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Initiation: This Is A Two-Step Process Involving: (1) The Decomposition of The Initiator Into Primary Radicalsb

(1) Polymerization involves initiation, propagation, and termination steps. Initiation involves the decomposition of an initiator into primary radicals and the addition of monomers to these radicals. (2) Propagation is the successive addition of monomers to the growing radical chain. All propagation steps have the same rate constant kp and it is a fast process. (3) Termination can occur via two mechanisms: combination of two radicals or disproportionation. The overall termination rate constant is kt=ktc+ktd, where ktc and ktd are the rate constants for termination by combination and disproportionation, respectively. At steady state, the rate of radical production equals the rate of radical destruction.

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Younis Muhsin
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140 views4 pages

Initiation: This Is A Two-Step Process Involving: (1) The Decomposition of The Initiator Into Primary Radicalsb

(1) Polymerization involves initiation, propagation, and termination steps. Initiation involves the decomposition of an initiator into primary radicals and the addition of monomers to these radicals. (2) Propagation is the successive addition of monomers to the growing radical chain. All propagation steps have the same rate constant kp and it is a fast process. (3) Termination can occur via two mechanisms: combination of two radicals or disproportionation. The overall termination rate constant is kt=ktc+ktd, where ktc and ktd are the rate constants for termination by combination and disproportionation, respectively. At steady state, the rate of radical production equals the rate of radical destruction.

Uploaded by

Younis Muhsin
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A.

INITIATION
This is a two-step process involving:
(1) The decomposition of the initiator into primary radicalsb

(2) The addition of a monomer to the primary free radical

The constants kd and ka are the rate constants for initiator dissociation and monomer
addition, respectively

Where [I] represents the initiator concentration.


B. PROPAGATION
The successive addition of monomers during propagation may be represented as follows

This means that all the propagation steps have the same rate constants kp. In addition,
propagation is a fast process.

Where [M*] =ΣRMx• i.e., the sum of the concentrations of all chain radicals of type RM

C. TERMINATION
Chain growth may be terminated at any point during polymerization by either or both of
two mechanism.

Combination (coupling) Disproportionation

If there is no need to distinguish between the two types of termination, which in any case
are kinetically equivalent, then termination may be represented as:
Where kt (ktc+ ktd), ktc, and ktd are the rate constants for overall termination process,
termination by coupling, and termination by disproportionation, respectively. The
termination rate is given by:

The factor of 2 arises from the fact that at each incidence of termination reaction, two
radicals disappear. Over the course of polymerization (at steady state), the total radical
concentration remains constant.
This means that radicals are being produced and destroyed at equal rates (i.e., Ri= Rt)

Since the overall polymerization rate is essentially the rate of monomer consumption
during propagation.

Example
The data for the bulk polymerization of styrene at 60°C with benzoyl peroxide as initiator are
[M] = 8.35 × 103 mol/m3
[I] = 4.0 mol/m3
kp2/kd = 1.2 × 10–6 m3/mol-s
If the initial rate of polymerization of styrene is 0.026 mol/m 3-s and the spontaneous
decomposition of benzoyl peroxide in styrene is 2.8×10–6 s–1, what is the efficiency of the
initiator?
Solution

 The kinetic chain length, ν, is defined as the average number of monomers


consumed by each primary radical. Obviously from this definition, the magnitude
of the kinetic chain length will depend on the rate of the propagation relative to the
termination rate, i.e.,

Equation 1
 At the steady state the kinetic chain length as

 Since at steady state the rate of initiation equals the rate of termination, Equation above can also
be written as

Substituting for [M·]

Equations quite general and do not depend on the nature of initiation. Notice the
inverse relation between the kinetic chain length and the radical concentration. For
reactions initiated by the decomposition of an initiator, thus

Recall that the rate of consumption of monomers by an active center (disappearance


of monomers) is, by definition, the rate of propagation, Rp. Now, in termination by
combination, two growing chains undergo mutual annihilation to produce a single
inactive polymer molecule, whereas for termination by disproportionation, a
biomolecular annihilation of active polymer chains results in two polymers.

But kinetic chain length is given by Equation 2

Example: - For pure styrene polymerized at 60°C, the value of the ratio k p2/kt is
0.0012 1/mol-s. The corresponding value for pure vinyl acetate polymerized at
60°C is 0.125 l/mol-s.
a. Estimate their relative kinetic length chain lengths.
b. Calculate the kinetic chain length for polystyrene if the rate of
polymerization is 10–4 mol/l-s and monomer concentration is 8.35 mol/l.

Solution

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