kinetics in chemistry this topic is very important and these are important to studying

1. UNIT-7
KINETICS AND
MECHANISM OF
POLYMERIZATION

2. POLYMERIZATION
TECHNIQUES
ADDITION
POLMERIZATION
CONDENSATION
POLYMERIZATION

3. Mechanism and kinetics of
copolymerization
Let two monomers, M1 and M2 be mixed together and polymerized using a
free-radical initiator. Once the initiator decomposes, the free-radical formed
can attack M1 or M2 as follows:
R+M1 → RM1
R+M2 → RM2
Two polymer chains can thus be initiated, one carrying a free radical site on
monomer unit M1 and the other on monomer unit M2. After initiation, comes
the process of propagation. Here, the chain carrying a free-radical site on M1
can either add another M1 or M2. Similarly, the chain carrying a free-radical
site on M2 can either add another M2 or an M1, i.e.:

4. 1. RM1+M1 → RM1M1
2. RM1+M2 RM1M2
3. RM2+M1 RM2M1
4. RM2+M2 RM2M2
Assuming that all the four types of addition take place, we still have the
growing chains ending with M1 or M2 and the possibility of all the four types
of propagation. Now, the rates of these four propagation reactions are as
follows:

5. The basic assumption is that the reactivity of any growing chain
depends only on the end monomer unit carrying the free-radical
site and not on the number or type of monomer units already
added to the chain.

6. The rate at which the monomers M1 and M2 are consumed
during the course of propagation can be expressed as follows:

7. Now, assuming a steady state, wherein the rate of a particular chain end (say,
M1) disappearing is equal to the rate of formation of the same chain end, we
can write
A combination of all the foregoing equations gives the
‘copolymer equation’:

8. The terms k11/k12 (denoted by r1) and k22/k21 (denoted by r2)
appearing in this equation are two important terms, known as the
reactivity ratios for any given pair of monomers M1 and M2.
These ratios (r1 and r2) indicate whether a growing chain carrying
a free radical on a particular monomer unit would prefer to add its
own monomer species or the comonomer species.

9. In other words, the composition of the copolymer formed at any
given instant is dependent not only on the concentration of the
monomer species present in the system at that instant but also on
their reactivity ratios. The point to be noted is that r1 and r2 for
any given pair of monomers are dependent purely on the nature
of the two monomers and temperature and independent of other
parameters such as the solvent, initiator and chain transfer
agent. The latter, however, will have a pronounced influence on
the molecular weight and the molecular weight distribution of
the polymer formed.

10. Addition polymerization.
Addition polymerization occurs in monomer units having double or triple
bonds. It involves the combination of a large number of monomer units by
addition reaction. In general, compounds containing –C=C-Z (Z is
substituent) undergo addition polymerization which is popularly known as
vinyl polymerization. This polymerization is also known as chain growth
polymerization. The monomer units may be joined in a head–tail
arrangement, or a head–head and tail–tail arrangement, or it may be a
random arrangement. The most favourable arrangement is head–tail
arrangement.

12. Mechanism of Addition
Polymerization
In the formation of addition polymers (chain growth polymers), the
reaction is initiated by a catalyst, which results in the formation of a
reactive intermediate. This intermediate then adds on to the monomer
unit to generate a new intermediate, which adds on to another monomer
unit and the process goes on. Depending upon the reactive intermediate
formed, the polymerization reaction may follow:
(a) Free radical mechanism
(b) Cationic mechanism
(c) Anionic mechanism
(d) Ziegler–Natta polymerization

13. (a) Free radical mechanism.
The addition polymerization reactions, which are carried out in
presence of peroxides or potassium perborate, follow a free
radical mechanism. The addition occurs through a free radical
intermediate and is also known as radical polymerization. The
radical polymerization occurs in head to tail manner. The
mechanism involves following steps:

14. Step 1. Chain initiation

15. Step 2. Chain propagation

16. Step 3. Chain termination

17. (b) Cationic mechanism.
The addition polymerization reactions, which are carried out in
the presence of strong acids like sulfuric acid, halogen acids or in
the presence of Lewis acids (BF3, AlCl3) follow a cationic
mechanism. The protonation of alkene results in the formation of
carbocation intermediates. Thus, the growing chains in
polymerization are cations and the process is known as cationic
polymerization. For example, the cationic mechanism for
polymerization of 2-methylpropene (isobutene) involves following
steps:

20. (c) Anionic mechanism.
The addition polymerization reactions, which are carried out in
presence of strong bases like sodamide or metal alkyls follow
anionic mechanism. The alkenes containing an electron
withdrawing group generally undergo anionic polymerization.
The electron withdrawing groups facilitate the attack of base on
the olefinic carbon to which they are attached. The mechanism
involves following steps:

24. (d) Ziegler–Natta polymerization
(Coordination polymerization).
The free radical, cationic, or anionic polymerizations generally
result in the formation of addition polymers, which are atactic.
In 1953, two scientists Karl Ziegler (Germany) and Giulio Natta
(Italy) independently synthesized “stereoregular” (isotactic and
syndiotactic) addition polymers. The stereoregular polymers are
high melting, crystalline solids. These have much better
properties and wider range of applications than atactic
polymers.

25. The Ziegler–Natta polymerization is carried out in the presence of
a mixture of titanium tetrachloride (TiCl4 ) and
triethylaluminium as catalyst, popularly called Ziegler–Natta
catalyst. The polymerization is also known as coordination
polymerization. The reaction is carried out at a low temperature
(<100 degree Celsius) and low pressure (~10 atm) in a non polar
solvent.

26. The reaction takes place as follows:

27. Mechanism:-
The catalyst has a complex structure. The alkyl titanium bond undergoes
insertion of monomer units. The π-electrons of monomer are coordinated
to a vacant site on metal (This being the reason that it is known as
coordination polymerization). The coordinated monomer then inserts
into titanium–carbon bond and the process goes on to form a polymer.
Since titanium is attached to different ligands, the coordination of
monomer units follow a stereoregular pattern. The various steps of
reaction mechanism are given below:

30. Ziegler–Natta polymerization: schematic and
simplified representation of (a) formation of
polyethylene (b) polypropylene formation
(further simplified); (c) mechanism for the
formation of polyethylene

31. Characteristics of Ziegler–Natta polymers.
The polymers manufactured by using Ziegler–Natta
catalyst have the following important characteristics:
(1) They are high melting, highly crystalline, isotactic
polymers having high molecular mass.
(2) They are tough and flexible, so can be moulded easily
and are resistant to acids and alkalis.

32. Synthesis and uses of vinyl
polymers
The polymerization of vinyl monomers generally occurs in
the presence of peroxides and follows a free radical
mechanism. However in case of polyethylene and
polypropylene an ionic or Ziegler–Natta mechanism are also
followed. The preparation of polypropylene is carried out
exclusively through Ziegler–Natta polymerization to obtain a
stereoregular polymer of high quality. The general reaction
for vinyl polymerization is represented as follows:

34. SOME OF THE VINYL POLYMERS, ALONG WITH THEIR MONOMER
UNITS AND COMMON USES ARE GIVEN BELOW:-

37. Condensation polymerization.
It involves the combination of a large number of monomer units
with the elimination of simple molecules like water, ammonia, and
so on. This polymerization is also known as step growth
polymerization. In condensation polymers the monomer units
contain two or more functional groups and generally the
condensation occurs between two different monomer units. For
example, polyesters, polyamides, polyurethane, and bakelite.

39. POLYAMIDES
Nylon 6,6.
Nylon 6,6 is a copolymer and is prepared by condensation of
hexamethylenediamine and adipic acid. During condensation,
loss of water molecule takes place to form a number of amide
linkages (polyamide) to yield nylon 6,6. Each monomer unit
(diamine and diacid) has six carbons each and therefore the
name, nylon 6,6. Adipic acid is obtained by oxidation of
cyclohexanone.

42. Polyesters
(i) Dacron [terylene].
Dacron, a copolymer, is prepared by the condensation of
methyl ester of terphthalic acid with ethylene glycol. The
condensation involves loss of a methanol molecule, which
results in the formation of a number of ester linkages
(polyester).

44. This polyester can be fabricated into a strong film,
which is known by the name Mylar. It is a light
weight, high strength, transparent film, which is used
for protection of artwork and important documents.

45. (ii) Polycarbonates.
These are polyesters formed by condensation of derivatives of
phenol and carbonic acid. For example, condensation of
bisphenol A with phosgene results in the formation of a
polycarbonate, known by the name of lexan. The reaction takes
place as follows:

48. Bakelite (Phenol-formaldehyde resin).
Bakelite is a copolymer of phenol and formaldehyde. It is a
thermosetting polymer and has highly cross-linked structure.
The methylene groups at ortho and para positions participate in
cross linkages. The condensation of phenol and formaldehyde
can occur in acidic or alkaline medium.
• In acidic medium, condensation results in the formation of a
linear polymer known as Novolak. This polymer further
polymerizes in basic medium to yield bakelite.

50. Here, polymerization is shown with o-methylolphenol (other type of linear
polymers are also formed). • In alkaline medium, condensation results in the
formation of a linear polymer known as Resol. Heating resol polymer results in
the formation of bakelite.

51. Both novolak and resol are thermoplastic polymers but they result in the
formation of a thermosetting polymer, namely Bakelite.

52. Urea-formaldehyde resin.
Urea on condensation with formaldehyde results in the
formation of urea-formaldehyde resin. Initially, the
reaction results in the formation of monomethylol,
which further condenses with urea and formaldehyde to
give the resin. The polymerization may occur in a linear,
three-dimensional, or cyclic structure.

54. Epoxy Polymers
The epoxy polymers are basically polyethers. One type
of epoxy polymer (or epoxy resins as they are
generally called) is prepared from epichlorohydrin
and bisphenol-A. The reaction is carried out with
excess of epichlorohydrin. The scheme is as follows:

56. Instead of bisphenol-A, many other compounds with hydroxyl
groups (such as glycols, glycerols and resorcinols) can also be used.
The epoxy resins obtained through these reactions will be either
highly viscous liquids or solids with high melting points. The epoxy
resins can be further cured with substances such as amines, poly
sulfides and polyamides. Epoxy resins find a large number of uses
because of their remarkable chemical resistance and good
adhesion. Epoxy resins are excellent structural adhesives. When
properly cured, epoxy resins can yield very tough materials. They
are used in industrial floorings, foams, potting materials for
electrical insulations, etc. One of the principal constituents in many
of the fibre-reinforced plastics (FRP) is an epoxy polymer.

57. POLYURETHANES
The reaction of an isocyanate and alcohol results in the formation
of a carbamate, which is known as urethane.

58. Polyurethanes are copolymers and are prepared by the reaction of alkyl or
aryl diisocyanates with diols. In general, the diol used in the preparation is a
copolymer of ethylene glycol and adipic acid, which has free hydroxy (– OH)
end groups. The commonly used diisocyanate is toluene-2,4- diisocyanate

59. The polymerization in the presence of water results in the formation of
polyurethane foam. Water reacts with the isocyanate group to produce
carbamic acid, which spontaneously looses carbon dioxide. The carbon dioxide
thus generates bubbles (giving porosity to polymer) to yield polyurethane foam.
The density of foam is dependent on the amount of carbon dioxide evolved
during the process.

60. Some condensation polymers along with their monomer units and
common uses are given below:-

62. QUESTIONS:
1. What is Zeigler-Natta catalyst? Explain the mechanism for the
polymerization of propene in presence of Zeigler- Natta catalyst.
2. Give the free radical addition mechanism involved in the
polymerization of chloroprene.
3. Name some vinyl polymers along with their monomer’s units
and common uses.
4. Give the mechanism of condensation polymerization with the
help of suitable examples.
5. What are the characteristics and advantages of Zeiger-Natta
polymerization?
6. Give the synthesis of polyurethanes.

63. THANK YOU