Toluenes living polymerization

A range of tetradentate Schiff-base ligands have also been employed to prepare discrete aluminum alkoxides. The most widely studied system is the unsubstituted parent system (256), which initiates the controlled ROP of rac-LA at 70 °C in toluene. The polymerization displays certain features characteristic of a living process (e.g., narrow Mw/M ), but is only well behaved to approximately 60-70% conversion thereafter transesterification causes the polydispersity to broaden.788 MALDI-TOF mass spectroscopy has been used to show that even at low conversions the polymer chains contain both even and odd numbers of lactic acid repeat units, implying that transesterification occurs in parallel with polymerization in this system.789... 

Living polymerization of trimethylenecarbonate (TMC) and 2,2-dimethyl-trimethylenecarbonate readily occurred in toluene at ambient temperature... 

Doi, Ueki and Keii47,48) have found that the soluble catalyst composed of V(acac)3 (acac = acetylacetonate anion) and A1(C2H5)2C1 polymerizes propylene in toluene at —78 °C to give a syndiotactic living polypropylene having a narrow molecular weight distribution (Mw/Mn = 1.05-1.20). This low-temperature polymerization of propylene was shown to satisfy all criteria for the living polymerization 47). 

The polymerization of dimethylsulfoxonium methylide (17) initiated by trialkylborane, which Shea et al. developed, is classified as the first category. Propagation involves insertion of 17 into the terminal C-B bond with elimination of DMSO. The polymerization was carried out in toluene at 70-80 °C, followed by oxidative workup to yield hydroxyl-terminated polymethylene (Scheme 71). The Mn values were very close to the calculated values from the feed ratio of 17 to trialkylborane, and the Mw/Mn ratio ranged from 1.04 to 1.17. These results are consistent with the character of living polymerization. 

Aluminum isopropoxide has been used for the preparation of block copolyesters [147, 148]. Tri-block poly(e-CL-b-DXO-e-CL) was prepared by the sequential addition of different monomers to a living polymerization system initiated with aluminum isopropoxide in THF or toluene solution [95]. An alternative route for the preparation of the tri-block copolymer was to react the diblock poly(e-CL-b-DXO) containing an -OH functionality at the chain end using a difunctional coupling agent such as isocyanate or acid chloride (Scheme 18). However, the molecular weights were low and full conversion of monomers was not achieved. 

As a complement to the stable metallacyclobutane catalysts, a series of stable alkylidene catalysts have been prepared and shown to be active living polymerization catalysts. The complex W(CHt-Bu)(NAr)(Ot-Bu)2 [47,48] (Ar = 2,6-diisopro-pylphenyl) (35) was reacted with 50-200 equivalents of norbornene in toluene at 25 °C, followed by end-capping with benzaldehyde, yielding polymers in which the major component has a molecular weight proportional to the number of equivalents of norbornene consumed, with dispersities of approximately 1.05,... 

Controiied/Living Polymerizations with Added Salts The two approaches discussed above are primarily useful in nonpolar solvents (like toluene and n-hexane) where the interactions of carbocations with nucleophiles are strong and favored. In relatively polar solvents like methylene chloride, these methods often fail to give controlled polymerizations, most likely because the interaction is weaker between the growing carbocations and nucleophiles [whether they are built-in (counteranions) or externally added (esters, etc.)], which facilitates dissociation of the carbocation. The effect of solvent in the latter system, however, is much weaker. 

The HB/MtX -initiated polymerizations of vinyl ethers are typically carried out in nonpolar media such as toluene and n-hexane (depending on the solubility of the products) at temperatures below 0° C. In some cases, however, polar solvents (e.g., methylene chloride) may be used at appropriate initiator/activator mole ratios [119] and, specifically with the HI/ZnI2 system, controlled/living polymerization is feasible even at room temperature ( + 25° C) [98,99]. 

Figure 1. Living polymerization of HC C/fo-SifCHajjCeHj by Mods or MoOCZ -fn-C Hgj Sn-CsHsOH (1 1 0.5) in toluene at 30 °C. [M]o = 1.0 or 0.10 M conversion = 100%. Abbreviations are defined as follows ...

An elegant alternative to living polymerization for the preparation of block polymers is to use functionalized Grignard initiators. The polymerization of methyl methacrylate to isotactic (in toluene at — 78"C) or syndiotactic polymers (in THF at — llO C) can be initiated by o-, m-, and p-vinylbenzylmagnesium chloride. The polymers had a low polydispersity and contained one vinylbenzyl group at the chain end, by H-NMR. The poly(methylmethacrylate) macromers thus obtained were polymerized or copolymerized with styrene to give graft and block polymers of controlled architecture [50,51]. 

Further evidence of the living polymerization nature was obtained by the fact that the GPC peaks of the PE produced shift to higher molecular mass on increasing the polymerization time. The monomodal shape is retained, and no shoulders or low molecular mass tails are detected.1134 The stability of the living polymer chain was investigated utilizing the MAO-activated complex 137 at 25 °C.1134 First, the activated complex is treated with ethylene-saturated toluene for 65 min. The values of Mn versus time clearly indicate that after 3 min all the ethylene is consumed. After 65 min under an N2 atmosphere, ethylene gas was fed to the system for 2 additional min. The Mw/Mn value resulting after the additional 2 min ethylene feed is 1.14, which indicates that no termination reaction occurred for at least 60 min in the absence of ethylene. This remarkable result opens the route to the controlled synthesis of ethylene-based block co-polymers. 

Problem 8.16 In a living polymerization experiment, 7.0 cm of 0.16 mol/L solution of s-butyllithium in toluene was added to a solution of 16.8 g styrene in 400 cm toluene. After complete conversion of the styrene, 56 g isoprene was added. When the isoprene had completely reacted, the polymerization was terminated by addition of 5.6 cm of a 0.10 mol/L solution of dichloromethane in toluene. 

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