Initiator-coinitiator complex

The initiator-coinitiator complex BF3-OII2 is often shown as H+(BF3OH). Initiation by aluminum chloride and f-butyl chloride is described by... 

Many polymerizations exhibit a maximum polymerization rate at some ratio of initiator to coinitiator [Biswas and Kabir, 1978, 1978 Colclough and Dainton, 1958 Taninaka and Minoura, 1976]. The polymerization rate increases with increasing [initiator]/[coinitiator], reaches a maximum, and then either decreases or levels off. Figure 5-1 shows this behavior for the polymerization of styrene initiated by tin(IV) chloride-water in carbon tetrachloride. The decrease in rate at higher initiator concentration is usually ascribed to inactivation of the coinitiator by initiator. The inactivation process in a system such as SnCl4-H20 may involve hydrolysis of Sn—Cl bonds to Sn—OH. There is experimental evidence for such reactions when comparable concentrations of coinitiator and initiator are present. However, the rate maxima as in Fig. 5-1 are observed at quite low [initiator]/[coinitiator] ratios where corresponding experimental evidence is lacking. An alternate mechanism for the behavior in Fig. 5-1 is that initiator, above a particular concentration, competes successfully with monomer for the initiator-coinitiator complex (V) to yield the oxonium salt (VI), which... 

Diaryliodonium and triarylsulfonium salts act as photoinitiators of cationic polymerization. Photolytic celeavage of an Ar—I or Ar—S bond yields a radical-cation (Eq. 5-8) that reacts with HY to yield an initiator-coinitiator complex that acts as a proton donor to initiate... 

Termination by combination differs from the other modes of termination in that the kinetic chain is usually terminated, since the concentration of the initiator-coinitiator complex decreases. 

Another consideration in the application of the various kinetic expressions is the uncertainty in some reaction systems as to whether the initiator-coinitiator complex is soluble. Failure of the usual kinetic expressions to describe a cationic polymerization may indicate that the reaction system is actually heterogeneous. The method of handling the kinetics of heterogeneous polymerizations is described in Sec. 8-4c. 

Lewis acids such as BF3 and SbCl5, almost always in conjuction with water or some other protogen, initiate polymerization of cyclic ethers. The initiator and coinitiator form an initiator-coinitiator complex [e.g., BF3 H20, H+(SbCl6) ], which acts as a proton donor (Sec. 5-2a-2). Cationic photopolymerizations are achieved when similar proton donors are formed by the photolysis of diaryliodonium and triarylsulfonium salts (Sec. 5-2a-4). 

The extent of formation of the initiator-coinitiator complex [i.e., the value of K in Eq. (8.111)] and its rate of addition to monomer [i.e., the value of ki in Eq. (8.112)] generally increase with increasing addiQ of the initiator and that of the coinitiator. Thus for the initiators the general order is... 

Spontaneous termination is a rearrangement of the propagating ion pair and involves regeneration of initiator-coinitiator complex by expulsion from the propagating ion pair the polymer molecule is thus left with terminal unsaturation. For example, for the system isobutylene/BF3/H20,... 

Water, when present in very small concentrations, acts as an initiator and initiates polymerization in combination with a coinitiator (e.g., SnCU). However, in larger concentrations, it inactivates the coinitiator (such as by hydrolysis of SnQ4) or competes successfully with monomer for the initiator-coinitiator complex to form... 

Even when conditions are scrupulously controlled, the kinetics of cationic polymerization are rarely simple. Water is highly reactive towards organic cations and if present as initiator, any excess will terminate polymer chains. Excess water may also destroy the coinitiator in some cases, or compete successfully with monomer for the initiator-coinitiator complex (see later). The kinetic influence of water is thus complicated. In some systems, the initial rate of polymerization increases with concentration of water at low concentrations and becomes independent as this concentration increases. Such behavior has been reported for the polymerization of isobutene in dichloromethane initiated by titanium tetrachloride and water [21]. In other systems, the initial rate of polymerization may rise to a maximum and then decline with increasing concentrations of water. Such behavior has been observed in the SnCl4/H20 initiated polymerization of styrene in carbon tetrachloride [22]. 

Lewis acids such as AlCl or BF together with small concentrations of water or other proton source are most often used to initiate cationic chain polymerization. The two components of the initiating system form an initiator-coinitiator complex which donates a proton to monomer... 

---