Polymerization, initiators cationic type

In cationic polymerization the active species is the ion which is formed by the addition of a proton from the initiator system to a monomer. For vinyl monomers the type of substituents which promote this type of polymerization are those which are electron supplying, like alkyl, 1,1-dialkyl, aryl, and alkoxy. Isobutylene and a-methyl styrene are examples of monomers which have been polymerized via cationic intermediates. 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

The actual species responsible for cationic polymerizations initiated by ionizing radiation is not established. The most frequently described mechanism postulates reaction between radical-cation and monomer to form separate cationic and radical species subsequently, the cationic species propagates rapidly while the radical species propagates very slowly. The proposed mechanism for isobutylene involves transfer of a hydrogen radical from monomer to the radical-cation to form the r-butyl carbocation and an unreactive allyl-type radical ... 

Carbonyl monomers can be polymerized by acidic initiators, although their reactivity is lower than in anionic polymerization. Protonic acids such as hydrochloric and acetic acids and Lewis acids of the metal halide type are effective in initiating the cationic polymerization of carbonyl monomers. The initiation and propagation steps in polymerizations initiated with protonic acids can be pictured as... 

Unlike cationic polymerization initiated by a conventional catalyst, the propagating species in the present system would bear different type of counter-ion or would be much more free. The counter-anion obtained in this entirely organic system would be large and unstable. The problem of the counterion in charge transfer ionic polymerization certainly requires further study. 

The cationic polymerization of styrene sulfide has been reinvestigated by Van Craeynest (15). With triethyloxonium tetrafluoroborate as initiator, a rapid and quantitative polymerization was observed, followed by a slow degradation of die polymer to a mixture of cis and tram 2,5-diphenyl-l, 4-dithiane and as and tram 2,6-diphenyl-1,4-dithiane. Since the BF4 counter ion is not capable of forming a covalent bond, a back-biting reaction via sulfonium ions seems the plausible mechanism for the dimer formation. The polymerization initiated with dimethyl sulfate showed the same characteristics a fast polymerization is followed by degradation to the same mixture of isomeric diphenyl- 1,4-dithianes. However, the mwts-2,5-diphenyl derivative was the only isomer that crystallized from the solution. It is therefore reasonable to accept that with dimethyl sulfate also, the cyclic dimers of styrene sulfide are formed by a back-biting type of degradation of the polymer and not by the mechanism shown above. 

On the basis of the profound difference in copolymer composition from a free radical or cationic type polymerization, it was stated that the sodium and potassium initiated polymerizations were carbanionic in nature. This has been one of the strongest arguments in favor of the anionic nature of the sodium and potassium polymerizations. The authors also suggested that the composition of a styrene-methyl methacrylate copolymer might be used as a criterion of the type of propagation induced by a given initiator. 

We have carried the theme of a primary electrophilic attack on monomer through the discussions on coordinated anionic polymerizations with simple alkyl metals, and on coordinated anionic and radical polymerizations with Ziegler type catalysts. When a Ziegler type catalyst is comprised of one or more strongly electrophilic components, the Lewis acidity can become great enough to initiate polymerization by a cationic mechanism. Naturally, this will occur most readily for those monomers which are prone to cationic initiation because of their ability to stabilize the carbonium ion 278). 

Table IV gives an example of our own work on the polymerization of a number of higher aldehydes. Potassium triphenylmethoxide—a soluble initiator—polymerized a number of higher aldehydes to crystalline isotactic poly aldehydes. Table V lists a number of alkali alkoxides and other related compounds used as initiators for the n-butyraldehyde polymerization. Neither the type of the alkoxide nor the cation is of any great importance for the polymerization rate, the polymer yield, and stereoregularity of the resulting polyaldehyde as long as the initiator is adequately soluble in the reaction mixture.

In cationic polymerizations, initiation occurs by attachment of a proton or some other Lewis-acidic cation X" to the H2C=CR2 double bond of a vinyl monomer to form a new carbon-centred cation of the type XH2C-CR2, which then grows into a polymer chain by subsequent H2C=CR2 additions (Figure 2, bottom). This type of polymerization works well - and is used in practice - only for olefins such as isobutene, where 1,1-disubstitution stabilizes the formation of a cationic centre. Since side reactions, such as release of a proton from the cationic chain end, occur rather easily, cationic polymerization usually gives shorter chains than anionic polymerization. 

Dormant Species and Pseudocationic Propagation The majority of propagating chain ends in most cationic polymerizations initiated by protonic acids and/or cocatalyzed by Lewis acids do not exist as carbenium ions, but are instead dormant species. The two major types of dormant species are onium ions and covalent esters or halides. The covalent species are formed by reversible reaction of carbenium ions with nucleophilic anions onium ions are generated by reaction of carbenium ions with noncharged nucleophiles such as ethers, sulfides, and amines. Because the majority of propagating chain ends exist as dormant species, they are often the only species that can be detected spectroscopically ... 

Only in two reports are there claims that the chains formed in a cationic polymerization are exclusively macrozwitterions when formaldehyde is polymerized by BF351) and vinyl ethers by TCNQ31). Neither of these polymerizations fit the criteria of an ideal macrozwitterion polymerization set out above. If cationic polymerizations of this type are to be successfully studied an initiator system must be developed which will generate macrozwitterions in a simple fashion from nucleophilic monomers. The organic chemical literature indicates the form such an initiator could take. 

The ABA-type block copolymers B-86 to B-88 were synthesized via termination of telechelic living poly-(THF) with sodium 2-bromoisopropionate followed by the copper-catalyzed radical polymerizations.387 A similar method has also been utilized for the synthesis of 4-arm star block polymers (arm B-82), where the transformation is done with /3-bromoacyl chloride and the hydroxyl terminal of poly(THF).388 The BAB-type block copolymers where polystyrene is the midsegment were prepared by copper-catalyzed radical polymerization of styrene from bifunctional initiators, followed by the transformation of the halogen terminal into a cationic species with silver perchlorate the resulting cation was for living cationic polymerization of THF.389 A similar transformation with Ph2I+PF6- was carried out for halogen-capped polystyrene and poly(/>methoxystyrene), and the resultant cationic species subsequently initiated cationic polymerization of cyclohexene oxide to produce... 

Recently, Yagci and coworkers investigated the initiation mechanism of a new type of cationic photoinitiator, namely, /V-phenacyl-/V,/V-dimethylanilinium hexafluoroan-timonate (PDA+ Sbl fi ) which initiates the polymerization of appropriate monomers [116]. The proposed mechanism includes irreversible fragmentation of the absorbent salt to yield the initiating species either via a heterolytic cleavage or via a homolytic cleavage followed by subsequent electron transfer between the preformed species still, forming the same cation that initiates cationic polymerization (Scheme 11.30). 

Polymerization studies with polar monomers indicate that some of these monomers can be polymerized at Ziegler-type sites (4, 5). Frequently secondary reactions often prevent propagation from occurring. The polar monomer may complex or react irreversibly with one or both of the catalyst components, or else one of the catalyst components may serve as a radical or cationic initiator for polymerization of the monomer. Prevention of these side reactions permits a more favorable Ziegler-type polymerization. 

The evolution of nitrogen on photolysis of the aryIdiazonium salts appears to have limited the use of these systems to thin film applications such as container coatings and photoresists (23). Other efficient photoinitiators that do not produce highly volatile products have been disclosed (24-27). These systems are based on the photolysis of diaryliodonium and triarylsulfonium salts. Structures I and II, respectively. These salts are highly thermally stable salts that upon irradiation liberate strong Bronsted acids of the HX type (Reactions 43 and 44) that subsequently initiate cationic polymerization of the oxirane rings ... 

The type of propagating species is mainly determined by the nucleophilicity of the monomer and basicity of the leaving group of the initiator, whereby covalent propagating species are present when the basicity of the counterion is higher than the nucleophilicity of the monomer and cationic propagating species are present if the basicity of the counterion is lower than the monomer nucleophilicity. The most nucleophilic monomer, 2-methyl-2-oxazoline, polymerizes via cationic propagating species with all counterions except... 

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