Page 256 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Free Radical Chain Polymerization 219

4. The rate-controlling step is the rate of initiation.
5. There is a steady-state concentration of growing chains.
6. The kinetic chain length, v, is equal to DP for disproportionation termination, but DP = 2v
for termination by coupling.
7. Chain-transfer reactions almost always decrease DP and will often introduce branching as
it occurs within or between polymer chains. Solvent also can act as effective chain-transfer
agents lowering chain length.
8. Vinyl monomers can be polymerized using solution, bulk, suspension, and emulsion tech-
niques. Each have their own characteristic strengths and weaknesses.
9. When polymerizations become viscous, termination slows, allowing an increase in the num-
ber of growing chains and rate of polymerization. This is known as the gel of Trommsdorff
effect. If such reactions are allowed to continue without cooling, explosions are possible.
10. Monomers may be polymerized using a water-soluble initiator while dispersed, by agitation,
in a concentrated soap solution. In this emulsion system, initiation occurs in the aqueous
phase and propagation occurs in the soap micelles. Since the growing macroradicals are
not terminated until a new free radical enters the micelle, high molecular weight products
are rapidly obtained. The rate of polymerization and DP are proportional to the number of
activated micelles.
11. Polyfluorocarbons are resistant to heat, solvent, and corrosives. The resistance is greatest in

the regularly structured PTFE and decreases as the geometry is upset by substitution of other
atoms for fl uorine.
12. PVC is an important polymer used for many commercial uses, including as pipes for water-
delivery systems within homes.

GLOSSARY
Backbiting: Hydrogen atom abstraction that occurs when a chain end of a macroradical
doubles back on itself forming a more stable hexagonal conformation.
Branch point: Point on a polymer chain where additional chain extension occurs producing a
branch.
Bulk polymerization: Polymerization of monomer without added solvents or water; also called
neat.
Ceiling temperature (T ): Characteristic temperature above which polymerization occurs but
c
the polymer decomposes before it is recovered.
Chain stopper: Chain-transfer agent that produces inactive free radicals.
Chain transfer: Process in which a free radical abstracts an atom or group of atoms from a
solvent, telogen, or polymer.
Chain-transfer constant (C ): Ratio of cessation or termination of transfer to the rate of
s
propagation.
Critical micelle concentration: Minimum concentration of soap in water that will produce
micelles.
Dead polymer: Polymer in which chain growth has been terminated.
Disproportionation: Process by which termination occurs as a result of chain transfer between
two macroradicals yielding dead polymers.
Half-life time: Time required for half the reactants to be consumed (generally for a fi rst-order
reaction).
Heterolytic cleavage: Cleavage of a covalent bond that leaves one electron with each of the
two atoms. The products are free radicals.
Homopolymer: Polymer composed of only one repeating unit.
Kel F: Trade name for polytrifl uorochloroethylene.
Kinetic chain length: Length of the polymer chain initiated by one free radical.

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