Page 35 - Engineering Plastics Handbook

Chemistry of Polymerization 9

The free electrons are relayed along the polymer chain to the end of the
polymer where the free electrons continue to chemically bond, forming poly-
mers. Monomers with reactive free radicals keep adding on to the grow-
ing polymer chains until either the reactive monomers or the initiator is
consumed. With chain-growth free-radical polymerization, monomers are
added to the free-radical ends of growing molecules which are effectively
long-chain free radicals. Two growing free radicals combine (coupling) or
disproportionate, causing termination of chain growth.
Free-radical polymerization can be terminated with a transfer agent
such as benzoquinone which consumes free radicals. Mercaptans (thiols)
such as butyl mercaptans are commonly used transfer agents. Free rad-
icals at macromolecule chain ends can selectively react with transfer
agents to terminate chain growth; but the free radical is transferred to
another macromolecule that continues to grow. Transfer can occur with
the initiator, monomer, macromolecule, and solvent. Monomer propa-
gation and transfer to the monomer are

Propagation R′+ CH = CHX → RCH = C′HX
2
2
Transfer R′+ CH = CHX → RH + CH = C′X
2
2
The relationship between kinetic chain length with chain transfer and
without chain transfer is expressed by the Mayo equation [9]

1 = 1  [ SX  ]
ν ν + C SX [  M ]  
tr

where ν = kinetic chain length with chain transfer
tr
ν= kinetic chain length without chain transfer
C SX = chain transfer constant
[SX] = concentration of chain transfer species
[M] = concentration of monomer

The Mayo equation is also expressed as

1 1
= + C transfer agent / monomer]
[
DP DP S
n n 0
where DP = degree of polymerization with a transfer agent
n
DP n0 = degree of polymerization without a transfer agent
C = chain transfer constant
S