The development of catalysts that are capable of polymerizing or copolymerizing functionalized vinyl monomers (CH2=CHX) by insertion mechanisms would enable the synthesis of new polyolefins with enhanced properties. The discovery by Brookhart and coworkers that (α-diimine)PdR+ catalysts copolymerize ethylene and acrylate monomers to highly branched copolymers was a seminal development in this field.
Vinyl ethers (CH2=CHOR) are attractive candidates for insertion copolymerization with olefins because their steric and electronic properties can be tuned by varying the R group, possibly enabling the deactivation reactions noted above to be avoided. Several potential problems can be anticipated for vinyl ethers in insertion polymerization. First, vinyl ethers are highly susceptible to cationic polymerization. Second, vinyl ethers can coordinate to metals not only through the C=C bond but also through the OR group. Third, the insertion barriers for LnMR'(CH2=CHOR) species are predicted to be high.
Recently we reported that (α-diimine)PdMe+ species (1, α-diimine = (2,6-iPr2-C6H3)N=CMeCMe=N(2,6-iPr2-C6H3)) copolymerize 1-hexene and CH2=CHOSiPh3 to OSiPh3-substituted polyhexene. This is possible because (α-diimine)PdCH2CH(OSiR3)R+ species are trapped by olefin and undergo subsequent insertion faster than they undergo β-OSiPh3 elimination leading to inactive Pd-allyl species, and no cationically polymerization was initiated. Alkyl vinyl ethers such as CH2=CHOtBu are not suitable comonomers in this system due to competing cationic polymerization and Pd0 formation. Phenyl vinyl ether is also unsuitable because (α-diimine)Pd(CH2CHMeOPh)+ species generated by CH2=CHOPh insertion undergo rapid β-OPh elimination, ultimately forming (α-diimine)Pd(η3-C3H5)+, which is catalytically inactive, and PhOH. Followed this, I studied the generality and the mechanism of the copolymerization and the microstructure of the copolymer with different VE (2a-g, Scheme 1), which showed the influence of the α-diimine ligands, vinyl ether, olefin, counter-anion, concentration, temperature, and other factors on the copolymerization behavior.
To understand the mechanism, and to broaden the scope of this chemistry, I studied the reactions of 1 with a set of VE with varying steric and electronic properties (Scheme 2). It is showed that 1 initiates the cationic polymerization of electron-rich VE 2a-c. Second, 1 reacts with less-electron-rich VE 2d-g, or stoichiometric quantities of 2a-c by sequential (i) VE coordination to yield 3a-g, (ii) 1,2 insertion to yield 4a-g, (iii) reversible chain walking to yield 5a-g, and (iv) irreversible β-OR elimination followed by allylic C-H activation to yield 6 and ROH. These studies show how the rate of cationic polymerization, the strength of VE coordination, the rate of VE insertion, the equilibration of chain-walk isomers, and the rate of β-OR elimination can all be controlled by variation of the VE, α-diimine ligand, concentration, temperature, counter-anion, etc.
Since (α-diimine)PdCH2CH(OSiR3)R+ species are easily trapped by olefin, the question arises whether it can be trapped by another equiv of 2f and undergo multiple insertion even insertion homopolymerization of the VE. For aryl-silyl vinyl ethers, insertion out-competes cationic polymerization, enabling copolymerization of these substrates with olefins, and multiple vinyl ether insertion reactions. Studies show that 1 undergoes up to three sequential insertions of 2f, ultimately forming Pd allyl products (7, Scheme 3 and Figure 1). The insertion is very sensitive to anion and steric effect. When more coordinating counter-anion (SbF6-) is used, only double insertion product 8 is observed. Further insertion was observed for smaller VE (CH2=CHOSiMePh2 and CH2=CHOSiMe2Ph).
A key issue for these studies is to avoid cationic polymerization of VE by the electrophilic Pd catalysts. The reactivity of (α-diimine)PdCl+ species with VE was studied . (α-diimine)PdCl+ species catalytically dimerize vinyl ethers (2a-f) to CH2=CHCH2CH(OR)2 acetals and cyclize divinyl ethers to analogous cyclic acetals. In situ-generated (α-Diimine)Pd(OR)+ alkoxide complexes may be the active species in these reactions. An interesting application of this reaction is olefin polymerization by (α-diimine)PdCl+/vinyl ether mixtures. When divinyl ethers were used, cyclic acetals were generated (Scheme 4). It is proposed that the mechanism involves in-situ generation of Pd-OR, double insertion of VEs, followed by β-OR elimination (Scheme 5). When ethylene was added to the reaction system, polyethylene with functionalized end groups are formed.
Group 4 metallocene catalysts have been used to produce polyolefins commercially and have achieved great success. Meanwhile, a huge number of studies have been conducted by academia and industry to construct high-performance post-metallocene catalysts. We found that The activation of group 4 'MCl3 complexes that contain sterically bulky tris(pyrazolyl)borate ligands (Tp') with methylalumoxane (MAO) generates highly active olefin polymerization catalysts. The second part of my research involves the synthesis and characterization of a series of group 4 poly(pyrazolyl)borate complexes and the study of their properties in olefin polymerization.
Two group 4 bis(pyrazolyl)borate complexes 9 and 10 were synthesized and characterized (Scheme 6). The reaction of 9 or 10 with 1 equiv of [Ph3C][B(C6F5)4] undergoes single electron transfer (SET) reactions to generate 11 or 12, trityl radical and benzyl radical, which coupled to give a series of organic proudcts. 11 and 12 are highly acitve in ethylene polymerization.
It is found that MeB(OiPr)2 catalyzes the reaction of Li[MeBH3] with 2 equiv of HPzMs, which is the general route for the synthesis of poly(pyrazolyl)borate ligands. Lewis acids catalyze the reaction of Li[MeBH3] and Na[BH4] with pyrazoles to yield poly(pyrazolyl)borates under mild conditions. Coordination of the pyrazole to the Lewis acid decreases the pKa of the pyrazole and increases the rate of B-H bond protonolysis. A more dramatic catalytic effect is observed in the synthesis of tris(pyrazolyl)borate (Tp'-) ligands. Coordination of the pyrazole to MeB(OiPr)2 decreases the pKa of the pyrazole and enhances the protonlysis of B-H bonds (Scheme 7). Traditional methods for Tp'- or Bp'- (bis(pyrazolyl)borate) synthesis require elevated temperatures (>190 °C), high boiling point solvents and a large excess of pyrazole; the current method overcomes these disadvantages.
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