Oxirane polymerization Initiators

In the first step the oxirane polymerization initiated by the isomerized EN dianion has been studied. The results reported in Table III and the low polydispersity observed (about 1.1 Figure 5) are conclusive for a living anionic system. Moreover the initiation rate of which is higher than the propagation one. 

From the kinetics of the oxirane polymerization initiated hy alcoholate (l6) and hy fluorenyl potassium (27) and as fluorenyl and dihydronaphthalene mono anion (28) have approximately the same basicity, the ka2 over kpr0p ratio may be estimated to 20. Therefore, the length of the two growing polyether chains must be largely independent on the nature of the initiating site. 

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

The epoxy or oxirane group is characterized by its reactivity toward both nucleophilic and electrophilic species and it is thus receptive to a wide range of reagents. Epoxy monomers polymerize through step-growth and chain-growth processes. The ionic polymerization initiated by both Lewis bases or acids will be discussed later (Sec. 2.3.4). The case of polyaddition polymerizations is mainly represented by epoxy-amine reactions. 

A very recent paper by Saegusa, 7) describes a method of synthesizing polyoxyethylene macromonomers bearing a polymerizable heterocycle at the chain end. Here again the method involves initiation of the oxirane polymerization by means of an alcoholate (derived from 2-p-hydroxyphenyl)-oxazoline). As metalation agent butyllithium, was used since lithium alcoholates are not very reactive towards oxirane. Indeed, the macromonomers obtained exhibit very low degrees of polymerization. Deactivation was performed with methyl iodide ... 

Bis(chlorodimethylsilyl)benzene-AgPF6 system was shown to act as a bifunctional initiator of substituted oxirane polymerization [42], Tri-methylsilyl iodide and triflate were used also as initiators of the cationic polymerization of oxazolines [43]. In this system, however, in contrast to typical initiation mechanism of oxazoline polymerization, O-silylation leads to initiation, because of the unfavorable charge distribution in N-siiyiated species ... 

Although there are some limitations on the molecular weights of the linear polymers which may be obtained by this method, AM polymerization offers an attractive, synthetic route for preparation of functional, medium molecular weight polymers by cationic polymerization of oxiranes. As the process involves the extension of the chain of hydroxyl group containing compound used to initiate the polymerization (initiator), the method is especially well suited for preparation of oligodiols (low molecular weight diols as initiators) and macromonomers (for example, hydroxy-ethyl acrylate as initiator) ... 

Tsuruta, T. Stereochemical behavior of oxirane and bioxirane in their polymerizations initiated with an organozinc complex having chair-type structure. Makromol. Chem., Macromol. Symp. 1991, 47, 277-283. 

Although in photocurable formulations, difunctional oxirane derivatives are employed for mechanistic studies, monofunctional oxiranes are used including cyclohexene oxide, styrene oxide, or phenyl glycidyl ether. These studies indicate that the cationic polymerizations proceeding as a result of photoinitiation by onium salts have typical characteristics of polymerizations initiated by strong protonic adds. Thus, initiation involves protonation of oxirane ring while propagation proceeds on tertiary oxonium ions as active species, that is, by the ACE mechanism. 

Calcium initiators are much weaker than the aluminum or zinc systems in forming oxirane complexes. As a result, they are particularly effective in polymerizing EO, which forms complexes more readily in comparison with the substituted oxiranes. Calcium initiators form weaker complexes with the substituted oxiranes, PO in particular, and initiate a low-rate polymerization. 

It was later shown that a small amount of water added to the FeCVpropylene oxide initiator system increased both the polymerization rate and the amount of crystalline product formed (67, 70) (Figure 2). This effect of an added reactive species such as water, an alcohol, or acetylacetone to one of the stereospecific initiator structures having a strong influence on polymerization rate and stereospecificity has been found in a number of the important oxirane polymerization systems. 

Early work identified Group II organometallic compounds such as diethylzinc and ethylmagnesium bromide as coordinate initiators of oxirane polymerization. Under carefully controlled conditions with anhydrous, high-purity monomer, the initiator activity was usually found to be weak. Bailey (80) found that with propylene oxide, distilled and maintained anhydrous with molecular sieves, the rate of polymerization of propylene oxide with dibutylzinc actually decreased as the concentration of dibutylzinc increased. This observation strongly suggested that a trace cocatalyst was needed to effect polymerization. 

The most active polymerization initiators for the polymerization of substituted oxiranes, particularly propylene oxide, produce a significant amount of product that is stereospecific and crystallizable. Most uses for polymers of propylene oxide, however, are in elastomeric systems (see chapter 2, table 4). For elastomers, the amorphous, noncrystallizable polymers are pre-... 

Initiation of the ionic polymerization by means of an unsaturated compound may also give access to macromonomers, provided the double bond cannot get involved in the polymerization. This method chiefly applies to heterocyclic monomers such as oxolane or oxirane, the initiator being respectively methacryloyl hexafluoroantimonate or the potassium alkoxide derived from p-isopropenylbenzyl alcohol (Scheme 29). 

Dimerization and polymerization can be initiated by electrophilic attack on an oxirane by a protonated or Lewis acid coordinated oxirane (Section 5.04.3.2.2). 

The use of an unsaturated anionic initiator—such as potassium p-vinyl benzoxide—is possible for the ring opening polymerization of oxirane [43]. Although initiation is generally heterogenous, the polymers exhibit the molecular weight expected and a low polydispersity. In this case, the styrene type unsaturation at chain end cannot get involved in the process, as the propagating sites are oxanions. 

This explanation turned out to be incorrect for this system, although it might apply to some future one. Indeed, recent studies of Teyssie8 b) revealed that in the polymerization of oxiranes initiated in non-polar solvents by Al—Zn oxyalkoxides the slower methyl oxirane is more strongly coordinated with the catalytic center than the faster epichlorhydrine. It is possible that the behaviour postulated by Korotkov might be observed in such systems. 

Our discovery that epoxides can initiate carbocationic polymerization led to the effective direct functionalization of PIBs with hydroxyl groups. Figure 7.18 shows our novel method of direct surface functionalization of SDIBSs using 4-(l,2-oxirane-isopropyl)-styrene, a new inimer. 

The initial formulation of Epon 828 and NMA must be such that oxirane equivalents equal the concentration of anhydride groups. A balanced stoichiometric ratio enables the polymerization to develop macromolecules at high extents of reaction (10). Therefore... 

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