Aqueous Dispersions of Polypropylene and Poly(1-butene) with Variable Microstructures Formed with Neutral Nickel(II) Complexes

Abstract

Polymerization of 1-olefins and the polymer microstructures formed have been studied intensly with early transition metal catalysts, and more recently with late transition metal cationic Ni(II) and Pd(II) diimine complexes.1,2 In contrast, while neutral Ni(II) polymerization catalysts are of general interest due to their specific functional group tolerance, they have been little studied for 1-olefin polymerization.3-5 The only notable study is Fink’s polymerization of C4 to C20 1-olefins (in a nonaqueous system) with an in situ catalyst [Ni(cod)2]/(Me3Si)2N-P{d NSiMe3)2 that most likely contains neutral Ni(II) active sites. 2,ω-incorporation was observed exclusively.3-5 Polymer dispersions, that is aqueous dispersions of polymer particles with sizes of ca. 50 nm to 1 μm, are employed on a large scale e.g. for environmentally friendly coatings and paints.6 Catalytic synthesis of polymer dispersions is an attractive aim, as it can enable control of polymer microstructures, by contrast to currently practiced free radical emulsion polymerization.7-11 Moreover, 1-olefins in particular are not susceptible to radical polymerization due to stable radical formation. Complexes [{κ-N,O-6-C(H)dNAr-2,4-R′2C6H2O}NiMe(L)] (Ar ) 2,6-{3,5-R2C6H3}2C6H3; 1: L ) tmeda, R ) CH3, R′ ) I; 2: L ) tmeda, R ) CF3, R′ ) 3,5-(F3C)2C6H3; 3: L ) tmeda, R ) CF3, R′ ) I; tmeda ) N,N,N′,N′-tetramethylethylenediamine) in ethylene polymerization afford a range of microstructures depending on the remote substitutents R (1, highly branched polymer; 3, nearly linear).12 1 has a high capability for chain running, and for insertion of ethylene into secondary metal alkyls, which prompted us to study 1-olefin polymerization. Exposure of 1-3 to 1-butene indeed resulted in the formation of low-molecular-weight polymers (Table 1). Productivities up to 103 TO (TO ) turnover ) mole monomer converted per mole of Ni) are observed, vs 105 TO in ethylene polymerization. The question arises whether catalyst deactivation occurs (intrinsically or, e.g., by impurities in the monomer) or whether chain growth is slow. Monitoring a reaction by continuously drawing samples reveals that polymerization continues for hours (Figure 1). By comparison to ethylene polymerization, insertion of the bulky 1-olefin into a Ni(II) alkyl bond slows down chain growth. Accordingly, 1 which has a pronounced capability for chain running (vide supra) to form a less hindered species after 1-olefin insertion is the most active. Molecular weight is timeindependent, that is chain transfer controls molecular weights. In accordance with this, the calculated number of chains formed per nickel(II) center present in the reaction mixture is .1, on the order of 102. To prepare polymer dispersions, a high degree of dispersion of the catalyst in the intial reaction mixture is a prerequisite.7,8,11 To a mixture of an aqueous surfactant solution and a solution of 1, 2, or 3 in a small amount of toluene a metered amount of liquid monomer was added (vapor pressure of 1-butene at 20 °C, 2 bar; vapor pressure of propylene, 9 bar). The mixture was sheared intensly by an ultrasound device in the pressure reactor, to form a miniemulsion. Colloidally stable dispersions were obtained (entries 7-9). Polymer solids content of entry 7 was 2.6 wt %. Volume average particles sizes, as determined by dynamic light scattering, are 50-100 nm. Catalyst activity is somewhat lowered by comparison to nonaqueous polymerizations. This is likely due to a partial, reversible or irreversible, deactivation of the catalyst by water13 (note that reaction conditions in terms of monomer concentration in the organic phase are similar in aqueous and nonaqueous polymerizations). However, half-lifes for deactivation in the aqueous system must be on the order of at least an hour.14 Microstructures of the poly(1-olefins) were studied by 13C NMR spectroscopy. Unexpectedly in view of Fink’s findings, 1,2-, 2,ω-,15 and also 1,ω-incorporation are found (Table 1 and Figure 2; cf. Supporting Information for NMR details). Concerning the underlying insertion steps, 1,2-insertion is well documented for catalytic polymerization, and observation of 2,ω-incorporation shows it to occur also with these neutral Ni(II) complexes. 1,ω incorporation shows that 2,1-insertion of olefin also occurs to a significant extent. The absence of ethyl and propyl branches in poly(1-hexene) shows that after a 1,2* Corresponding author. E-mail: stefan.mecking@uni-konstanz.de. Table 1. Polymerization Resultsa