ABSTRACT
The mechanism of free-radical polymerizations belongs to the class of so-called chain reactions. Chain reactions are characterized by the fast subsequent addition of monomers to an active center at the chain end. The activity of the growing chain is transferred to the adding unit. The active centers are present in very low concentrations (10-5-10-8 mol/L). The rate of addition is very high (103-104 units per second) and the time of growth of a chain (time between initiation and termination of a chain) is quite short (0.1-10 seconds) relative to the total reaction time, which can be in the order of several hours. This means that composition of the chain and chain length is determined in seconds. Terminated chains, in principle, do not take part in further reactions (except when transfer-to-polymer events occur). The final chemical composition distribution and molecular mass distribution is determined by the accumulation of rapidly produced dead chains (chains without an active center). In free-radical polymerization the active center is a free radical. During the formation of a polymer chain a number of subsequent kinetic events take place: (1) radical formation, (2) initiation, (3) propagation, and (4) termination. Transfer of the radical activity to another molecule is a complication that will often take place also. Sometimes a chain transfer agent is added deliberately to control and limit the molecular weight. 2.1.1 Radical Formation in Free-Radical PolymerizationThe formation of free radicals can take place in a number of ways. Radicals can be produced by photoinitiation and radiation (g-radiation or electron beams), electrochemically, and by thermal initiation. Well-known examples of the thermal decomposition of initiators are: j – C – O – O – C – j 80 90∞- ∞æ ÆæææC 2j – C – O• Æ 2j• + CO2 benzoyl peroxide (BPO) NC C(CH ) N N C(CH ) CN 2NC C(CH ) N
3 2 2 - - = - - æ Ææææ - ∑+- aa¢ azobis(isobutyronitrile) (AIBN)
A schematic representation of the decomposition of the initiator (I) into two radicals (R•), with a decomposition rate coefficient (kd), and an expression for the rate of decomposition of the initiator (vd) is given below: = d dt
= kv k
I 2R with rate [R ]
æ Ææ • • (2.1) This reaction has a high activation energy (140-160 kJ/mol, depending on the initiator), so kd depends strongly on temperature. In fact not all radicals will initiate a polymeric chain; some of the radicals are lost in side reactions (like recombination of the initiator fragments). For this reason the efficiency factor f is introduced. f is the fraction of radicals that actually initiate a polymeric chain, the rate of radical production (leading to an actual initiation step) riequals then: r i d = 2 [I]k f (2.2)
2.1.2 InitiationThis is the addition of the first monomeric unit to the initially formed free radical: R M RMi• •+ æÆæk with rate R k i i
[R ][M]= • (2.3) 2.1.3 Propagation This is the process for the growth of the chains: RM + M MM + M M
M + M M
• •
• •
• •
æ Ææ
æ Ææ
æ Ææ
-
- -
- -with rate R k p p
M ][M= [ ]• (2.4) 2.1.4 Termination Termination takes place via two types of bimolecular free-radical reactions: In combination the two radicals form a new bond, connecting the two growing chains to form one dead chain with the combined length of the two growing chains.
