Covalent initiators

The scales of solvent polarity discussed above are based on rate processes (6) in which a polar transition state is formed from a covalent initial state. There are also several scales based on an electronic transition in which an electron is transferred from one species to another or from one end of a molecule to another [eqn (7) Reichardt, 1965 Reichardt and Dimroth, 1968 Kosower, 1968]. All these scales represent aspects of the microscopic behav-... 

AU four of the elementary reactions in a cationic polymerization involve electrophilic or cationic intermediates. Thus, initiation, propagation, transfer, and termination may be classified as either nucleophilic substitution, electrophilic addition, elimination, rearrangement, or possibly as a pericyclic reaction. Initiation occurs in alkene polymerizations by either addition of acid to the alkene, or by ionization of a covalent initiator followed by addition of the resulting carbocationic intermediate to an olefin s double bond. Although initiation is an electrophilic addition (AdE) reaction in... 

The rates of initiation and propagation are comparable when the covalent initiator and dormant chain ends have similar structures. Therefore, 1-phenylethyl precursors are useful initiators for styrene polymerizations, but are poor initiators for a-methylstyrene and vinyl ether polymerizations. Similarly, cumyl derivatives are good initiators for isobutene and styrene, but are poor initiators for vinyl ethers their initiation of a -methylstyrene is apparently slow [165]. 1-Alkoxyethyl derivatives are successful initiators for vinyl ethers, styrenes, and presumably isobutene polymerizations [165,192]. /-Butyl derivatives initiate polymerization of isobutene slowly [105]. This is mirrored in model studies that show that /-butyl chloride undergoes solvolysis approximately 30 times slower than 2-chloro-2,4,4-trimethylpentane [193]. This may be due to insufficient B-strain in monomeric tertiary precursors [194]. In contrast, monomeric and dimeric or polymeric structures of secondary esters and halides apparently have similar reactivity. 

Because the covalent initiator is incorporated into the polymer chain as an end group, end-functionalized polymers can be prepared using functionalized initiators. This has been used extensively in vinyl ether polym-... 

Clearly, carbanionic macrozwitterions are not likely to be stable when they are generated from a monomer with a proton transferring group. This study of the polymerization of acrylic add has been included for the sake of completeness and to illustrate an intelligent approach to the problem of identifying the polymer molecules formed by covalent initiators. 

The covalent initiators employed are almost always tertiary amines or phosphines. However, Kunzel, Giefer, and Kern43), who studied formaldehyde polymerization, compared triphenylamine, phosphine, stilbene, and arsine. They found a rough correlation... 

Thus, one could, in principle, construct tte scale of catenating power and nucleophilicities from which it would be possible to conclude a priori whether polymerization of a given monom can be initiated by a chosen covalent initiator. 

Covalent Initiators. The initiation with alkylating compounds depends both on the ability of the initiator to form a cation and on the monomer nucleophilicity. Strong alkylating agents such as esters of superacids (CF SOoR, FSO2R, and CISO2R) are able to initiate directly without sides reactions both strong and weak nucleophilic monomers (44. 45). 

Table 2. Relative rates of initiation in cationic polymerizations initiated by covalent initiators in the presence of a Lewis acid catalyst [113].

The choice of initiator is also very important for a controlled cationic polymerization since initiation is often not quantitative in classic systems. Covalent initiators will be reactive enough to initiate polymerization if... 

As in anionic copolymerizations, the order of monomer addition is extremely important in achieving efficient crossover reactions in cationic copolymerizations. However, in contrast to anionic polymerizations the most efficient crossover reaction is not achieved by reacting the more reactive monomer with the more reactive propagating species. Instead, initiation of the second monomer is apparently fast and quantitative if the most reactive monomer is polymerized first. As demonstrated by the examples in Scheme 28, this generates the more easily ionized covalent chain end, which therefore corresponds to the efficiency of covalent initiators discussed in Sec. 2.3 of this chapter and... 

According to this interpretation, then, Br0nsted acids consist of a subclass of covalent initiators which would also include the acyl chlorates. It is characteristic of covalent initiators that they first form a cation and an anion only in the presence of the monomer. 

In contrast to covalent initiators, salts ionize readily in the presence of solvent into cations and anions. To some extent, the cations can be identified spectroscopically. Trityl chloride (triphenyl methyl chloride), for... 

Application of bulky ligands that fill the space around the countercation metal atom in covalent initiators prevent the aggregation - initially by Inoue and co-workers " and then by Spassky and co-workers ° - resulted in new mechanistic implications. Inoue introduced a notion of immortal polymerization for the already known processes with reversible chain transfer (the use of this expression is discouraged by lUPAC ). Polymerizations initiated with bulky chiral Schiff base Al alkoxides revealed a possibility of the stereodective polyester synthesis starting from the chiral cydic ester... 

When an excess of methyl triflate was used it was possible to establish that the covalent initiator is slightly more reactive than the trialkoxycarbenium ion. " Thus, the propagation reaction involves the cleavage of the alkyl-oxygen bond in the monomer. The active cationic chain end and the dead methyl-carbonate end-groups were identified by means of NMR spectroscopy. 

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