Helical Microstructure of Polynorbornene

Abstract

Some time ago [1] we reported a metallocene-catalyzed hydrooligomerization of norbornene (NB) in which a C7 linkage is formed as a result of s-bond metathesis. Based on the crystal structure of the pentamer, we were able to show that two of the norbornene molecules are joined by 2-exo-7'-syn linkages and we proposed a mechanism in which a change of conformation of the chain brings the syn hydrogen atom on C7 of the second last inserted monomer into the vicinity of the Zr atom of the metallocene unit so that a s-bond metathesis takes place (Scheme 1: 1!2!2'!3).The crucial question remained unanswered-whether this s-bond step is repeated regularly in the growing chain and in this way determines the as yet unknown structure of the polynorbornene and its properties. Herein we show through norbornene oligomerizations in a deuterium atmosphere and structural analysis of the higher oligomers that this is indeed the case.The oligomerization reactions were carried out in a 250-mL Büchi autoclave under a 0.5-1.25 bar hydrogen or deuterium atmosphere.[2] The oligomers were preparatively fractionated according to their hydrodynamic volumes (! 100 mg; gel permeation chromatography (GPC)) and the samples obtained were analyzed by NMR spectroscopy.First let us consider the hydrotrimer (Scheme 2): If 2 (Scheme 1 with R = D) were present at the moment of chain termination in a deuterium atmosphere, then C3a and C3c would both be deuterated, the rings A and C would be enantiotopic, and the product would have a mirror plane. Figure 1 shows the 13 C{ 1 H} NMR spectra of the meso,mesocoupled NB hydrotrimers prepared in a hydrogen atmosphere and a deuterium atmosphere.As can be clearly seen, the resonance at d = 33.68 ppm of the bridging carbon atom C7b of the second monomer to be inserted (Figure 1; upper spectrum) has almost disappeared in the deuterated compound (Figure 1; lower spectrum), while a new signal with the characteristic 1:1:1 triplet of a carbon

Synthesis, characterization and olefin polymerization of the nickel catalysts supported by [N,S] ligands

“…The norbornene polymerization was typical addition-vinyl type through the FT-IR spectra and 1 H NMR and 13 , attributed to carbons 2 and 3, carbons 1 and 4, carbon 7, and carbons 5 and 6, respectively (Chart 1). These data indicate that the obtained PNB was an addition-type (2,3-linked) product, which was not 2-exo, 7 0 -syn linked norbornene polymer [42][43]. Their 13 C NMR spectra show that the PNB are exo enchained; the spectra do not exhibit resonances in the 20-24 ppm region [23].…”

Addition Oligomerization of Dicyclopentadiene: Reactivity of Endo and Exo Isomers and Postmodification

“…In general, analogously to norbornene (NB), the polymerization of DCPD can follow three pathways: (i) ring opening metathesis polymerization (ROMP) which gives polymers containing double bonds in the main chain, (ii) vinyl‐type addition in which the bicyclic structure remains intact and only the π‐bond of the NB ring is involved in the reaction, and (iii) cationic polymerization, giving 2,3‐ and rearranged 9,10‐enchained units through successive migration of the carbocation (Figure ) . In addition, Fink and co‐workers found that the hydro‐oligomerization and polymerization of NB catalyzed by a Zr–metallocene gave oligomers (i.e., tetramers and pentamers), and a helical poly(norbornene) (PNB), respectively, with a new type of linkage involving the C7 carbon atom formed as a result of σ‐bond metathesis.…”

Stereoselective synthesis and polymerization of Exo‐5‐trimethylsilylnorbornene

“…The exo-face is less sterically hindered and in the case of model compounds the insertion was only on the exo-face of norbornene in the presence of Pd-catalysts. 31 The exo-insertion was also confirmed by the X-ray analysis of oligomers prepared from norbornene [32][33][34][35] or dicyclopentadiene. 36,37 To the best of our knowledge, there are not published examples of the endoface insertion of norbornenes during addition polymerization in the presence of transition metal catalysts.…”