g., vinylchloride-vinylacetate,

vinylidine chloride-vinylchloride, styrene-butadiene, ethylene-propylene-isoprene polymers.

In vinylchloride-vinylacetate copolymers the vinylacetate content is 3 to 40%. These

copolymers are more soluble and pliable than polyvinylchloride homopolymer. They can be

shaped mechanically at lower temperatures than homopolymers with the same degree of

polymerisation and are used mainly in surface coatings and products where exceptional flow

and reproduction of details of a model surface are needed. This example shows that random

copolymers are more superior than homopolymers.

ALTERNATING COPOLYMER

In an alternating copolymer each monomer of one type is joined to monomers of a

second type. An example is the product made by free radical polymerisation of equimolar

quantities of styrene and maleic anhydride :

It may be noted that this structure could be regarded as a homopolymer having only the

following repeating unit :




However, it is usual to consider as copolymer products obtained from a mixture of

monomers when each of the monomer is separately capable of forming a homopolymer under

appropriate conditions. Thus such polymers as polyamides derived from diamines and dibasic

acids are not counted as alternating copolymers since the monomers are not separately

polymerisable.

Alternating copolymers are low molecular weight polymers. These low molecular

weight polymers have a variety of special uses including the improvement of pigment

dispersions in paint formulations.

GRAFT COPOLYMER

Graft copolymers are formed by growing one polymer as branches on another

preformed macromolecule. Graft copolymers may be prepared in three general ways, namely

transfer grafting, irradiation grafting and chemical grafting. Transfer grafting is most

commonly free radical initiated. Typically, a vinyl or diene polymer is treated with a peroxide

in the presence of vinyl monomer. Transfer occurs between the polymer chain and radicals

derived from the initiator; the resultant polymer chain radical than initiate polymerisation of

the monomer, e.g.,

CH2─CHR' is grafted on the polymeric chain of ~~CH2─CHR~~, Grafting is

invariably accompanied by formation of homopolymer of the monomer to be grafted. In




irradiation grafting, an essentially similar process is involved except that the reactive sites on

the polymeric substrate are created by irradiation with UV light.

In chemical grafting, reactive groups present along the polymer chain are used as sites

for grafting. Both free radical and ionic reactions have been utilised in this technique. One

method involves irradiation of the polymeric substrate in the presence of oxygen to produce

peroxide groups which can be subsequently decomposed thermally in the presence of

monomer to initiate free radical grafting, e.g.,

BLOCK COPOLYMER

Block polymers have backbone consisting of fairly long sequences of different

repeating units. These copolymers can be prepared by several techniques, of which anionic

polymerisation offers the best possibilities for controlling the product. In this method the first

step is to polymerise a single monomer, allowing reaction to proceed until, the monomer is

exhausted. To the 'living polymer' is added a second monomer which then forms the second

block. When the second monomer is exhausted a third monomer may be added, and so on.

Many combinations of monomers have been investigated and a few block copolymers are

now commercially available, e.g., the styrene-butadiene copolymer.

REFERENCES 1. G. Odian: Principles of Polymerization, 4th Ed. Wiley, 2004. 2. J. Sing & R. C. Dubey: Organic Polymer Chemistry, Revised Ed. Pragati Praksahan, 2009.

3. Charles E. Carraher, Jr.: Polymer Chemistry, 6th Ed. Marcel Dekker, Inc., 2003. 4. Fred W. Billmeyer, Jr.: Textbook of Polymer Science, 3rd Ed. John Wiley & Sons, Inc., 1984. Acknowledgement: Special thanks to Dr. Soumendu Bisoi for his contribution.

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COMPILED AND CIRCULATED BY DR. SK MOHAMMAD AZIZ, …