Lithium Living polymer

Since the alkyl ligand is incorporated in the final polymer produced, it seemed feasible that the polymerization system outlined above could be adapted to effect an anion to Ziegler-Natta transformation process in order to prepare novel block copolymers. Thus lithium living polymers could be used in lieu of lithium alkyls—such combinations, used to examine the effect of alkyl size on catalyst solubility, had already shown catalytic activity—and polymer-aluminum derivatives could be prepared to replace the small molecule aluminum alkyls in Ziegler-Natta catalyst combinations. The latter derivatives could be synthesized by successive alkylation of aluminum halides with living anionic polymers. ... 

When the styrene has been consumed, to give living polymers of narrow molecular mass distribution, more styrene and more catalyst is added. The styrene adds to the existing chains and also forms new polymer molecules initiated by the additional sec-butyl-lithium. 

The observations discussed above suggest that the kinetic order of lithium poly-isoprene propagation should vary with the living polymer concentration. The effect is imperceptible in aliphatic hydrocarbons, but is observed in benzene solutions. The apparent propagation constants of lithium polyisoprene (MW 2 2 10 ) were determined in benzene and the results are displayed in Fig. 16 in the form of a plot of log kapp vs log c, c denoting the total living polymer concentration. 

Similar divergences are found for lithium poly-2,4-hexadiene solution (1 10-3 M in living polymers) for which a sixfold decrease of viscosity upon protonation corresponding to a degree of association of 1.7 was reported 113), whereas only a threefold decrease, i.e. a degree of association of 1.4 was indicated earlier 1,8). The difference between the 1.7 and 1.4 values was tentatively attributed to a slow decomposition of the active ends over a period of two weeks U8) notwithstanding their reported good... 

The observation of Tsuji et al. 148) concerned with copolymerization of 1- or 2-phenyl butadiene with styrene or butadiene illustrates again the importance of the distinction between the classic, direct monomer addition to the carbanion, and the addition involving coordination with Li4. The living polymer of 1- or 2-phenyl butadiene initiated by sec-butyl lithium forms a block polymer on subsequent addition of styrene or butadiene provided that the reaction proceeds in toluene. However, these block polymers are not formed when the reaction takes place in THF. The relatively unreactive anions derived from phenyl butadienes do not add styrene or butadiene, while the addition eventually takes place in hydrocarbons on coordination of the monomers with Li4. The addition through the coordination route is more facile than the classic one. 

Anionic polymerisation of hydrocarbon monomers is initiated by lithium butyl to produce a living polymer the association number of which in solution is required to elucidate the kinetics. When the living polymer (for example polystyryl lithium) is terminated, the polystyrene can be isolated and a solution then made to determine its molecular weight, M. If the living polymer is associated in solution, the ratio of its... 

Actually it is well known that ionic polymerization need not terminate. They have been termed living polymers. If further monomer is added, weeks or months later there will be a further molecular weight increase as the polymer chains grow longer. As long as the counterion is present (lithium in the preceding case), the anionic end group is perfectly stable. 

Difunctional initiators such as sodium naphthalene are useful for producing ABA, BABAB, CAB AC, and other symmetric block copolymers more efficiently by using fewer cycles of monomer additions. Difunctional initiators can also be prepared by reacting a diene such as /n-diisoprope ny I benzene or l,3-bis(l-phenylethenyl)benzene with 2 equiv of butyl-lithium. Monomer B is polymerized by a difunctional initiator followed by monomer A. A polymerizes at both ends of the B block to form an ABA triblock. BABAB or CABAC block copolymers are syntehsized by the addition of monomer B or C to the ABA living polymer. The use of a difunctional initiator is the only way to synthesize a MMA-styrene-MMA triblock polymer since MMA carbanion does not initiate styrene polymerization (except by using a coupling reaction—Sec. 5-4c). 

Preparation of the Living" Polystyrene. 18 g of the living polymer was prepared by standard anionic polymerization using n-butyl lithium. The reaction was carried out by the dropwise addition of 20 ml of styrene to 5 ml of the initiator solution in 150 ml of neat THF at -78°C. The styrene drip was adjusted to take approximately 30 min for completion and then the reaction was allowed to stir for two hours before the grafting reaction with mesylated lignin was carried out. The number average molecular weight of the polystyrene, as determined by HPSEC, was 9500 with polydispersity of 1.2. 

Let me pass now to some problems of co-polymerization of lithium salts of living polymers. With Dr. Zdenek Laita we studied the rate of cross-over reaction converting lithium poly-styryl into 1,1-diphenyl ethylene", D end-groups in benzene(23). The stoichiometry of the reaction is... 

From the Table IV, it also shows that the low styrene content in the copolymer may relate to the polymerization temperature. As the polymerization temperature was increased from 5° to 70°C, the styrene content of the butadiene-styrene copolymer decreased from 21.7% to 9.1%, respectively. The decreasing in styrene content at higher temperature is consistent with the paper reported by Adams and his associates (16) for thermal stability of "living" polymer-lithium system. In Adams paper, it was concluded that the formation of lithium hydride from polystyryllithium and polybutadienyllithium did occur at high temperature in hydrocarbon solvent. The thermal stability of polystyryllithium in cyclohexane is poorer than polybutadienyllithium. From these results, it appears that the decreasing in styrene content in lithium morpholinide initiated copolymerization at higher temperature is believed to be associated with the formation of lithium hydride. 

Unpublished work of Halasa and co-workers 31) on the study of live chain ends using 13C-NMR as a probe into their structures has led to some interesting new findings. These workers studied the lithium live ends of polybutadiene in the presence of a new polar modifier, dipiperi-dylethane (DPE). This modifier forms a complex with n-BuLi which initiates the polymerization of 1,3-butadiene to give polymer having 100% 1,2 microstructure. 

For example, polymers having hydroxyl end groups can be prepared by reaction of polymer lithium with epoxides, aldehydes, and ketones III-113). Carboxylated polymers result when living polymers are treated with carbon dioxide (///) or anhydrides (114). When sulfur (115, 116), cyclic sulfides (117), or disulfides (118) are added to lithium macromolecules, thiol-substituted polymers are produced. Chlorine-terminus polymers have reportedly been prepared from polymer lithium and chlorine (1/9). Although lithium polymers react with primary and secondary amines to produce unsubstituted polymers (120), tertiary amines can be introduced by use of p-(dimethylamino)benzaldehyde (121). 

Amino-terminated telechelic polybutadiene was prepared by LiAlH4 reduction of amidino end-group in polybutadiene, which was polymerised by a water-soluble initiator, 2,2 -azobis(amidinopropane)dihydrochloride. The structure was analysed by 1H- and 13C-NMR, but functionality of 2.0 was obtained by a titration method [70]. Synthesis of co-epoxy-functionalised polyisoprene was carried out by the reaction of 2-bromoethyloxirane with living polymer that was initiated with sec-butyl lithium. The functionality of the resulting polyisoprene was 1.04 by 1H-NMR and 1.00 by thin layer chromatography detected with flame ionisation detection [71]. 

The general characteristics of these polymerizations are those of the so-called living polymer (24). Under suitable conditions every lithium alkyl molecule will initiate one chain, and there is no evidence for chain transfer or termination 14, 27). The molecular weight distributions will be narrow— Mw/Mn of 1.3 or less according to the relative rates of chain initiation and... 

The study of the spectra of living polymer systems is valuable from a more practical point of view and indicates that the term has some limitations. At room temperature all the polymer-lithium compounds in hydrocarbon solvents show spectra which are stable for considerable time intervals. At elevated temperatures spectral changes occur at least for polystyryllithium, which indicate that isomerization reactions are occurring 4). Most of them display instability in solvents containing appreciable amounts of more polar constituents such as tetrahydrofuran. This effect was first noticed for poly-sty rylsodium 11) and has been attributed to the elimination of sodium hydride, followed by a subsequent reaction to form the more stable substituted allyl anion 21). 

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