Nickel initiators

A major limitation of such Group IVB metallocene catalysts is that they are air- and moisture-sensitive and not tolerant to heteroatom-containing monomers. In the case of heteroatom-containing monomers the unbonded electron pairs on the heteroatom, such as oxygen, preferentially coordinate to the Lewis acid metal center in place of the carbon-carbon double bond. Some so-called middle and late transition metal organometallics are more tolerant to the presence of such heteroatoms and can be used as effective cocatalysts. These include some palladium, iron, cobalt, and nickel initiators. 

With a few exceptions, 1,2-disubstituted alkenes are not polymerized because of steric hindrance. The exceptions include 1-deuteropropene (Sec. 8-4g) and cydoalkenes. Polymers are obtained from some 1,2-disubstituted alkenes, but the reactions involve isomerization of the monomer to a 1-alkene prior to polymerization, e.g., 2-butene yields poly( 1-butene) [Endo et al., 1979]. There is one report of polymerization of trans-2-butene to poly( ranv-2-butene) using the a-diimine nickel initiators described in Sec. 8-8b [Leatherman and Brookhart,... 

Figure 18 Efiect of ammonia to nickel ratio (/tA)Ni on the apparent first-order reaction rate constant k. Also drawn is the line (n = 2) showing the fraction of total nickel initially present as Ni(NH3)22"1" ions (after Needes and Burkin, ref, 412)...

Deming and Novak then began to develop a modified nickel-initiated system for a highly efficient polymerization of isocyanides [16]. They used the 73-allylnickel trifluoroacetate dimer 14, whose allyl group may serve as an initial migrating group, thus facilitating the initiation step (Scheme 17). 

Synthesis of amphiphilic copolymers having a hydrophobic polystyrene tail with a charged helical poly(isocyanide) headgroup were prepared by polymerization of the peptide-derived isocyanide 50a with the macromolecular nickel initiator 63 (Scheme 42) [71]. The macromolecular initiator was prepared by reaction of (f-BuNC)4Ni(II)(Cl04)2 (46a) with polystyrene, which was end-capped with an amino group. The block copolymer 64 thus ob-... 

In wet atmospheres, nickel initially forms NiO and (NiOH)2 [35,36]. Nickel sulfates are present as corrosion products on the surface in outdoor exposures [37].Jouenefatmospheric corrosion of nickel in industrial, urban, and rural atmospheres. Nickel corrodes through a pitting corrosion process. The highest corrosion rates were observed in industrial areas. The corrosion products were mainly sulfates, chlorides, and n ligible amounts of nitrates surrounded by carbonate species. The pitting corrosion process occurs in two steps on nickel surfaces exposed to an outdoor atmosphere, as shown in Fig. 10.9 [38]. 

Sheina, E.E., Liu, J., lovu, M.C., Laird, D.W, Mccullough, R.D., 2004. Chain growth mecharusm for regioregular nickel-initiated cross-coupling polymerizations. Macromolecules 37,3526-3528. 

A. Smeets, Van den K. Bergh, De J. Winter, P. Gerbaux, T. Verbiest, G. Koeckelberghs, Incorporation of Different End Groups in Conjugated Polymers Using Functional Nickel Initiators. Macromolecules 2009,42,7638-7641. 

Scheme 7.5 Proposed mechanism for the nickel-initiated chain growth of P3HT.

Figure 2.2 Conversion (filled symbols) and logarithm of monomer concentration (open symbols) vs time plots for 2-bromo-3-hexylthiophenepolymerization at different concentration ofNi(dppp)Cl2 initiator (23-25°C) [M]o = 0.075 mol L (M, [M]o [Ni(dppp)Cl2] = 136 1 ( , o) [M]o [Ni(dppp)Cl2] = 57 1 (A, A) [M]o [Ni(dppp)Cl2] = 49 1. The McCullough method was used for polymerization of2-bromo-3-hexylthiophene. Reprinted with permission from E. E. Sheina, J. Liu, M. C. lovu, D. W. Laird and R. D. McCullough. Chain growth mechanism for regioregular nickel-initiated cross-coupling polymerizations. Macromolecules 37(10), 3526-3528 (2004). Copyright 2004 American Chemical Society...

Terminal ring functionalization has been explored as a route to block copolymer synthesis. The Stille coupling approach affords Br and RsSn termini on an isolable product. Iraqi and Barker isolated a low-A/ fraction (DP 14) then homo-cross-coupled to increase Af (DP ra 28), thus demonstrating telechelic utility [43]. The nature of the nickel-initiated cross-coupling polymerization (GRIM method) allows chain extension from the quasi-living end until the reaction is terminated. Controlled blocks of HT-PHT with HT-PDDT were prepared by changing the monomer [63]. 

E. E. Sheina, J. Liu, M. C. lovu, D. W. Laird and R. D. McCullough. Chain growth mechanism for regioregular nickel-initiated cross-coupling polymerizations. Macromolecules 37(10), 3526-3528 (2004). 

It may be deduced that in the en-Si02+INi and ENien+INi samples, impregnated nickel initially as [Ni(H20)6l + in the impregnation solution, is coordinated by ethylenediamine when silica contains adsorbed ethylenediamine or exchanged [Ni(en)3] +. 

FIGURE 16.5 (a,b) Neutral bis(pentafluorophenyl)nickel initiators for the polymerization of norbornene and... 

Sheina EE, Liu JS, lovu MC, Laird DW, McCullough RD. Chain growth mechanism for regioregular nickel-initiated... 

By knowing the active intermediates in these polymerizations, Cheng and Deming were also able to use chiral donor ligands to prepare optically active nickel initiators for the enan-tioasymmetric polymerization of NCAs. Since polypeptides are chiral polymers, the ability to control stereochemistry during polymerization is a feature worth pursuing. This is especially true since the self-assembly and properties of polypeptides are critically dependent on the stereochemistry of the amino acid components. Due to constraints imposed by the initial... 

Smeets A, Van den Bergh K, De Winter J, Gerbaux P, Verbiest T, Koeckelberghs G (2009) Incorporation of different end groups in conjugated polymers using functional nickel initiators. Macromolecules 42 7638-7641... 

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