Norbornene End Groups

In attempts to improve processibility and introduce tack and drape into PI prepregs and adhesives without using solvent, a PMR-type resin, LARC-160, was developed at NASA-Langley. This resin uses a commercially available liquid aromatic diamine, Jeffamine AP-22 or Anacamine DL (19), 

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Macromonomers with norbornene end groups were synthesized by living anionic polymerization. The norbornene groups were polymerized by molybdenum catalysts. A series of other a>-nor-bornenyl macromonomers were synthesized and polymerized by metathesis polymerization. 

Prepolymeric imides with norbornene end groups, I, cross-link during normal polymerization, and those with acetylene end groups, II, presumably cross-link by trimerization to benzene rings ... 

To improve the processability of PI and to alleviate or eliminate volatiles, work began in the late 1960 s under NASA Lewis sponsorship (23), to end-cap oligomers with reactive groups. The first report involved the use of 3,6-endomethylene-l,2,3,6-tetrahy-drophthalic anhydride (5-norbornene-2,3-dicarbo ylic anhydride, nadic anhydride) and alkyl derivatives thereof (e.g. citraconic anhydride) and 1,2,3,4-tetrahydrophthalic anhydride (4-cyclohexene-1, 2-dicarboxylic anhydride) as the reactive end group on imide oligomers (23-25). This initial effort led to the development of P13N [with T for polyimide, 13 for Mn of 1300g/mole and N for nadic end cap] whose structure is shown below. 

Having elucidated fhe end group structure for norbornene polymerized in the presence of ethylene, a catalytic cycle showing initiation, propagation, and chain transfer can be constructed (Fig. 4.14). 

Disulfide-bridged oligoferrocenes 71 (d/ < 3,500) were prepared in an attempt to co-polymerize ferrocenedithiol with norbornadiene in the presence of AIBN in toluene. Rather than the expected radical-induced polyaddi-tion to give 70 (Equation (28)), condensation of the ferrocene monomer was observed with norbornene and nortricyclane units as end groups (Equation (29)). The isolated oligomers 71 were characterized by NMR spectroscopy and field-desorption mass spectrometry, and possessed a chain length of 2-12 repeat units as determined by GPC. 

Nomura and co-workers reported that norbomene-based macromonomers of the following structure (Fig. 15) containing ring-opened poly(norbornene) derivatives in the side chain can be prepared efficiently by the ROMP using well-defined molybdenum catalysts and subsequent end-group reactions (412,413). 

H.R. Lubowitz of TRW32.33 discovered polyimide systems that were end-capped with norbornene anhydride groups. The synthetic route used was the classical amic acid precursor method whose molecular weight could easily be adjusted by varying the molar ratio between the reactants. The prepolymer (P13M, CIBA-GEIGY) showed a real melt transition at around 270 C and could therefore be processed in laminates. 

Norbornenes bearing a chain substituent in 5-position and a methacrylate end-group have also been polymerized to give polymers with electro-optical applications [442]. 

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