A cationic polymerization

Besides being used as initiators and monomers, azo compounds may also be used for terminating a cationic polymerization. Thus, the living cationic polymerization... 

A. Cationic Polymerization with Si-Cl Containing Initiator/Et2AlCl Initiating Systems... 

Initial work was carried out with 3,9-bis(methylene-2,4,8,10-tetraoxaspiro[5,5] undecane) where R = H (11). However, this monomer contains two electron donor alkoxy groups on one double bond which is thus highly susceptible to a cationic polymerization. For this reason, the monomer is extremely difficult to handle and cannot be analyzed by gas chromatography since it does not survive passage through the column. It is prepared by the dehydrohalogen-ation reaction of the reaction product of pentaerythritol and chloro-acetaldehyde,... 

It is unfortunate that many workers have not appreciated how essential a clue to the kinetics can be provided by the kinetic order of the whole reaction curve. The use of initial rates was carried over from the practice of radical polymerisation, and it can be very misleading. This was in fact shown by Gwyn Williams in the first kinetic study of a cationic polymerization, in which he found the reaction orders deduced from initial rates and from analysis of the whole reaction curves to be signfficantly different [111]. Since then several other instances have been recorded. The reason for such discrepancies may be that the initiation is neither much faster, nor much slower than the propagation, but of such a rate that it is virtually complete by the time that a small, but appreciable fraction of the monomer, say 5 to 20%, has been consumed. Under such conditions the overall order of the reaction will fall from the initial value determined by the consumption of monomer by simultaneous initiation and propagation, and of catalyst by initiation, to a lower value characteristic of the reaction when the initiation reaction has ceased. 

PVA film/ Tetraborate (TB)/ Proteolytic enzyme Protease C (Pr)+, a cationic polymeric antimicrobial (AM=Metacid) 235... 

An alkoxyamine with an appropriate functional group can be used to terminate a non-NMP polymerization to yield a polymer with an alkoxyamine end group, which subsequently initiates NMP of the second monomer, such as termination of a cationic polymerization by LX... 

The most nucleophilic of the reagents discussed, such as water, alcohol (often with KOH), ammonia, and amines, are often used in excess to quench a cationic polymerization. This is typically carried out after complete (or at least maximum) conversion has been reached in order to inactivate the coinitiator by the process described above. 

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. 

Under what reaction conditions might a cationic polymerization with SnCl4 plus water show a dependence of the polymerization rate that is... 

It has previously been shown that large changes can occur in the rate of a cationic polymerization by using a different solvent and/or different counterion (Sec. 5-2f). The monomer reactivity ratios are also affected by changes in the solvent or counterion. The effects are often complex and difficult to predict since changes in solvent or counterion often result in alterations in the relative amounts of the different types of propagating centers (free ion, ion pair, covalent), each of which may be differently affected by solvent. As many systems do not show an effect as do show an effect of solvent or counterion on r values [Kennedy and Marechal, 1983]. The dramatic effect that solvents can have on monomer reactivity ratios is illustrated by the data in Table 6-10 for isobutylene-p-chlorostyrene. The aluminum bromide-initiated copolymerization shows r — 1.01, r2 = 1.02 in n-hexane but... 

A main feature of the melt polymerization is the sharp increase in electrical conductivity at temperatures (>220°C) where polymerization occurs. This observation together with those described above support a cationic polymerization mechanism based on ionization of a P—Cl bond (Eq. 7-113) followed by electrophilic attack of P+ on monomer (Eq. 7-114). Boron... 

Traditional Ziegler-Natta and metallocene initiators polymerize a variety of monomers, including ethylene and a-olefins such as propene, 1-butene, 4-methyl-1-pentene, vinylcyclo-hexane, and styrene. 1,1-Disubstituted alkenes such as isobutylene are polymerized by some metallocene initiators, but the reaction proceeds by a cationic polymerization [Baird, 2000]. Polymerizations of styrene, 1,2-disubstituted alkenes, and alkynes are discussed in this section polymerization of 1,3-dienes is discussed in Sec. 8-10. The polymerization of polar monomers is discussed in Sec. 8-12. 

In addition to the primary application of PTMEG in polyurethanes, polyureas, and polyesters, a considerable number of reports of other block and graft polymers highlighting PTME units have appeared. Methods have been developed that allow the conversion of a cationically polymerizing system to an anionic one or vice versa (6,182). 

An example of a grafting reaction via cationic active centres is the reaction of the allylic —Cl of polyvinylchloride (formed by partial loss of HC1 from the polymer) with A1R2C1, which leads to a carbocation along the polymer chain which, in the presence of a suitable monomer, can initiate a cationic polymerization 20). 

As an illustration of initiation of a cationic polymerization by a zwitterionic tetramethylene, the polymerization of JV-vinylcarbazole (NVCz) in the presence of dimethyl 2,2-dicyanoethylene-l,l-dicarboxylate was studied in great detail [136] (Scheme 3). The cationic homopolymerization of NVCz could be initiated by adding either the electrophilic olefin or the cyclobutane adduct. The proposed mechanism involves bond formation to the zwitterionic tetramethylene, which closes reversibly to the cyclobutane adduct, and can be trapped with methanol. 

Kennedy and Kelen studied a cationic polymerization with transfer proceeding by reversible reactions [28], The process can be represented by the scheme... 

It was demonstrated that the P-centered radical of BAPO and its derivatives is oxidized by cationic photoinitiators (onium salts in their ground state). Phosphonium ion, a BAPO fragment, can efficiently initiate cationic polymerization. Thus, under certain conditions, PI used mfree radical polymerization can initiate a cationic polymerization as well. ... 

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... 

Polymers produced by cationic vinyl polymerizations include poly(A/-vinylcarbazole) and poly(vinyl ether). However, polyisobutylene and its copolymer with isoprene (butyl rubber) is probably the most important commercial polymer produced by a cationic polymerization. Other industrial polymers such as poly(styrene) can be prepared by cationic polymerization, although they are usually produced radically or anionically. Many low molecular weight polymers produced by cationic polymerizations of... 

In general, photolysis either leads to protonic acids or Lewis acids, which iratiate a cationic polymerization of oxies. However, the formation and initiation step of the active species of several initiators are not yet fully dari/ied. 

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. 

In general, an alternating eopolymer is formed over a wide range of monomer compositions. It has been reported that little chain transfer occurs, and in some cases, conventional free radical retarders are ineffective. Reaction occurs with some combinations, like styrene-acrylonitrile, when the monomers are mixed with a Lewis acid, but addition of a free-radical source will increase the rate of polymerization without changing the alternating nature of the copolymer. Alternating copolymerizations can also be initialed photochemically and electrochemically. The copolymerization is often accompanied by a cationic polymerization of the donor monomer. 

Such a process constitutes the initiation reaction of a cationic polymerization, all pre-initiation phenomena being absent. This situation can be regarded as a model for the reaction which must follow the charge formation process of any chemical initiation, and marks the point at which some generalizations can be introduced into cationic reaction mechanisms, e.g. 

Fukui et al. [37] had found that CO2 above 2 mole % inhibited the spontaneous polymerization of formaldehyde. A kinetic study of the 7-ray polymerization of formaldehyde was carried out in the presence of CO2 (no copolymerization between formaldehyde and CO2 could be detected) at temperatures from +13 to —17°C. Experimentally the spontaneous contribution could be separated from the 7-ray initiated polymerization, presumably a cationic polymerization. 

The nature of the active sites is open to discussion. Toby et al. chose to follow a possible earlier suggestion and used the addition of formaldehyde to a neutral polymer chain as the propagation mechanism. Because of a slight inhibition of the polymerization by oxygen, a radical mechanism was not completely ruled out. Looking at the formaldehyde polymerization as a whole and accepting some of the observations of Toby et al., it must be concluded that their formaldehyde polymerization was a cationic polymerization. Active centres, or active sites were actually oxonium... 

Tetrahydrofuran, just as other cyclic ethers, can be submitted to a cationic polymerization. It leads to polytetrahydrofuran (PTHF). Schematically, this can be expressed as follows ... 

For obtaining a cationic polymerization, the new carbocation generated between R and the monomer should have enough stability to be relatively easily formed and to continue the polymerization (for example, CH2=CH2 is not polymerized using a cationic initiator, while (CH3)2C=CH2 can be polymerized because the species RCH2-C(CH3)2 is stable enough to be formed). The stability of the carbocation increases as the chain length increases. Chain transfer reactions are common in carbocation polymerization. The termination reactions typically occur because of the combination of the cationic component with a counterion. 

The polymerization shown in Scheme 12 was observed at room temperature and explains the stoichiometric formation of vinylidene end groups. This reaction seems to be a cationic polymerization rather than an example of CCT. 

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