Spontaneous terminations

A first step towards acquiring such knowledge is the work of Chien who polymerized ethylene with homogeneous bis cyclopentadienyltitanium dichloride (cp2TiCl2) and Et2AlCl. He assumes termination to be a bimolecular reaction of the active centres 

It appears that the reduction of the transition metal oxidation state is a more frequent cause of activity loss of the centre than is usually acknowledged. Such reduction can occur as a consequence of a bimolecular reaction according to eqn. (92) but it may also be caused by the organoaluminium (organomagnesium) catalyst component, by H2 intentionally added in order to reduce the molecular mass of the product, and perhaps also by other, so far unknown, processes. 

As mentioned earlier, living polymers do not, in reality, have in nite life times even in the complete absence of terminating agents as they undergo decay on aging, a process known as spontaneous termination. Polystyryl carbanions, known to be the most stable of all anionic chains as they can survive for weeks in hydrocarbon solvents, undergo spontaneous termination by a mechanism known as hydride elimination, as shown by the equation (Odian, 1991)  

This may be followed by other reactions (Odian, 1991), snch as abstraction of an allylic hydrogen from (XIII) by a carbanion center (to yield an nnreactive anion) or hydrogen abstraction by the sodium hydride (XIV) formed in Eq. (8.20). 

The sodium hydride eliminated in Reaction 5-73 may also participate in hydrogen abstraction from XXI. The order of stability of polystyryl carbanions depends on the alkali metal in the order K Na Li. 

The stability of polystyryl carbanions is greatly decreased in polar solvents such as ethers. In addition to hydride elimination, termination in ether solvents proceeds by nucleophilic displacement at the C—O bond of the ether. The decomposition rate of polystyryllithium in THF at 20°C is a few percent per minute, but stability is significantly enhanced by using temperatures below 0°C [Quirk, 2002], Keep in mind that the stability of polymeric carbanions in the presence of monomers is usually sufficient to synthesize block copolymers because propagation rates are high. The living polymers of 1,3-butadiene and isoprene decay faster than do polystyryl carbanions. 

Polar monomers, such as methyl (meth)acrylate, methyl vinyl ketone, and acrylonitrile, are more reactive than styrene and 1,3-dienes because the polar substituent stabilizes the carba-nion propagating center by resonance interaction to form the enolate anion. However, the polymerizations are more complicated than those of the nonpolar monomers because the polar 

Several different nucleophilic substitution reactions have been observed in the polymerization of methyl methacrylate. Attack of initiator on monomer converts the active alkyl-lithium to the less active alkoxide initiator (Eq. 5-75). Further, methyl methacrylate (MMA) is converted to isopropenyl alkyl ketone to the extent that this reaction occurs. 

The resulting polymerization is a copolymerization between the two monomers, not a homopolymerization of MMA. More importantly, this results in a slower reaction (and lower polymer molecular weight) since the carbanion derived from the ketone is not as reactive as the carbanion from MMA. Nucleophilic substitution by intramolecular backbiting attack of a 

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Finally, two termination steps are considered a spontaneous termination with acid expulsion and restoration of a terminal 2,3-dihydro- or furan ring and a recombination of the growing ion pair to give a tetrahydro- or a 2,3-dihydrofuran ester. 

Interest in anionic polymerizations arises in part from the reactivity of the living carbanionic sites4 7) Access can be provided to polymers with a functional chain end. Such species are difficult to obtain by other methods. Polycondensations yield ro-functional polymers but they provide neither accurate molecular weight control nor low polydispersity. Recently Kennedy51) developed the inifer technique which is based upon selective transfer to fit vinylic polymers obtained cationically with functions at chain end. Also some cationic ring-opening polymerizations52) without spontaneous termination can yield re-functional polymers upon induced deactivation. Anionic polymerization remains however the most versatile and widely used method to synthesize tailor made re-functional macromolecules. 

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

Chain transfer to counterion, also called spontaneous termination, involves transfer of a P-proton to the counterion. The initiator-coinitiator is regenerated by its expulsion from the propagating species and, as in chain transfer to monomer, the polymer molecule has a terminal double bond... 

The rates of spontaneous termination and the two transfer reactions are given by... 

Mechanism of the Chain Transfer Process with the Monomer and of the Spontaneous Termination Process... 

Fig. 33. Values of the ratio between the rate of the chain-transfer processes depending on the catalyst concentration and the rate of the chain-transfer process with the monomer (plus the rate of the spontaneous termination process) (I = 70°, pc,ut = 950 mm. Hg, ground a-TiCU sample A). The values were calculated assuming for the isotactic polymeric fraction x = Kih] = KiC. ...

The spontaneous termination by the cyclization of chain end seems to occur only in the growing chain of short length in the polymerization, although the reason is not clear at present. 

A catalyst-monomer complex and spontaneous termination were postulated. 

Higginson and Wooding 277) also reported a transfer reaction to solvent for the case of the polymerization styrene in ammonia initiated by potassium amide. There was no termination event in their kinetic scheme, i.e., active center deactivation via a spontaneous termination event was not considered to be a significant event. 

Enantioselective Hydrolysis with Arthrobacter Lipase. The results of the enantioselective hydrolysis of the acetate of racemic CPBA are summarized in Table V for several commercial lipases that liberate very optically pure CPBA. The experimental conditions were chosen to give approximately 50% hydrolysis for each enzyme. It is noticed that all of the lipases in Table V hydrolyzed the ester of (S)-CPBA preferentially to give the insecticidally active (S)-isomer. This is apparently different from the case of HMPC. The highest activity and optical purity were again given by the Arthrobacter lipase. Spontaneous termination of the reaction at 50% hydrolysis was observed with this enzyme as was the case of HMPC. 

Due to the large differences in reactivities of the comonomers the chains are mostly composed of isobutene units with minor amounts of 1-butene and traces of the even less reactive Z-2-butene. They are linear and present several types of unsaturations. Spontaneous termination and transfer involving proton abstraction lead to the expected and largely predominant exo/endo terminal double bonds (A, B) but some other tri- and tetrasubstituted olefinic structures (C, D) together with internal vinyli-denes were also detected by H and nC NMR spectroscopy [30-34]. 

Eventually, the anion will spontaneously terminate by mechanisms that are apparently not yet completely established. There are a lot more interesting things about anionic polymerization—the effect of polar groups, the fact that not all monomers can be used to make block copolymers, the ability to make certain polymers with very narrow molecular weight distributions, and so on—but these topics are for more advanced treatments, so now we will turn our attention to cationic polymerization. 

Equation 10.80 assumes spontaneous termination for termination by hydrogeno-lysis, JctTm should be replaced by k timpH2. [The right-hand side of eqn 10.80 also expresses the probability that an adduct will continue to add monomer rather than split off catalyst and produce a dead polymer molecule.] Since all adducts Pj produce polymer Pj with the same rate coefficient klrm, the ratio in eqn 10.80 is also that of the concentrations or mole fractions of the respective polymer molecules Pj+1 and Pj that are formed ... 

Ph = organic phosphine). The mechanism is as in 10.77, with spontaneous termination. Table 10.2 shows a typical Schulz-Flory distribution obtained with this process. 

A kinetic analysis based on four processes, initiation, propagation, termination on water and spontaneous termination, led to the following expression for the initial rate of polymerization. 

In the absence of side reactions the number average degree of polymerization will be c/[M]/r/[I] if initiation is by nucleophilic attack on the monomer or 2 d[M]/c/[I] if initiation is by electron transfer followed by dimerization of the monomeric radical anions (r/fM] and d[I] are the reacted concentrations of monomer and initiator, respectively). If the rale of initiation is very rapid compared to the propagation rate and the initiator is mixed very rapidly and efficiently into the reaction mixture, then all macroions should start growing at almost the same time and should add monomer at equal rates. The active centers can be terminated deliberately and simultaneously since there are no spontaneous termination reactions under appropriate experimental conditions. Polymers made in such reactions have molecular weight distributions which approximate the Poisson... 

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