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In this video I'm
going to describe
some basic of polymerization reaction and we will start with
chain growth polymerization and so
I have indicated an example here of polyethylene
So we have ethylene here and it's reacting with a
chain of a number of ethylene molecules that have already reacted
it is a radical this electron is indicated here and "i" just means we have some
number
of these C2H4 groups
100, 1000 and were are going to add one more to it
when we are finish we still have something that's reactive
this one paired electrons or we'll refer
to has a radical. So the idea then of chain growth polymerization
is to keep adding a monomer. So this ethylene is a monomer
and so what we do is add the monomer to the end of the chain
and we sequentially add these monomers
and what I'm showing you as an example of free radical
chain growth polymerization. It is also refereed to as addition
because we continued to add a monomer unit onto growing
polymer chain. Now there are other types reactions beside just free radical
addition polymerization but we will just concentrate on this
example for now. There are several steps involve. We have to get this
started. We have to initiate the reaction
and normally we use some molecule
that's not the monomer. That can split apart to form radicals
and there is a species that can do this. So peroxide
is one but what matters is we now have
this species that can react with a monomer
and form something that is active
radical for further reaction. We will write this as
making radical one. It is the start of the polymer chain
This initiation reaction is normally
slow, the limiting step of the two reactions.
This is followed by propagation and the idea here is we have
R1 reacts with the monomer and makes
R2 which is still reactive. R2 react to the monomer
to make R3 and in general we have R with
"i" number of monomer unit in it reacting with the monomer
to grow the chain we have "i" plus one
and this is still radical. This is still
a radical. So we're continuing to add monomer
to the end of this growing chain. Something that's assumed in analyzing the kinetics
of such reactions
is the rate of this step is the same as the rated this step
and it doesn't matter Rj plus M
is the same as the rate of this step independent of what
j and i values we have. So these continue to grow until typically we have
termination or we form an inactive species. So for example
Ri plus Rj, two growing polymer chains are
radicals can form a polymer
We can call this a deactivated polymer
in the sense that it no longer will react with the monomer
We can also get reactions where Ri
plus Rj form polymer i plus polymer j
and this case one of these for example this would be saturated
and this would be unsaturated. This would have a double bond
on the end. Now typically the rate that
polymerization takes place is related to the rate constant for
this propagation step so kp being the rate constant for this step
times some fraction of the initiation that's effective. Some of this
initiation
takes place here we lose gets deactivated. So it is some fraction less than
one. Let's say 0.6
and rate constant call this kd for the
decomposition of the
initiator and then kt
is often some combined value for these different types of
steps that terminate the reaction from the inactive chain
and then the concentration of this initiator
to the 1/2 power and the concentration a monomer
and so the rate of polymerization typically is first-order
in the monomer concentration so typically the
propagation step is fast and so the lifetime that one of these
radicals exist before it's exterminated
is short. One thing to note is when a reaction is taking place
it's much more likely that a radical reacts with a monomer
because we have a lot of monomer small number concentration to radicals
rather than the radicals terminating
but clearly they terminate to form our polymer
chains.