Polymeric Chemical Reaction- Polycondensation Reaction.
Polymeric Chemical Reaction
Polymer can be further classified by
the type of chemical reactions which the monomers undergo in producing
molecular weight compounds.
1. Poly condensation reaction –Monomers
react by repeated splitting off H2O, NH3, CH2O, NaCl, or other small molecules
on a step by step reaction to build of the molecular size.
2. Addition reaction – occurs among
molecules containing double or triple bonds or with oxo-ring compounds. No
small molecules are liberated and reaction is the rapid chain type.
A. Poly condensations Reactions.
a. Representative groups
i.
Aldehyde
ii.
Amide
and Amine
iii.
Anhydride,
iv.
epoxy,
v.
carboxyl,
vi.
chloride,
vii.
hydroxyl,
viii.
sulfide and
ix.
Isocynate
A monomer must contain two or more of such
reaction groups.
b. Fundamental reaction mechanism ;
Same mechanism and chemical activation energies
are found in preparing low molecular weight condensation products. E.g. Esterification
reaction
Reacts under condition of T, P and
C.
c. Complexity of poly condensation
reaction.
Function groups presents:
Three types of polymer molecules
are formed from pairs of bifunctional reactions e.g. terephhthalic acid and
ethylene glycol.
R1 is a polyester with one terminal hydroxyl and
one terminal H, both reactive occurs only when reactant mole ratio is unity
with degree of polymerization is m
R2 is polyester formed in excess of
acid with degree of polymerization is n
R3 is polyester formed in excess of
gylycol with degree of polymerization is p
Such is characteristic of phenol
formaldehyde polymerization.
d. Equilibrium conditions for poly-condensation
reactions
Reverse reaction or de-polymerization
must be considered in final equilibrium achieved at an T, p, CI
There are only two sites on which
the ester bond can be formed – (1) or (2) but there are m1+m2 sites where it
may be hydrolyzed. This leads to difficult industrial problems in removing
water and maintaining very low water concentrations on final polymer mix.
e. Kinetic consideration for poly
condensation reactions.
Polycondensations are
relatively slow, step-wise reactions in which functional group reactions occur
repeatedly to build up a polymer. Since reactions occur between function groups,
reaction is not restricted to monomer adding to a multimer of given DP but may
also occur between two multimers of DP m1 and m2 . It has
been shown that the rates constant for all functional groups, in the
condensation, are equal, whether on a monomer or multimer.
With this simplification, Polycondensation
reactions parallel the rate and reaction order conditions for monocondensation
between two functional groups, e.g., OH and COOH in esterification
r
= K1CaOH CbCOOH
Catalysts
are not generally required but, if used, are of the acid base water soluble
type.
Experimental
data on esterification do not follow these simple equations exactly because of
change in polarity of media. Functional group are polar while the product is usually
non polar.
For
bulk polymerizations, viscosity increases markedly affect the measured rate constant
which change from chemically controlled to diffusion controlled values during
the polymerization.
Third
difficulty is related to the different reactivity of functional groups on the
same molecules. For example, different
esterification rate exist for primary and secondary glycol hydroxyl groups.
The
average degree of polymerization, DP , for poly-condensation can be derived
from probability statistic as ration of smallest number of reacting groups(N0)
present at the start to the number of same reacting groups (N) present at time
t
DP
= No / Nt = 1/(1-p)
Where p = fractional
conversion of reacting group = (N0-N)/N0
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