Some aspects of the synthesis and characterization of special polymers and oligomers by olefin metathesis

Abstract

Homopolymers of 2-norbornene and 2,3-bis(trifluoromethyl)-2,5-norbornadiene and copolymers of these bicyclic olefins with 1,5-cyclooctadiene and cyclopentene were prepared via ring opening metathesis polymerization. The molecular weight distributions of the polymers were estimated by gel permeation chromatography. The polymers were degraded in a cross metathesis reaction with E-4-octene; only poly[2,3-bis(trifluoromethyl),2,5-norbornadiene] was not degradable. All reactions were carried out with WC16/(CH3)4Sn as the catalyst system. The low molecular cyclic oligomers in the polymerization mixtures and the degradation products were analyzed by gas chromatography and identified using a gas chromatography/mass spectrometry coupling. The degradation experiments show reactivity differences for the double bonds situated in the polymer backbone. On the basis of these differences and the fact that this is the first time that a metathesis degradation of such polymers has been reported, the consequences on the ring opening metathesis copotymerization of cycloolefins are discussed in general terms, leading to some new aspects in the planning of the synthesis of special copolymers and oligomers.

Self-Inhibition of Propagating Carbenes in ROMP of 7-Oxa-bicyclo[2.2.1]hept-2-ene-5,6-dicarboxylic Acid Dendritic Diesters Initiated with Ru(CHPh)Cl2(PCy3)(1,3-dimesityl-4,5-dihydroimidazol-2-ylidene)

“…This secondary poly-5 exhibits exceptional stability toward further metathesis with complex 1 (Table , entry 9) most probably because its number of cis double bonds is very low. In line with this observation, a sample of poly-5 does not undergo any detectable cleavage with complex 1 in a CH 2 Cl 2 solution for 24 h. The secondary metathesis, i.e., the reversible cleavage and formation of the double bonds (Scheme ), shortens the chain length and changes the polymer microstructure. 9c, In particular, we assume that the trans/cis ratio increases significantly with progress in these secondary reactions, in analogy with a macrocyclization catalyzed by 1 …”

“…The molecular weight of polymers formed by ROMP initiated by complex 1 decreases significantly with time (Table , entries 1−5). This observation together with the presence of complex 1 in the polymerization mixture suggests that the secondary metathesis reaction is a statistical scission of double bonds in the polymer backbone by the free catalyst. 1a,5c, To gain further support for this mechanism, depolymerization experiments with samples of poly-2 isolated after short reaction times were performed (Scheme ). For instance, in a reaction of poly-2 ( M n = 82 000, M w / M n = 2.6) with catalyst 1 a slow decrease in the polymer molecular weight and polydispersity was observed.…”