Stereochemistry and Intermediates in the Ring-Opening Polymerization of Norbornene and Its Derivatives

Abstract

In the ring-opening polymerization of norbornene (NBE) and its derivatives, catalysed by W(=CHCMe₃)(OR)₂ Br₂ /GaBr₃ or $W[ = \overline {\mathop C\limits_1 {{\left( {C{H_2}} \right)}_3}C} {H_2}]$ (OR)₂Br₂/GaBr₃ (R = CH₂CMe₃ or CHMe₂) in CD₂Cl₂ at 220–250K, H and ¹³C n.m.r. spectra reveal the presence of both tungstenacyclobutane complexes X and tungsten-carbene complexes P as chain carriers. In some systems one sees distinct spectra for X₁ and X_n (n>2) or for P₁ and P_n (n>2), where n denotes the number of monomer units added to the initiating carbene complex. When the monomer is unsymmetrically substituted, as in endo-5-methylnorbornene (endo-5-MNBE), both head and tail species are seen for X₁ (X_1H and X_1T) and P₁ (P_1H and P_1T), and also for X_n (X_nH and X_nT) and P_n (P_nH and P_nT), where H and T refer to the species in which the methyl substituent is closer to, and further away from, the tungsten centre respectively. X_1T is considerably more stable than X_1H, having a relatively long life (several hours) at 235K, but converting to P_1T at 250K. With cyclopentylidene as carbene ligand, P₁ is seen first, followed by X_n. The non-appearance of X₁ is attributed to a destabilising effect of the spiro structure at the α-carbon of the tungstenacyclobutane. With R = CHMe₂, X₁ but not X_n is seen, so that the reaction appears to be propagated by a very small number of centres. The tungsten-carbene complexes also initiate polymerization in the absence of GaBr₃ but the temperature must be raised by about 30–40° to obtain activity which is comparable with systems in which GaBr₃ is present. In this case, neither X₁ nor X_n are detected, but P₁ and P_n are again observed.