Syndiotacticities, living polymerization

In 1992 Yasuda et al. [236, 237] reported that organolanthanide complexes of the type Cp 2Sm-R (where Cp is pentamethyl cyclopentadienyl, and R is either an alkyl, alkylaluminum or a hydride) initiate highly syndiotactic, living polymerizations of methacrylates. It was also reported that lanthanide complexes such as Cp 2Yb(THF)i 3, Cp 2Sm(THF)2, and (indenyl)2Yb(THF)2 can also initiate polymerizations of methylmethacrylate [238]. Although very low initiator efficiencies were... 

Characteristic initiation behavior of rare earth metals was also found in the polymerization of polar and nonpolar monomers. In spite of the accelarated development of living isotactic [15] and syndiotactic [16] polymerizations of methyl methacrylate (MMA), the lowest polydispersity indices obtained remain in the region of Mw/Mn = 1.08 for an Mn of only 21 200. Thus, the synthesis of high molecular weight polymers (Mn > 100 x 103) with Mw/Mn < 1.05 is still an important target in both polar and nonpolar polymer chemistry. Undoubtedly, the availability of compositionally pure materials is a must for the accurate physical and chemical characterization of polymeric materials. 

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 low-temperature polymerization of propylene has been performed with various types of vanadium compounds instead of V(acac)3. The results are summarized in Table 1. Vanadium compounds such as VC14, VOCl3, VO(acac)2 and VOCl2(acac) afford syndiotactic polypropylenes with rather broad MWDs, close to the most probable distribution (Mw/lVln = 2.0). At present, V(acac)3 is the sole vanadium compound being effective for the living polymerization of propylene. 

The chain tacticity of PMMA synthesized by GTP catalyzed by nucleophiles at different temperatures was analyzed by Webster and coworkers The syndiotactic content increases from 50% at 60 °C up to 80% at —90°C in THF, using tris(dimethylamino)sulfonium bifluoride [(Me2N)3S+ HF2 ] as catalyst . In contrast to the anionic polymerization of MMA, the stereoselectivity of GTP is less sensitive to solvent. It must be noted that PMMA is less syndiotactic when the GTP is catalyzed by nucleophiles rather than by Lewis acids . GTP was extended to the living polymerization of many acrylates and methacrylates, such as nBuMA, glycidyl-MA, 2-ethylhexyl-MA, Me3SiOCH2CH2-MA, sorbyl-MA, allyl-MA, lauryl-MA), acrylates (EA, BuA), acrylonitrile, methacrylonitrile and Al,A-dimethylacrylamide . 

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]. 

Syndiotactic poiy(methyl methacryiate) Anionic living polymerization Mn = 2000 14500 M /Mn = 1.13-5.5 effect of synthetic variables on tacticity, molecular weight and distribution and yield discussed. [139,147]... 

In this chapter, recent advances in stereoregular propylene polymerizations with nonmetallocene complexes are briefly reviewed. The greatest emphasis is placed on phenoxy-imine ligated group 4 complexes, which have been used to achieve both highly isotactic and highly syndiotactic (living) propylene polymerization. 

The use of dMMAO enabled the living polymerization of propene in heptane, which afforded PP with the highest syndiotacticity amcnig the solvents employed. We therefore synthesized a series of 2 analogues that possessed alkyl substiments on the fluorenyl ligand 2,7- Bu2, 3 3,6- Bu2, 4 2,3,6,7- Bu4 analogue, 5. 

SCHEME 2.11 Titanium-based phenoxyimine catalysts as reported by Fujita in 1999 (R=H, Ar=CgHj) for living polymerization of ethylene, Coates in 2000 (R= Bu, Ar=CgHj) for syndiotactic polypropylene. Subsequently, fluorinated versions were reported by Coates (R=[Pg.34]

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