1.1- Diphenylhexyllithium initiator

The propagation reaction in the 1,1-diphenylhexyllithium initiated polymerization (toluene, —30°C) shows simple first order kinetics in monomer and initiator over a limited range of concentrations [174], The precipitant-soluble (and presumably inactive) polymer chains always form, in this case, a constant fraction of the added initiator. These represent about 80% of the potential polymer chains, fep values evaluated on the basis that the active chain concentration is always 20% of that of the added initiator are shown in Fig. 19. The results correspond to an activation energy of 5.0 kcal mole" together with a pre-exponential factor of 10 . Extrapolation to 25°C leads to a value of fep of 20 1 mole" sec". The values are reasonable for an ion-pair process, but may represent minimum values since it is not sure that all high polymer chains are active. Figure 19 also includes a point at —60°C determined from data... 

Some indications of what may be happening are gained from analysis of the acetic acid terminated products at various reaction times (Fig. 18). With butyllithium initiation a large amount of methanol (based on initiator) can be isolated at very short reaction times, whereas in the diphenylhexyllithium initiated process only a slower and smaller methanol formation is observed. Now this must correspond to lithium methoxide formed either directly in step (20), or in the termination of product (20), or from the cyclization reaction, or from termination of active chains [177] in reaction (21),... 

The most studied catalyst family of this type are lithium alkyls. With relatively non-bulky substituents, for example nBuLi, the polymerization of MMA is complicated by side reactions.4 0 These may be suppressed if bulkier initiators such as 1,1-diphenylhexyllithium are used,431 especially at low temperature (typically —78 °C), allowing the synthesis of block copolymers.432,433 The addition of bulky lithium alkoxides to alkyllithium initiators also retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised.427 LiCl has been used to similar effect, allowing monodisperse PMMA (Mw/Mn = 1.2) to be prepared at —20 °C.434 Sterically hindered lithium aluminum alkyls have been used at ambient (or higher) temperature to polymerize MMA in a controlled way.435 This process has been termed screened anionic polymerization since the bulky alkyl substituents screen the propagating terminus from side reactions. 

With diphenylhexyllithium 121) (the product of addition of butyl-lithium to 1,1-diphenylethylene) kinetic results are the same as found for fluorenyllithium initiation in the presence of moderate amounts of ether. Even in pure toluene, the rates are first order with respect to initiator concentration and monomer concentration. This simple behaviour is caused by a constant fraction of the initiator forming low molecular weight polymer. If butyllithium is used as initiator, the kinetic behaviour is too complex for analysis. 

Figure 7.1.3, in comparison to Figure 7.1.2, demonstrates the influence of the initiator, in this case 1.1-diphenylhexyllithium (DPHLi), on the microstructure. 

The structure of the hydrocarbon group also affects reactions leading to the start of polymerization. Initiation by butyllithium leads to the rapid formation of a relatively large amount of lithium methoxide, whereas by the reaction of diphenylhexyllithium with methyl methacrylate only a small amount of methoxide is formed slowly in both cases, the reaction proceeds according to scheme (36) or by cyclization or termination of the active centres [170] by 1,2 addition [i. e. again in analogy to reaction (36)]... 

The described behaviour of both initiators can be interpreted by the preferential reaction of diphenylhexyllithium according to scheme (34) and of butyl-lithium according to scheme (36). The subsequent phases of macromolecule formation are affected by the liberated methoxide. 

Quirk and Seung used Bu3NaMg for initiating the copolymerization of oxirane with styrene. They obtained block copolymers while with poly(sty-ryl)lithium or 1,1-diphenylhexyllithium, oxirane did not polymerize at all... 

The progress of the reaction with methylmethacrylate depends somewhat on initiator, temperature and solvent. Investigations have been carried out using fluorenyllithium [167, 168, 170], phenylmagnesium bromide [171, 172], butyllithium [173] and 1,1-diphenylhexyllithium [174] in toluene solution with or without the presence of ethers. Product analysis shows that two basic reactions occur with the monomer both with magnesium compounds [171, 175] and with butyllithium [176], viz. 

Whatever the explanation, there is no doubt that polymer of molecular weight 500—800 is formed extremely rapidly at the start of polymerization and can be isolated from the final product. With fluorenyllithium [168], (toluene—ether, —60°C) a first order disappearance of monomer is observed, which extrapolates at zero time, not to the original added monomer concentration but to a concentration corresponding to the immediate loss of three molecules of monomer per initiator molecule. With 1,1-diphenylhexyllithium [174] (toluene,—30°C) this extrapolation corresponds to the rapid addition to the initiator of about five monomer units. In this case termination at various times and isolation of precipitant-soluble material confirms that polymer of molecular weight 830 is formed rapidly and does not change appreciably in amount throughout the polymerization. With butyllithium [173] (toluene, —30°C) the course of reaction is more complex in the initial stages but eventually a steady concentration of active centres is probably formed as the reaction settles down to first order decay in monomer. A second addition of monomer at the end of the reaction then produces a first order disappearance of monomer immediately. The two first order rate coefficients are identical. Evidently products are produced with butyl-lithium which disturb the reaction, and until these are removed a steady concentration of active centres is not achieved. 

Fig. 18. Percentage of initiator appearing as methoxide ion in the polymerization of 0.125M methylmethacrylate in toluene. (A) Initiator 7.6 x 10 M butyllithium at 10°C (O) initiator 7.6 x 10 M butyllithium at —30°C ( ) initiator 3.2 x 10 M diphenylhexyllithium at —30 C. Arrows indicate time for 70% conversion of monomer to polymer [174].

Fig. 19. Temperature dependence of ftp for methylmethacrylate polymerization in toluene, (o) Initiated by diphenylhexyllithium [174] (+) initiated by butyl-lithiumdiethylzinc [180] ( ) initiated by fluorenyllithium [170] (0.1% THF present).

The polymerization of MMA by alkyllithium has been studied in greater detail than that by any other initiator. For bulky alkyllithiums such as 1,1-diphenylhexyllithium, the polymerization in THF proceeds in an ideal manner to give a living polymer ... 

Fig. 16 Flow microreactor system for anionic polymerization of alkyl methacrylates initiated by 1,1-diphenylhexyllithium. Ml, M2 T-shaped micromixers Rl, R2 microtube reactors...

Table 4 Block copolymerization of alkyl methacrylates initiated by 1,1-diphenylhexyllithium...

The block copolymer poly[block-poly(4-vinylpyridine], abbreviated P[MG8-4VP], was prepared by anionic living polymerization techniques by the synthetic pathway shown in Scheme 3 [49]. The polymerization of macromonomer MGS was initiated by 1,1-diphenylhexyllithium and the polymerization was allowed to proceed for up to 3 h at — 60°C and followed by the addition of the 4-vinylpyridine monomer, at which time the solution turned yellow-orange indicating the formation of the 4-pyridyl anion. The 4-vinylpyridine monomer was allowed to react for up to 3 h, after which time the living polymer was terminated with methanol. The block copolymers were purified by precipitation using a mixture of hexane and THF (90 10 v/v) followed by a second precipitation from diethyl ether. 

Methyl Methacrylate. The most generally usefiil initiator for anionic polymerization of MMA and related compounds is 1,1-diphenylhexyllithium which is formed by the quantitative and facile addition of butyllithium with 1,1-diphenylethylene (DPE) (eq. 17) (46). Using this initiator in THF at -78°C, it is possible to polymerize MMA to obtain polymers and block copolymers with predictable molecular weights and narrow molecular weight distributions. Controlled polymerizations are not effected in nonpolar solvents such as toluene, even at low temperatures. Other usefiil initiators for polymerization of MMA are oligomers of (a-methylstyryl)lithium whose steric requirements minimize attack at the ester carbonyl group in the monomer. These initiators are also useful for the polymerization of 2-vinylpyridine (see Methacrylic Ester POLYMERS). 

The adducts of either butyllithium with DPE (1,1-diphenylhexyllithium) or polymeric organolithiums with DPE are the preferred initiators for polymerization of a wide variety of alkyl methacrylates and (ert-butyl acrylate [3]. For example, Stiihn and coworkers [77] recently reported the synthesis of well-defined, narrow molecular weight distribution polymers from methyl, ethyl, n-propyl, -butyl, n-pentyl, and n-hexyl methacrylates using the adduct of scc-butyllithium and DPE as initiator in THF at -78 °C in the presence of lithium chloride. This initiator was also used recently to copolymerize 2-tri-methylsiloxyethyl methacrylate and butyl methacrylate using analogous reaction procedures [78]. 

In the polymerization of methyl methacrylate(MMA) with BuLi the initiator disappears almost Instantaneously on mixing the reactants(2,7) and reacts first with many more carbonyl groups to produce lithium methoxide than with the monomer vinyl double bond at lower temperature(8). When 1,1-diphenylhexyllithium was... 

When the polymerization of EMA was carried out in toluene with 1,1-diphenylhexyllithium as initiator at -78 C, a large amount of methanol-soluble, isotactic polymer formed with a very small amount of methanol-insoluble fraction. The latter fraction was also isotactic in this case. The results indicate the possibility that in the polymerization of EMA in toluene with BuLi the propagating species for syndiotactic polymer exist in the form of ion pairs complexed with one or more molecules of lithium ethoxide, and the isotactic active species are less connected with the ethoxide or free from it. At higher polymerization temperatures the coordination of ethoxide weakens and the rate of propagation at the chain ends becomes very rapid, which results in a decrease in the formation of methanol-insoluble, syndiotactic fraction. 

Anionic polymerization of alkyl methacrylates cannot be conducted using i-BuLi, which is used as an initiator of anionic polymerization of styrenes. To suppress the nucleophilic attack to the carbonyl group, anionic polymerization of alkyl methacrylate is usually initiated by 1,1-diphenylhexyllithium (DPHLi), which is prepared by the reaction of w-BuLi and 1,1-diphenylethene. This initiator has greater steric effects and is less reactive than i-BuLi, and thus is less likely to give a nucleophilic attack to the ester carbonyl group. Anionic pol3mierization of methyl... 

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