Influence of external nitrogen-containing donors on polymerization behavior of neodymium-based cis-1,4-polybutadiene rubber

Abstract

"Neodymium(Nd)-based catalyst in butadiene (Bd) polymerization has drawn interests due to its availability in affording higher cis-1,4-unit selectivity than transition metal (Ti, Co, Ni, etc.)-based catalysts［1-2］. Such outstanding high cis-1,4-unit selecti-vity is hypothetically originated from the presence of 4 f orbitals, that can participate in monomer coordination and thereby govern subsequent enchainment manners. This unique characteristic also renders the active species highly susceptible to Lewis bases, and may impact the overall selectivity as well as polyme-rization behavior after coordination. Nevertheless, it is still a virgin area in such a field, and the influence of Lewis bases on Nd-based diene polymerizations is still a black box. Based on this consideration, how nitrogen-containing donors (D) impacts the overall behaviors of Nd-mediated Bd polymerizations is disclosed. Systematic evaluation was commenced by incorporation different amounts of trioctylamine(TOA) into neodymium neodecanoate［Nd(vers)3］/triethylaluminum (AlEt3) /diisobutyl aluminum hydride ［Al(i-Bu)2H］/ethyl sesquialuminum chloride (Al2Et3Cl3) mediated Bd polymerization. As shown in Table 1, with the increase of the n(TOA)/n(Nd) ratio from 0 to 5, the polymerization performances was negligible influenced, giving polybutadiene rubber with similar yields, molecular weights (■■) and polydispersity indexes. Nevertheless, it was astonishingly found that cis-1,4-unit stereoselectivity of the system was remarkably enhanced from 93.66% to 96.18%, with simultaneously decreasing of trans-1,4-unit selectivity from 6.00% to 3.48%. Further increa-sing n(TOA)/n(Nd) to 8 witnessed an even higher cis-1,4-unit selectivity up to 99.29%. Such a result was solidly confirmed from carbon nuclear magnetic ersonance (13C-NMR) spectra (Fig 1) that one predominant peak was observed that assigned to cis-1,4-unit of polybutadiene rubber sample. These positive results were hypothetically originated from the changes in monomer enchainment fashion after the Nd metal was coordinated by TOA. After the vacant site on Nd metal was occupied by TOA, a sterically congested active species would be resulted, which then forced Bd monomer coordinating the Nd atom in a cis-1,4-η4 manner because such a conformation required much smaller space than trans-1,4-η4 manner. The cis-1,4-η4 coordinated monomer was more favo-rable to be inserted to form anti-structured π-allyl propagation species, with which a cis-1,4-unit was concurrently generated. For the present system, changing the addition order of different components during the aging process by selecting (Nd, D, Bd, Al, Cl), or (Nd, Bd, Al, D, Cl), or (Nd, Bd, Al+D, Cl), or (Nd, Bd, Al, Cl, D) resulted in little influence on the selectivities, all of them gave cis-1,4-unit selectivities higher than 96%, implying the similar structured active species formed therein. Besides TOA, other types of nitrogen-containing donors could also give parallel results. For instance, incorporation of N,N,N',N'-tetramethylethylenediamine, 2,6-dimethylpyrdine and 2,6-dichloropyridine could enhance the cis-1,4-unit selectivities to 97.79%-98.94%, suggesting the universal influence of nitrogen-donors."

Keywords

• adding nitrogen-containing electron donors， rare earth neodymium catalyst， butadiene， cis-1
• 4-structural content， aggregation mechanism，