1.
Polymerization
1.1 Classification of Polymers and Polymerization
Mechanisms
In 1929 W. H. Carothers suggested a classification of
polymers into two groups, CONDENSATION and
ADDITION polymers.
Condensation polymers are those in which the molecular
formula of the repeat unit of the polymer chain lacks
certain atoms present in the monomer from which it is
formed(or to which it can be degraded) for example, a
polyester is formed by typical condensation reaction
between bifunctional monomers, with elimination of water
xHO  R  OH  xHOCO  R'COOH  HO[ R  OCO  R'COO ]X H 
( 2 X  1)H 2O
The most important group of addition polymers includes
those derived from unsaturated vinyl monomers
CH 2
CH
X
 CH 2  CH  CH 2  CH  , etc.
X
X
Carother's original distinction between addition and
condensation polymers was amended by Flory, who placed
emphasis on the mechanism by which the two types of
polymer are formed. Condensation polymers are usually
formed by the stepwise intermolecular condensation of
reactive groups; addition polymers ordinarily result from
chain reaction involving some sort of active center.
Some of the consequences of the differences between the
mechanisms of chain and stepwise polymerization are
shown in Table 2.1
Table 1.1 Distinguishing features of chain- and steppolymerization mechanisms
Chain Polymerization
vi) 1.1
Only growth reaction adds
repeating units one at a time
1.2
to the chain.
vii) 1.3
Monomer concentration decreases steadily throughout
1.4
reaction.
viii)1.5
High polymer is formed at
once; polymer molecular
1.6
weight
changes
little
throughout reaction.
1.7
ix) Long reaction times give
1.8
high yields but affect molecular weight little.
1.9
x) Reaction mixture contains
only monomer, high polymer, and about 10-8 part of
growing chain.
Step polymerization
i)
Any two molecular species
present can react.
ii) Monomer disappears early
in reaction : at DP 10, less
than 1% monomer remains.
iii) Polymer molecular weight
rises steadily throughout
reaction.
iv) Long reaction times are
essential to obtain high
molecular weights.
v) At any stage all molecular
species are present in a
calculable distribution.
1.2 Step-Reaction(Condensation) Polymerization
The type of product formed in a condensation reaction is
determined by the functionality of the monomers, that is by
the average number of reactive functional groups Per
monomer molecule.
Mono functional monomers give only low-molecularweight product.
Bifunctional monomers give linear polymers
Polyfunctional monomers, with more than two functional
group per molecule, give branched(three dimensional)
polymers. The properties of the linear and threedimensional polymers differ widely.
The mechanisms of some step-reaction polymerization is
discussed below.
i)
Carbonyl Addition-Elimination mechanism
The most important reaction that has been used for the
preparation of condensation polymers
O
R-C-X + Y :
O:
R-C-Y
O
R-C-Y + X:
X
Where R and R'( below) may be alkyl or aryl groups, X
may be OH, .OR', NH2 NHR', OCOR' or Cl; Y may be
R'OH, R'NH2 or R2COO-.
i)
Interchange
The reaction between a glycol and an ester,
XHO-R-OH + xR''OCO-R'-COOR''
R''O(-CO-R'-COO-R-O)xH + (2x-1)R''-OH
Is often used to produce polyester, especially where the
dibasic acid has low solubility. Frequently the methyl ester
is used, as in the production of poly(ethylene
terephthalate) from ethylene glycol and dimethyl
terephthalete. The reaction between a carboxyl and an
ester link is much slower, but other interchange reactions,
such as amine-amide, amine-ester, and acetal-alcohol, are
well known.
iii) Carbonyl Addition – Substitution Reaction
The reaction of aldehydes with alcohols, involving first
addition and then substitution at the carbonyl groups, is of
great practical and historic importance in stepwise
polymerization. The general reaction, leading to acetal
formation, is
O
RCH +R'OH
OH
RCH
OR'
OR'
R'OH
RCH
OR'
In addition to polyacetals, important polymers formed in
this way include those of formaldehyde and phenol, urea
or melamine
iv) Nucleophilic Substitution Reaction
These reactions are important from the standpoint of
commercial organic polymers, primarily because of their
lose in the polymerization of epoxides. The most common
epoxide monomer is epichlorohydrin which reacts with a
nucleophile N: as follows
N: + H2C
CHCH2Cl
NCH2CHCH2Cl
O:
O
Typically, the nucleophile is a bifunctional hydroxy
compound such as bisphenol A and reaction proceeds as
described below
O
__
CH2 CHCH2Cl + HO_
CH3
_ _
C
_
OH
CH3
v) Double-Band Addition Reaction
Although addition reaction at double bands are often as
associated with polymerization by chain mechanism this is
not necessarily the case, and many important stepwise
polymerization are based on this reaction. Of major
interest among is the addition of dials to diisocyanates in
the production of polyurethanes
O
xHOROH +xOCNR'CNO
[ OROCNHR'NHC-]x
1.3 Radical Chain (Addition) polymerization
The characteristics of chain polymerization listed in Table
2.1 of several postulated types of active center, three have
been found experimentally: cation, anion and free radical.
Free – radical polymerization is discussed in this chapter,
and the related cases of ionic and coordination
polymerization may be discussed later.
The concept of vinyl polymerization as a chain mechanism
is not new, dating back to Staudinger’s work in 1920. In
1937
Flory
showed
conclusively
that
radical
polymerization proceeds by and requires the steps of
Initiation
Propagation and
Termination
Typical of chain reactions in low-molecular weight species.
The carbon-carbon double band is, because of its relatively
low stability, particularly susceptible to attach by a free
radical. The reaction of the double band with a radical
proceeds Well for compounds of the type CH2=CHX and
CH2=CXY called vinyl monomers.
Mot cell vinyl monomers yield high polymer as a result of
radical aliphatic hydrocarbons of her then ethylene not at
all.
1.3.1 Generation of Free Radicals
Many organic reactions take place through intermediates
having an add number of electrons and, consequently, an
unpaired electron. Such intermediates are known as FREE
RADICALS. They can be generated in a number of ways,
including thermal decomposition of organic peroxides or
hydroperoxides (Mageli 1968 or azo or diazo compounds
(Zand 1965)
Two reactions commonly used to produce radicals for
polymerization
are
thermal
or
photochemical
decomposition of benzoylperoxide
2C6H5COO-
(C6H5COO)2
2C6H5. + 2CO2
And azobisisobutyronitrile (AIBN)
(CH3)2CN
CN
NC(CH3)2
2(CH3)2C. + N2
CN
Free radicals can be produced; by
Thermal or photochemical decomposition of initiators
such as Benzoylperoxide or AIBN
High energy radiation; from a wide variety of sources
including electrons, gamma rays, X-rays, and slow
neutrons, is effective in producing free radicals that can
initiate polymerization
1.3.2 Polymerization steps
i)
Initiation
When free radicals are generated in the presence of vinyl
monomer the radical adds to the double band with the
regeneration of another radical If the radical formed by
decomposition of the initiator I is designated R.
I
2R.
H
2R. + CH2
CHX
RCH2C.
X
ii)
Propagation
The chain radical formed in the initiation step is capable of
adding successive monomers to propagate the chain.
H
R
(CH2CHX
)XCH2C. + CH2
H
H
CHX
R
(CH2CHX )X+1CH2C .
X
iii) Termination
Propagation would continue until the supply of monomer
was exhausted were it not for the strong tendency of
radicals to react in pairs to form a paired-electron covalent
band with loss of radical activity. This tendency is
compensated for in radical polymerization by the small
concentration of radical species compared to monomers.
The termination step can take place in two ways:
Combination or coupling
H
H
H
CH2C. + .CH2___
___
X
___
H
CH2C____CH2 ____
X
X
X
or disproportionation
H
H
H
.
CH2C. + CH2___
___
X
CH2C___H
H
+
C = CH___
X
X
X
In which hydrogen transfer results in the formation of two
molecules with one saturated and one unsaturated end
group
iv) configuration of monomer Units in Vinyl Polymer
chains
The addition of a vinyl monomer to a free radical can take
place in either of two ways:
R-CH2-CH.
2R + CH2= CHX
(I)
X
R-CH-CH2.
X
(II)
The reaction leading to the more stable product being
favored. Since the unpaired electron can participate in
resonance with the substituent X in structure I but not II,
reaction I is favored. Steric factors also favor reaction I.
The occurrence of reaction I exclusively would lead to
head-to-tail configuration in which the substituents occur
on alternate carbon atoms.
_
_ _
_ _
_ _
_
CH2 CH CH2 CH CH2 CH CH2
X
X
X
Alternative possibilities are a head-to-head, tail-to tail
configuration
_
_ _ _
_
_ _ _
CH2 CH CH CH2 CH2 CH CH
X
X
X
X
Or random structure containing both arrangements.
1.4 Copolymerization
Although the polymerization of organic compounds has
been known for over 100 years, the simultaneous
polymerization (Copolymerazation) of two or more
monomers was not investigated until about 1911, when
copolymers of olefins and diolefins were found to have
rubbery properties and were useful than homopolymers
made from single monomers.
In the 1930’s it was found that monomers differed
markedly in their tendencies to enter into copolymers.
Staudinger (1939) fractionated a vinyl chloride-vinyl
acetate copolymer made from a mixture of equimolar
Quantities of the two monomers. He found no polymer
containing equal amounts of each monomer but, instead,
found vinyl chloride: vinyl acetate ratios of 9:3, 7:3, 5:3
and 5:7 among the fractions.
In 1936 Dostal made the first attach on the mechanism of
copolymerization by assuming that the rate of addition of
monomer to a growing free radical depends only on the
nature of the group on the chain. Thus monomers M1 and
M2 lead to radicals of types M1. and M2. There are four
possible ways in which monomer can add
Reaction
M1. + M1
M 1.
rate
k11[M1.][M1]
M1. + M2
M 2.
k12[M1.][M2]
M2. + M1
M 1.
k21[M2.][M1]
M2. + M2
M 2.
k22[M2.][M2]
The monomer reactivity ratios r1 and r2 are the ratios of
the rate constant for a given radical adding its own
monomer to the rate constant for its adding the other
monomer.
r1>1 means that the radical M1. prefer to add M1
r1<1 means that it prefers to add M2
In the system styrene (M1)-methyl methacrylate (M2), for
example, r1= 0.52 and r2 = 0.46 each radical adds the other
monomer about twice as fast as its own.
Ideal system :A copolymer system is said to be ideal the
two radical show the same preference for adding one of the
monomers over the other k11/k12 = k21/k22 or r1=1/r2 or
r1r2=1
In this case the end group on the growing chain has no
influence on the rate of addition, and the two types of units
are arranged at random along the chain in relative
amounts determined by the composition of the feed and
the relative reactivities of the two monomers.
1.4.1 Polymerization Conditions and Polymer Reactions
As the industrial production of polymers has increased
over the years, there has developed an increasing interest
in the application of chemical engineering principles to
polymerization.
Table 1.2 Comparison of polymerization System
Type
Bulk( batch type)
Bulk(continuous)
Solution
Heterogeneous
Suspension
Emulsion
Advantages
Minimum contamination
simple equipment for
making casting
Disadvantages
Strongly
exothermic
Broadened
molecular
weight. Distribution at
high conversion.
Complex
is
small
particles required
Lower conversion per Requires
agitation,
pass leads to better
material transfer,
heat control and
Separation and
narrower
molecular Recycling
weight distribution
Ready control of heat of Not use for dry polymer
polymerization. Solution because of difficulty of
may be directly useable. complete solvent removal
Ready control of heat of Continuos agitation
polymerization.
required.
Suspension of
Contamination by
resulting granular
stabilized possible
polymer may be
washing, drying,
directly useable
possibly compacting
required
Rapid polymerization to
high molecular weight
and narrow distribution,
with ready heat control.
Emulsion
may
be
directly useable.