Ether cationic polymerization of cyclic

Studies on the cationic polymerization of cyclic ethers, cyclic formals, lactones and other heterocyclic compounds have proliferated so greatly in the last few years that a detailed review of the evidence concerning participation of oxonium and analogous ions in these reactions cannot be given here. Suffice it to say that there is firm evidence for a few, and circumstantial evidence for many such systems, that the reactive species are indeed ions and there appears to be no evidence to the contrary. A few systems will be discussed in sub-sections 3.2 and 4.4. 

Carbon-13 NMR Studies on the Cationic Polymerization of Cyclic Ethers... 

Propagation in the cationic polymerization of cyclic ethers is generally considered as proceeding via a tertiary oxonium ion, for example, for the polymerization of 3,3-bis(chlor-omethyl)oxetane (R = CH2C1)... 

It is generally agreed that propagation in the cationic polymerization of cyclic ethers occurs after nucleophilic attack by the monomer oxygen atom (equation 3). Therefore, many authors attempt to explain their copolymerization data by noting that the more basic monomer has the higher reactivity with the active chain end. The order of basicity which has been established (36, 38) is ... 

Table 2. Oligomer formation during the cationic polymerization of cyclic ethers... 

The initiation mechanism for cationic polymerization of cyclic ethers, vinyl amines, and alkoxy styrenes has been investigated by A. Ledwith. He used stable cations, like tropylium or triphenylmethyl cations with stable anions, like SbCl6, and distinguished between three initiation reactions cation additions, hydride abstraction, and electron transfer. One of the typical examples of cationic polymerization, in which the propagating species is the oxonium ion, is the polymerization of tetra-hydrofuran. P. and M. P. Dreyfuss studied this polymerization with the triethyloxonium salts of various counterions and established an order of... 

It is well known that cyano derivatives of anthracene form charge transfer (CT) complexes with certain aromatic compounds. It was reported [67] that the radical cations formed upon irradiation of these complexes played an important role in initiation of cationic polymerization of cyclic ethers. Pyridinium salts were also found [68] to form CT complexes with hexamethyl benzene and trimethoxy benzene which result in the formation of a new absorption band at longer wavelengths where both donor and acceptor molecules have no absorption. This way the light sensitivity of the pyridinium salts may be extended towards the visible range. According to the results obtained from the... 

UV irradiation of the resulting prepolymers caused a-scission, and benzoyl and polymer bound electron donating radicals are formed in the same manner as described for the low-molar mass analogues. Electron donating polymeric radicals thus formed may conveniently be oxidized to polymeric carboeations to promote cationic polymerization of cyclic ethers. It was demonstrated that irradiation of benzoin terminated polymers in conjuction with pyridinium salts as oxidants in the presence of cyclohexene oxide makes it possible to synthesize block copolymers of monomers with different chemical natures [75] (Scheme 19). 

Another novel class of silicon-based initiators has been described [44]. Compounds containing Si—H bond in the presence of platinum catalysts (Ptl2, H->PtBr6, PtCl2(C6HsCN)2) are effective initiators of cationic polymerization of cyclic ethers. 

Diazonium Salts. The first use of a diazonium salt as an initiator for cationic polymerization of cyclic ethers was reported in 1965 by Dreyfuss and Dreyfuss (1 ), who announced the polymerization of tetrahydrofuran initiated by thermal decomposition of a "new catalyst" which they identified as benzene-diazonium hexafluorophosphate. In a later publication (13) this material was correctly identified as p-chlorobenzenediazomum hexaf1uorophosphate). 

Dreyfuss and Dreyfuss (13) showed that the cationic polymerization of cyclic ethers has the characteristics of a "living" polymerization, in that there appears to be a lack of termination except through reaction of the cationic growing chain end with impurities and that eventually a steady state is attained where the living polymer is in equilibrium with its monomer. 

In115 116) the cationic polymerization of cyclic ethers was examined theoretically and experimentally with regard to the nature of MWD variation. A theoretical analysis was made of how MWD is affected by the depolymerization reactions, monomole-cular deactivation of active centers, recombination of active centers, chain transfer by hydroxyl-containing compounds, chain transfer to the monomer, and ether oxygen of the polymer chain, as well as via the end hydroxyl group. 

Cationic Polymerization of Cyclic Ethers Initiated by Macromolecular Dioxoleniuni Salts... 

P. Kubisa, Activated monomer mechanism in the cationic polymerization of cyclic ethers. Makromol. Chem. Macromol. Symp. 1988, 13(4), 203-210. 

Cationic and anionic polymerizations of heterocyclic monomers provide many examples in which the concurrent formation of cyclics of various sizes is observed during the ring-opening polymerization. As illustrated in Scheme 1, in these systems active species follow three pathways they can react with a functional group of the monomer, of its own polymer chain, or of other chains. When the function / involved belongs to a linear polymer chain, intramolecular chain saambling or intermolecular macrocycle formation takes place, as observed in the cationic polymerization of cyclic ethers, acetals, esters, amides, siloxanes, and so forth. 

Propagation in the cationic polymerization of cyclic ethers proceeds as a nucleophilic attack of an oxygen atom on the carbon atom in a-position in an oxonium ion (Scheme 1). 

Knowledge of the order of basicities of cyclic and linear ethers is important for understanding certain phenomena in cyclic ether polymerization. As indicated earlier, chain transfer to polymer is a general feature of the cationic polymerization of cyclic ethers because the nucleophilic site of the monomer molecule (oxygen atom) is transferred to the polymer unit. To what extent chain transfer to polymer competes with propagation depends on the relative nucleophilicity of monomer and polymer unit. Thus, for five-membered THF, the polymer unit is a weaker base than the monomer. This makes the polymer less reactive than the monomer in nucleophilic substitution type reactions. Consequently, for this monomer, chain transfer to polymer is slow as compared to propagation. In contrast, in the polymerization of three-membered EO, the polymer unit is more basic than monomer. Therefore, reactions involving the polymer chain are important in this system. Practical consequences will be discussed in the subsequent sections devoted to polymerization of different classes of cyclic ethers. 

Propagation proceeds by nucleophilic attack of an oxygen atom in a monomer molecule on a carbon atom in a-position to an oxygen atom bearing formally the positive charge in a tertiary oxonium ion located at the chain end. Even such a simplified scheme indicates the possibility of a side reaction that is a typical feature of cationic polymerization of cyclic ethers. A nucleophilic center (oxygen atom) is present not only in monomers but also in polymer chains. Therefore, the attack of an oxygen atom from the chain on a carbon atom in a... 

Most commonly initiation proceeds as direct addition of initiator to monomer molecule (route 1). Cationic polymerization of cyclic ethers may be initiated by both Bronsted and Lewis acids. Most commonly used initiators include strong protic acids such as trifluoromethanesulfonic (triflic) acid (also its anhydride or esters), fiuorosulfonic acid, perchloric acid, or heteropolyacids, oxonium salts such as triethyloxonium (e.g., EtsC A ), carbenium (e.g., Ph3C A ), or carboxonium (e.g., CeHsCO A ) salts where A should be stable, weakly nucleophilic counterion (e.g., BF4, PFg, and SbFg) or Lewis acids (most commonly used is BF3 -Et20). Several other initiation systems have been used (e.g., rare earth triflates) but the advantages over typically used simple and easily available initiators have not always been shown. 

As already discussed, propagation in cationic polymerization of cyclic ethers by the ACE mechanism proceeds on tertiary 0x0-nium ion active species. Ionic species in general may exist in the form of ion-pairs (contact or solvent separated) and free ions. The fraction of each form is governed by a corresponding equilibrium constant that depends on the polarity of the medium. The knowledge of the fraction of different ionic forms, which is essential for the proper analysis of kinetics of anionic vinyl polymerization in which different forms show different reactivity, is less crucial in analyzing the kinetics of cationic polymerization of cyclic ethers because available data point out to equal reactivity of ion-pairs and free ions in propagation. 

In the cationic polymerization of THF, the concentration of active species was measured by capping the growing chain with sodium phenoxide and the determination of phenoxy groups in polymers by UV spectroscopy. More general methods have been developed in the Lodz group. Active species of cationic polymerization of cyclic ethers (and other heterocyclic monomers) were trapped by reaction with tertiary phosphine. It was shown that oxonium ions are fast and irreversibly converted to corresponding phosphonium ions that could be quantitatively analyzed by NMR using a known excess of phosphine as an internal standard without the need for polymer isolation. The principle of the method, which allows determination not only of concentration but also of the structure of active species, is outlined in Scheme 12. 

Although the chemistry of acetal and ether bonds is quite different, cyclic acetals bear some resemblance to cyclic ethers thus the polymerization of both groups of monomers shows some similarities. Essentially the same groups of initiators that initiate cationic polymerization of cyclic ethers (i.e., strong protonic acids, Lewis acids, or oxonium, carbenium, or oxocar-benium salts) are also effective in the polymerization of cyclic acetals. On the other hand, there are distinct differences. 

Scheme 9 Unimolecular opening of oxonium ion in the cationic polymerization of cyclic ether (THF) and cyclic acetal (DXL).

In contrast, in the cationic polymerization of cyclic ethers (e.g. THF or seven-membered oxepane), the dissociation equilibrium constants have considerably higher values, depending on the solvent polarity Ro-lO molh (CH2CI2) and - 10 molT (CH3NO2, QH5NO2) with SbClg as a counterion (see Table 8 in Ref [3a]). 

During the early 1980s, the cationic polymerization of cyclic ethers in the presence of low-molecular-weight diols as chain-transfer agent was studied with the aim of preparing polyether diols [82]. A more detailed investigation of this process revealed that the addition of alcohols to the polymerization of some oxiranes reduced the proportion of cyclics which was known to be formed by back-biting. The explanation for this observation was based on the activated monomer (AM) mechanism shown in Scheme 1.5 [171, 172]. 

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