End-capped oligomers

The major distribution, centred at approximately 2000 Da as expected, can be assigned as from di-hydroxyl end-capped oligomers of PPG (18). Addition of lithium salts in the MALDI sample preparation procedure leads to the generation of [18 + Li]+ ions as the major species. A series of low-intensity peaks below... 

Figure 25 MALDI-TOF mass spectrum from di-hydroxyl end-capped PPG, indicating the presence of hydroxyl and ally end-capped oligomers (see text for description of peak annotation).

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. 

Thus the additional stabilization of the eliminated free radical by the phenyl group promotes essentially quantitative cleavage. The mechanism of the production of the end-capped oligomer is probably as follows (24) ... 

Although there are other unsaturated compounds that will undergo addition-elimination with free radicals, the benzyl ketene acetal XIV appears to be the most active double bond as far as rate of addition is concerned and the most efficient as far as regards to the extent of elimination is concerned. A comparison with the list of chain transfer agents listed in the Polymer Handbook (23) indicated that only the sulfur compounds appear to be more effective than XIV. Hydrolysis of the end-capped oligomer gives a macromer that is terminated with a carboxylic acid group. 

The nadimide and maleimide end-capped oligomers belong to the family of thermosetting telechelic oligomer precursors of thermostable networks shown in Fig. 1. These products were developed in order to fulfill the requirements of the aerospace industry in the domain of high performance adhesives and matrices for laminates. 

MononadimideSjbisnadimides and nadimides end-capped oligomers are generally prepared in a one-stage process. The starting materials are dissolved in a polar solvent (NMP, DMF, diglyme,...). Intermediate amic acids are formed by heat-... 

Fig. 4. Chemistry of the first bisnadimide end-capped oligomer (theoretical n=1.67 Mn= 1300 Da)...

The first nadimide end-capped oligomers were prepared using the unbalanced classical reaction of a dianhydride (BTDA) with an excess of diamine (MDA) and a quantity of nadic anhydride (NA) corresponding to the diamine excess (Fig. 4). The reaction was performed in NMP with a molecular ratio calculated in order to get a Mn of 1300 g mol-1 [12]. However the amic acid intermediate is not stable enough to allow a long term storage of the NMP solution... 

Model compounds (mono- and bisnadimides) as well as nadimide end-capped oligomers have been extensively studied. In this section we will examine first the characterization of BNIs, then the monitoring of their thermal polymerization and finally the characterization of polymers. 

The usual difficulties in structurally characterizing crosslinked materials are mainly related to their insolubility in the usual organic solvents. Solid-state FTIR spectroscopy is the most convenient analysis to perform several acute techniques allow accurate measurements to be obtained for several kinds of polymers including nadimide end-capped oligomers [30]. Nevertheless, only fragmentary structural information is obtained the attendance in the network of some functional groups can be evidenced. However, it is not possible to determine exactly the polymer microstructure by FTIR. 

The reaction of a BMI with an acetylenic end-capped oligomer has been undertaken. The mechanism was thought to proceed according to the two pathways indicated in Fig. 21 either by a kind of ene synthesis with formation of an hypothetic cycloallene leading to a dihydronaphthalene derivative (pathway a) or by a condensation concerted with an hydride shift (pathway b). No experimental proof was given for the structure but the condensation of both oligomers gave an expected linear product with a better Glc than the one of BMI alone (324 J m 2 vs. 34 J m 2) [73]. 

Relationship Between Structure of BMI and BNI End-Capped Oligomers and Properties of the Network... 

The conclusions of these works are very similar firstly the crosslinking mechanism is stopped when the vitrification takes place, secondly the Tg values of the networks are mainly dependent on the nature of the starting end-capped oligomer. 

Table 7. End-capped oligomers used for the preparation of thermostable semi-IPNs...

Ageing and thermal degradation of crosslinked nadimide end-capped oligomers have been studied by means of solid infrared 13C NMR spectroscopy as well as by thermogravimetric measurements. They are mainly related to PMR and BBN families. 

Fig. 15.2.35. Solid state C CP/MAS spectra of three samples of poly(methyl methacrylate) crosslinked with 5 (lower), 20 (mid) and 50 (top) % of f-butyl acrylate end-capped oligomer containing ca. 2-3 HDDA repeat units.

When p-nadimidochlorobenzene as a monofunctional dichloro compound is added to the reaction mixture of sulfide and aromatic dichloro compound, an end capped oligomer with a pendant double bond is obtained. Further, a trifunctional compound, such as trichlorobenzene, may be added to compensate the monofunctional compound with respect to molecular weight. OUgomers with a molecular weight of 1,000-6,000 Dalton can be readily obtained. The oligomerization is achieved at 225-260 C. At this temperature, the double bonds are still not reactive. 

PEKs that are subsequently end capped with nitrile functions have been prepared by Friedel-Crafts polymerization. For example, m-hexaphenyl ether are reacted with p-cyanobenzoyl chloride with AICI3 as catalyst in dry 1,2-dichloroethane to form a nitrile end capped oligomer. In another study, 5-cyanoisophthaloyl chloride was used as a nitrile-containing comonomer for the Friedel-Crafts polymerization. ... 

To broaden the processing window and to improve processability, the synthesis of phenylethynyl terminated oligoimides has been also studied (Scheme 71). Phenylethynyl terminated oligomers normally have reduced reactivities compared to the ethynyl compounds and, consequently, provide a wider processing window to the end-capped oligomers [347-349]. 

In this chapter, we would like to review the recent development of preparation and applications of fluoropolymer nanocomposites by the use of fluoroalkyl end-capped oligomers as key intermediates. 

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