Cycloaliphatic Polymers via Late Transition Metal Catalysis - Late Transition Metal Polymerization Catalysis - page 131

Ni(C 6 F 5 ) 2 . In an effort to lower the molecular weight of the polymer chain and
simplify analytical characterization, homopolymerization of norbornene was car-
ried out in the presence of ethylene pressure. Norbornene polymerizations cata-
lyzed by cationic nickel catalysts such as [(

3 -crotyl)Ni(1,5-COD)]PF 6 are quite sus-
ceptible to acyclic olefins as chain transfer agents and produce substantially re-
duced molecular weight vinyl-terminated poly(norbornenes) (Section 4.2.3.5).
Thus, norbornene was polymerized under 5 psig ethylene on a 5 g scale using

6 -toluene)Ni(C 6 F 5 ) 2 at a 500 : 1 norbornene to nickel molar ratio. The polymer
was precipitated into methanol and a low yield (8%) of solid was collected by cen-
trifugation. The methanol solution was evaporated to dryness and a smaller
amount of a methanol soluble fraction was collected. Although the yield of the
methanol-insoluble fraction was low, NMR analysis of this fraction was quite in-
formative. While the aliphatic region of the 1 H NMR spectrum was identical to
the higher molecular weight polymer, there were three new resonances observed:
broad singlets at 3.3 and 5.0 ppm and a pair of broad singlets at 5.7 and 6.0 ppm.
The relative intensities of these signals were about 1 : 2 : 1, respectively. The two
downfield resonances are consistent with vinyl groups at the end of the polymer
chain. The assignment of the resonance at 3.3 ppm was not clear at this stage of
our investigation and required further analysis (see below).
The 19 F NMR spectrum of the methanol-insoluble fraction was especially illumi-
nating. Four distinct groups of resonances were observed: singlets at -162.7 (2 F),
-158.1 (1 F), and -139.3 (1 F), and two signals at -133.1 and -133.8 (1 F). These
resonances were consistent with the presence of a C 6 F 5 group appended to the
polymer, presumably at the head of the polynorbornene chain.
The methanol-soluble fraction was of sufficiently low molecular weight that
peaks could be observed in the field ionization mass spectrometry experiment.
Analysis of the predominant series of peaks were consistent with the structure
shown in Fig. 4.28, where the end and head groups are -CH=CH 2 and -C 6 F 5 and
n =2,3,4,5,6,and7( m/z =382, 476, 570, 664, 758 and 852, respectively). The 1 H
and

(

19

F NMR spectra confirmed the presence of vinyl and pentafluorophenyl
groups in the methanol-soluble fraction.
Exact assignment of the resonances observed in the 19 F, 1 H and 13 C NMR spec-
tra of the norbornene oligomers required separation of each member of the se-
ries. First, norbornene polymerization with (

6 -toluene)Ni(C 6 F 5 ) 2 in the presence

Fig. 4.28 Norbornene oligomers made using
bis(pentafluorophenyl)nickel.