Oxiranes, propagation

Consequently, linear polymer cannot be usually obtained because of kinetic enhancement in macrocycles connected with conversion of the active center of oxirane propagation to more stable nonstrained oxonium cations. Dale et have... 

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. 

Initiation resulting from insertion of the monomer into the Al—Cl bond is followed by propagation involving insertion between the porphinato-aluminum and the alkoxide group of the growing polymer, coupled with P-scission of the C—O bond of the oxirane monomer (demonstrated by nmr results) it yields a polyether terminated by a CH2C1 end-group. 

Oxirane57). If the conditions are chosen such as to avoid propagation (low temperature, Li+ or Na+ are better counterions than K+ in this respect) one single oxirane unit is added, yielding after protonation a polymer fitted at chain end with an alcohol function. 

These results combined with the total suppression of copolymerization in the presence of hydroquinone as inhibitor indicate that hydrostannylation takes place upon the polyaddition of diorganostannane to the epoxyolefine by a radical mechanism accompanied by hydrogen atom migration in each chain propagation, No addition of organostannanes to the oxirane ring was observed 98>. 

The additional complexity present in block copolymer synthesis is the order of monomer polymerization and/or the requirement in some cases to modify the reactivity of the propagating center during the transition from one block to the next block. This is due to the requirement that the nucleophilicity of the initiating block be equal or greater than the resulting propagating chain end of the second block. Therefore the synthesis of block copolymers by sequential polymerization generally follows the order dienes/styrenics before vinylpyridines before meth(acrylates) before oxiranes/siloxanes. As a consequence, styrene-MMA block copolymers should be prepared by initial polymerization of styrene followed by MMA, while PEO-MMA block copolymers should be prepared by... 

Polymerization of the oxiranes is typically propagated from a starter molecule that is chosen to define the functionality if) of the final polyol. The functionality and the molecular weight of polyols are the main design features that define the polyurethane properties in the end-use applications. Additionally, the balance of EO and PO in the polyether polyols, mainly for flexible foam polyols, is tailored to enhance the compatibility of formulations and the processability of the foam products. The exact composition of the polyols defines the crucial performance features of the final polyurethane product. Even seemingly small differences in polyol composition can result in changes to polyol processabihty and polyurethane performance. This becomes a crucial issue when replacing conventional petrochemical polyols with polyols from different feedstocks. To demonstrate the sensitivity of commercial formulations to changes in feedstocks, a simple example is offered below. 

Some cationic ring-opening polymerizations take place without termination and are reversible. Oxirane and oxetane polymerizations are seldom reversible, but polymerizations of larger-sized rings such as tetrahydrofuran are often reversible. The description of reversible ROP is presented below [Afshar-Taromi et al., 1978 Beste and Hall, 1964 Kobayashi et al., 1974 Szwarc, 1979]. It is also applicable to other reversible polymerizations such as those of alkene and carbonyl monomers. The propagation-depropagation equilibrium can be expressed by... 

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. 

It seems that the initiation step of the copolymerisation most likely involves the oxirane reaction [according to scheme (3)]. Zinc alcoholate species formed in this reaction can easily propagate the copolymer chain, coordinating and enchaining both the oxirane [scheme (3)] and the cyclic carbonate [scheme (15)] comonomers. However, in the case of the cyclic carbonate, its enchainment may also proceed according to scheme (14), leading to decarboxylation. Thus, the obtained poly(ether-carbonate)s are characterised by a lower content of carbonate units with respect to the ether units [82,146]. 

Taking the above into account, oxirane-carbon dioxide-tetrahydrofuran terpolymerisation should be considered to involve active sites that change in nature as the reaction progresses an account of cross-propagation, like those appearing and disappearing in schemes (6) and (7) [41,74,244],... 

The first step is the formation of an alkoxide anion by the initiating alcohol (allyl alcohol is the initiator most commonly used, although other initiators have been suggested). The appropriate oxide(s) is (are) then added to the alcohol initiator. This causes the opening of the oxirane ring in the oxide and propagates the chain growth of the alkylene oxide on the initiator. The last step is the neutralization of the alkoxide anion to terminate the polymerization. 

So far, not many systems are known to meet the conditions of the growth mechanism with activated monomer. Polymerizations of oxirane, chlo-romethyloxirane [145, 146] and of lactams [147] belong to this class. The existence of an electroneutral propagating macromolecule and of an activated monomer results in reduced probability of back-biting reactions and in easier preparation of macromers [145] by means of exchange reactions. 

Cationic polymerization is induced by Lewis acid catalysts. This type is mainly used in connection with higher cyclic ethers, since oxiranes produce only low-molecular-weight polymers. The propagation step is illustrated in Eq. 398. 

Small amounts of primary hydroxyl groups observed in PECHG are probably due to opening of the oxirane CH-0 linkage in the propagating species ... 

Most cationic ring-opening polymerizations of cyclic ethers involve the formation and propagation of oxonium ion centers. Reaction involves the nucleophilic attack of monomer on the oxonium ion, e.g., for 1,2-epoxides (oxiranes) ... 

End-labelling of the active centres with diphenyl chlorophosphate, combined with31P NMR spectroscopy, was shown to be an effective method for the determination of structure and concentration of the active species in the polymerization of several oxirane compounds.96 98 The chemical shifts of31P signals from the diphenylphosphoryl end-groups are sensitive to the structure of polymer chain end to which it is attached. Thus the 31P NMR spectrum provides information whether the propagating anion is secondary or primary. A similar... 

In139) the researchers studied the nonisothermal kinetics of polyaddition reactions of oxiranes to aromatic amines up to high conversion levels, as well as the kinetic laws governing the curing of epoxide oligomers by diamines under the conditions of a propagating reaction front. 

The mechanism of alkylene oxide anionic polyaddition to hydroxyl groups, catalysed by alkali hydroxides, is discussed in chapters 4.1-4.1.5, the real active centre being the alkaline alcoholate, and the propagation reaction being the repeated SN-2 attack of the alcoholate anion on the a-carbon atom of the oxirane rings. The rapid equilibrium of the alcohol - alcoholate assures that each hydroxyl group from the reaction system is a chain initiator. 

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