Lewis Acids in Cationic Initiations

Complexation of Lewis acids with water is another case of formation of electrophiles that can initiate chain growth. MeX_n represents a Lewis acid

The above can be shown on boron trifluoride initiation of the polymerization of isobutylene:

In the same manner, are action of a Lewis acid with an alkyl halide and subsequent initiation can be illustrated as follows:

In the above shown examples the Lewis acid is actually the co-initiator and the water or the alkyl halide are the initiators. Numerous studies confirm the need by many Lewis acids for other molecules like, water or alkyl halides to form initiating ions. For instance, pure TiCl4 will not initiate the polymerization of isobutylene [16, 17]. This led many to believe that none of the Lewis acids are capable of initiating cationic polymerizations of olefins by themselves [18]. Subsequent investigations, however, demonstrated [28, 30] that some strong Lewis acids, are capable of initiating such polymerizations. Whether a Lewis acid is capable of initiating these polymerizations by itself was tested with the aid of an early discovery. It has been known for some time that hindered bases like crowded pyridine derivatives exhibit specificity toward reactions with protons [19]. Such bases might be used to discriminate between two types of initiating mechanisms encountered with Lewis acids [20]. The base will not interfere with direct electrophilic additions of the Lewis acids to the monomer. On the other hand, it should prevent an initiation process by protons from taking place. If both pathways are operative, then the pyridine derivatives can only quench the protonic initiation and will offer a means of assessing the relative importance of each process. Kemedy et al. investigated polymerizations initiated by Lewis acids-water in the presence of one such "proton trap," 2,6-di-t-butylpyridine [21]. The presence of this base markedly decreased conversion from almost 100%, in some cases, to only a few percent. At the same time, there was a marked increase in the molecular weight and a narrowing of the molecular weight distribution of the products. This shows that the protonic reaction is by far the most important mode of initiation. It also suggested to Kemedy et al., that not all the protonic initiations occur exclusively by free protons. They might also proceed through concerted protonations. These would not be blocked by the "proton traps" [21]:

where, MeX3 represents a Lewis acid. The di-t-butyl pyridine does not block the above reaction due to steric compression. It also suggested that protonic initiations by water in the presence of Lewis acids are probably unlikely when sufficient quantities of materials that act as "proton traps" are present. This was felt to be true even in polar media [21]. Sigwalt and coworkers, however, disputed that [28]. They based their opinions on cationic dimerizations of 1,1-diphenylethylene with AlCl3-H2O in the presence of 2,6-di-t-butyl-4-methylpyridine. The results suggested to them that sterically hindered amines do not inhibit co catalytic initiation by AlCl3 H2O. Also, that the free hindered amine is a powerful terminating agent and that the sterically hindered pyridine forms a strong complex with AlCl3. When engaged in a complex with AlCl3 the reactivity of the hindered amine in termination is much reduced [28].

Alkyl halide solvents in the polymerization reaction mixture may cause an initiation to result from a transfer of AIX, or (RX)AIX₂ to the more basic monomer [23]:

where X ¼ halogen; R ¼ alkyl. In summarizing the initiation mechanism by complexed Lewis acids [24], the catalysts must be present in the reaction mixture in three forms: (1) as ionized molecules, (2) as ion pairs, and (3) as free ions. All three forms are in equilibrium. When the monomer is introduced, it complexes with some of the ionized molecules and a new equilibrium is established. Additional ion pairs form by a slow process. This depends upon a change in the monomer concentration [25]. Somestrong Lewis acids are actually capable of initiating carbon cationic polymerizations without a co-catalyst. As a result, many mechanisms were offered to explain the reaction paths of initiations by Lewis, like aluminum halides and some others, without the aid of any co-reactant. Following are some more prominent ones. One mechanism [26] was based Onan observation that olefins with allylic hydrogens like isoprene, methyl styrene, indene, and cyclopentadiene can be polymerized by super dry, pure Lewis acids, alone. This led to a suggestion that the process may involve an allylic self-initiation [26]:

The above suggested mechanism can only apply to polymerizations of monomers with allylic hydrogens. Also, it is contradicted by the ability of 1,1-diphenylethylene to dimerize in a super dry system in the presence of aluminum halides. Yet, this compound lacks allylic hydrogens [27]. Two other very similar mechanisms [28] are based on a concept that initiation takes place by a process of halo metalation:

Depending upon the substituents on the olefins, the metal organic compound may ionize in the presence of a second molecule of a Lewis acid, or it may eliminate HX:

The above mechanism [29] is based on an observation that strong Lewis acids can form stable molecularly bound complexes with hindered alkenes, as for instance with adamantyladamantane:

The carbon cation, which forms, from the above shown complex, actually cannot initiate a cationic polymerization of excess alkene molecules that are present in the reaction mixture, due to steric hindrance [29, 31]. The less hindered alkenes, however, like isobutylene polymerize rapidly under the same conditions with antimony pentafluoride. It was, therefore, suggested [29, 31] that initiation involves an opening of a previously formed л complex of SbF5 with the double bond, through halide participation. The a,ß-haloalkylantimony tetrafluoride ionizes with excess Lewis acid to form a related carbon cation and is capable of initiating cationic chain growth:

Alternately, the intermediate could be shown as splitting of HF that can then react with another molecule of Lewis acid, because the metalated alkene is a tertiary halide and can eliminate hydrogen halide:

The initiation mechanism of olefins by AlB3 was explained similarly [29]. The intermediate compound of aluminum bromide and the olefin is expected to lose HBr. Although initially the reaction mixture is free from protonic acid, it could form under conditions where initiation takes place by a conjugate acid catalyzed system. In addition, all cationic polymerizations of olefins should be considered as typical examples of general carbocationic reactivity in electrophilic reactions [29]. The separate mechanisms are to be looked upon as various examples that differ only in the nature of the initial electrophile. They always lead to the related trivalent alkyl cation when polymeric chain growth is initiated [29]:

Much of the evidence gathered to date, however, supports the concept of initiation due to auto ionization of Lewis acids. This was originally proposed in 1948 [30]. Lewis acids can aggregate, generally into dimers, and then auto ionize. The electrophilic portion adds to the olefin:

Afterwards, in 1965, a separate investigation [32] also led to the conclusion that the evidence does support the above mechanism. The additional data [33] shows that isobutylene can be polymerized by

super dry, pure AlBr3. At the same time, however, BF3 and TiCl4 require addition of water to initiate polymerizations of this monomer. Furthermore, the rate of AlBr3 initiated polymerization of isobu- tylene is enhanced considerably by addition of other Lewis acids, like SnCl4, SbCl5, TiCl4, and VCl4 [34, 35]. Significantly, these Lewis acids by themselves are incapable of initiating polymerizations of isobutylene [16, 17]. It is believed, therefore, that mixed Lewis acids interact and generate ions that lead to rapid polymerizations.

There were many investigations of isobutylene [36] polymerization. The earlier ones yielded additional supporting evidence. In this study, aluminum chloride and bromide catalysts were used in alkyl halide solutions. The reaction was monitored by conductivity measurements and tritium radiotracer techniques. Conductivity changes during and after polymerization and unexchanged tritium in the polymer support the theory that initiation does take place by addition of AIX2 to the double bonds of the monomers. This results in formation of the carbon cations [36]. It was also observed [36] that: (1) Only a very small fraction of the aluminum halide is ionized. (2) "Active species" are formed from reactions of AIX, with the double bonds and covalent bonds are formed between aluminum and carbon atoms. The cations that form start the polymerizations. Other cations may form from some impurities and may also act as initiators. The polymerizations that follow are terminated fairly rapidly because the concentration of the initiating cations is much smaller than the amount of aluminum halide. This explains the low efficiency of these particular initiators. Only a little polymer forms per mole of aluminum halide used. (3) Most of the unionized aluminum halide complexes with unreacted monomer molecules. These complexes do not act as initiators. The result is that the concentration of aluminum halide and the formation of AIX, becomes very small and there is no further initiation. Such conclusions explain why yields are low when the reactions are started by additions of the initiator solutions to the monomers, but are high, however, when the monomers are added to the initiator solutions [36]. Faust et al. carried out a detailed kinetic investigation to compare polymerization rates of isobutylene with Et2AICI, EtAIC12, and TiCI4. They demonstrated that EtAICI, induced faster polymerizations than did TICI in hexanes/CH3C at -80°C. Polymerization with EtAIC2 was also examined in dichloromethane or chloroform at low temperatures. The feasibility of living cationic polymeriza- tion with aluminum-based Lewis acids was examined in hexanes/CH3Cl also at -80°C. MeAICI2 induced rapid and less controlled reactions were completed within 30 s. On the other hand, slow and well-controlled polymerizations took place with Me2AICI. Despite rapid reactions, product polymers had narrow molecular weight distributions [36].

It is not clear why other Lewis acids like BCl3 require water or other compounds and must be added to the monomer to obtain high conversions [37]. Boron trifluoride can initiate polymerizations of styrene, presumably by itself, in a methylene chloride solvent [38]. One explanation that was offered for the different behaviors of various Lewis acids is based on differences in the electronic configurations around the metal atoms [12].

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مواضيع ذات صلة

One Electron Transposition Initiation Reactions

Metal Alkyls in Initiations of Cationic Polymerizations

Initiation by Protonic Acids

Cationic Polymerization

The Chemistry of Ionic Chain-Growth Polymerization

Polymer Preparation Techniques

Special Types of Controlled/Living Polymerizations

Combinations of Click Chemistry and ATP as Well as ATP and RAFT Polymerizations

Reversible Addition-Fragmentation Chain Transfer Polymerization

Nitroxide-Mediated Radical Polymerizations

Atom Transfer Radical Polymerizations

Controlled/“Living” Free-Radical Polymerization