Showing papers by "Andreia Valente published in 2010"

Rare earths/main group metal alkyls catalytic systems for the 1,4-trans stereoselective coordinative chain transfer polymerization of isoprene

Abstract: A series of lanthanum and neodymium complexes comprising the half-lanthanidocenes Cp * La(BH 4 ) 2 (THF) 2 (Cp * = C 5 Me 5 ) (1) and Cp * Nd(BH 4 ( 2 (THF) 2 (2), the trisborohydrides La(BH 4 ) 3 (THF) 3 (3) and Nd(BH 4 ) 3 (THF) 3 (4), the trichlorides LaCl 3 (THF) 3 (5) and NdCl 3 (THF) 3 (6), the triisopropoxides La(O i Pr) 3 (7) and Nd(O i Pr) 3 (8), and the triaryloxide Nd(OC 6 H 3 - t B U2 -2,6) 3 (9) has been assessed for the chain transfer polymerization of isoprene. A transmetalation process is occurring efficiently with the borohydride complexes in the presence of magnesium dialkyl. A gradual decrease of the 1,4-trans stereoselectivity of the reaction is observed at the benefit of 3,4-selectivity with increasing quantities of magnesium dialkyl. This can be at least partially attributed to the growth of 3,4 polyisoprene units onto the magnesium atom. By combining dialkylmagnesium and tri-alkylaluminum, a 1,4-trans stereospecific reversible coordinative chain transfer polymerization of isoprene is reached using the half-lanthanocene Cp * La(BH 4 ) 2 (THF) 2 .

Reversible coordinative chain transfer polymerization of styrene by rare earth borohydrides, chlorides/dialkylmagnesium systems

Abstract: The ability of various rare earth borohydride and chloride complexes/n-butylethylmagnesium systems to operate styrene chain transfer polymerization in mild conditions has been assessed. Thirteen precatalysts have been considered: the rare earth trisborohydrides Ln(BH4)3(THF)x (x = 3, Ln = Nd (1), La (2), Sm (3), x = 2, Ln = Y (4), Sc (5)), the rare earth chlorides LnCl3(THF)x (x = 3, Ln = Nd (6), La (7), Sm (8), Y (9), x = 2, Ln = Sc (10)), the mixed La(BH4)2Cl(THF)2.6 (11) and the half-lanthanidocenes Cp*Ln(BH4)2(THF)2 (Ln = Nd (12), La (13)). Six systems were found to be active precatalysts for the polymerization of styrene. 1, 2, and 11 led to an efficient transmetalation of the growing polystyrene chain with the simultaneous occurrence of βH elimination, whereas 7, 12, and 13 led to catalyzed chain growth behavior. It is noteworthy that the catalyzed chain growth obtained with 12 and 13 occurs with significant stereoselectivity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 802–814, 2010