Stable oligomers

Neutral benzol,3,2-diazaphospholes or their tetrameric cycloaddition products react with hard Lewis acids to give N-coordinated Lewis acid-base complexes [13, 80, 81] this reaction can be used to disassemble the otherwise stable oligomers into monomeric units at ambient temperature. 

It is possible to exactly identify and characterize the radical species and chain structures of the reaction intermediates, which are determined by their different reactive or unreactive chain ends. The reactive intermediates are best described by diradical (DR), asymmetric carbene (AC) and dicarbene (DC) oligomer molecules of different lengths. The respective singlet (S = 0), triplet (S = I) or quintet (S = 1) states and their roles in the polymerization process are investigated in detail by solid state spectroscopy. A one-dimensional electron gas model is successfully applied to the optical absorption series of the DR and AC intermediates as well as on the different stable oligomer SO molecules obtained after final chain termination reactions. 

From spectroscopic data, presented in the following, we conclude that the mechanism of polymerization is described by three series of intermediate states differing by the number of reactive radical or carbene chain ends these are the diradicals DR , the dicarbenes DC , and the asymmetric carbenes AC . Via a final chain termination reaction an additional series of reaction products is obtained. These are the stable oligomers SO with two unreactive chain ends. The schematic structures of the DR, DC, AC, and SO molecules are shown by example of the trimer in Table 2. The lengths of the dimer-, trimer-, tetramer-... units are characterized by the numbers n = 2, 3,4,... of the respective monomer molecules. The symbols and the schematic structures as well as the notation of the optical and the ESR absorption lines, are summarized in Table 2. 

The absorption lines of the low temperature photoreaction products in TS-6 monomer crystals are summarized in the diagram of Fig. 7. The correlation of the A, B, C,. .. photoproduct series to diradical DR intermediates and of the b, c, d,... photoproducts to asymmetric carbene AC intermediates is based on the ESR experiments discussed below. The correlation of the y, 8,6,... series to stable oligomers SO is based on their thermal and optical stability. The correlation of dimer, trimer, tetramer,... molecules follows from the chemical reaction sequences observed in the time resolved optical and ESR measurements as well as from the widths of the one-dimensional potential wells used in the simple electron gas theory , which already has proved successful in its application to dye molecules. Following Exarhos et al. the explicit dependence is given by... 

The asymmetric AC species are composed of a reactive carbenoic chain head and an unreactive chain tail with fully saturated bonds. They are formed by optical transformation of the diradical molecules DR In a first chain termination reaction one reactive chain end is destroyed and an unreactive chain end is formed by addition of a monomer molecule following DR -> AC + i. In a second chain termination reaction the remaining reactive chain end of an AC oligomer may be destroyed in a further photoaddition reaction. In this way the AC oligomers are transformed to stable oligomers SO (7, 6, ,...) with two unreactive chain ends. This transformation reaction has been observed in the optical spectra. As expected no corresponding ESR spectra could be detected since radical electrons were absent. 

Direct excitation of the molecules M is performed with UV-light energies below the monomer absorption edge hv < hv (with wavelength 320 nm < K < 370 nm).. The photoaddition polymerization reaction obtained in this way at 10 K is shown for the optical absorptions of the AC-centres (b, c, d,...) in Fig. 15. After preparation of the b photoproduct (see the procedure of Fig. 6) the sequence of the AC centres b->c->d->e->f gis obtained upon 364 nm-irradiation. Simultaneously chain termination reactions leading to the stable oligomers 5 and e are observed. The same reactions are also observed with the diradicals in the ESR and optical spectra. After preparation of the dimer diradical and subsequent 364 nm-irradiation, very effective addition polymerization reactions are observed parallel with chain termination reactions leading to AC-molecules. The same effects are valid for the dicarbene DC species described below (Section 3.2). 

For a quantitative calculation of the time dependencies of the concentrations of the intermediates [DR ], [DCJ, [AC ], and of the stable oligomers [SOJ observed during the photochemical and thermal reactions in the course of the solid state polymerization reaction we still have to solve the corresponding rate equations. These are differential equations of first order and follow from the reaction Eqs. (9) to (13). 

As we have seen above there are certain probabilities x in all low temperature photoaddition reaction steps that the first termination reaction will occur, i.e. the transformation of the diradical intermediates to the asymmetric carbene intermediates (DR -+AC +i). The second termination reactions, in which finally the stable oligomers are produced from the AC intermediates (AC -+ SO +i) may be characterized by the probabilities y . Therefore, a simplified reaction scheme is given by ... 

Figure 23 shows the stationary distribution of the DR- and AC-intermediates and of the SO molecules obtained with Eq. (21) for x = 0.2 and y = 0.1. The distribution of the DR and AC molecules has been normalized on the DR2 concentration, representing the shortest DR intermediate, which are stable at low temperatures in contrast to the short-lived excited DRj molecules. The distribution function of the SO molecules has been normalized with [SOj] = 1. The SOj molecules are the shortest stable oligomer molecules. 

A direct consequence of this is that, for a given conformation, the most stable oligomer will be the one containing the highest number of aluminum atoms, provided it is consistent with Loewenstein s rule (31). This trend becomes weaker as the pH decreases or the overall Si/Al ratio in solution increases, in agreement with the usual observations made in zeolite synthesis (32). These correlations are shown in table V referring to the cyclic tetramers. The numbers in brackets represent the concentration ratio between the species considered and the purely silicic form. 

The precipitation is usually achieved when approaching the pHpzc- Because the electronegativity for Classes III-V is intermediate in character (amphoteric) they correspond to network formers producing stable oligomers and extended networks also in the dry state. 

A brief list of ionization methods is given in Table 1. (One may quibble a bit about the dates given in the table, but we believe these are more or less accurate.) Up xmtil about 1970, the only ionization method in common use was electron impact (El). Field ionization (FI) was developed in the 1950s, but it was never very popular, and chemical ionization (Cl) was just getting started. These three methods (El, Cl, FI) depend upon vaporization of the sample by heating, which pretty much limits polymer applications to small, stable oligomers or to polymer degradation products (formed by pyrolysis or other methods). Field desorption (FD-MS), invented in 1969, was the first "desorption/ionization" method. FD- and FI-MS are often very useful (particularly for analysis of less polar polymers), but they have never been in widespread use. 

Triazine and Other Heterocyclic Ring Formation. Several types of reactions can be used to form heterocyclic rings in which multiple C-N bonds contribute high thermal stability. When these are used to cross-link heat-stable oligomers, the resulting thermoset polymers may have high thermal stability and other useful properties. These include cyanate/cyanurate, isocyanate/isocyanurate, hexaazatriphenylene trianhydride, and phtha-lonitrile/phthalocyanine. 

Polarized optical absorption spectra of the diradical (DR), the asymmetric carbene (AC) and the stable oligomer (SO) molecules. The dimer (n=2) spectra (A,a,ot) are given by the upper part. The spectra with n > 2 are shown in the lower part. 

Fig. 13 Absorption energy of diradicals (DR), asymmetric carbenes (AC) and stable oligomers (SO) as a function of chun length in partially polymerized PDA-TS.

Spedfic kinetic enhancement in one type of maaocyde can sometimes be observed in enzymatically catalyzed processes. For instance, in lipase-catalyzed polycondensation of dimethyl terephthalate and diethylene glycol at first a series of cydic oligomers is formed. Further, due to polymerization/depoly-merization of them, almost exdusivdy a cydic dimer is obtained while the most stable oligomer is trimer. ... 

Cb] asymmetric carbenes, (c) stable oligomers, (d) and (e) dicarbenes and (f) stable polymer, from (30). 

The results of these studies are summarised in Figure 5. For TS, the first intermediate observed is a dimer diradical, which can be converted thermally or by selective irradiation into a trimer, tetramer, etc.. Irradiation, however, also produces an asymmetric carbene trimer, which in turn can be converted into a series of oligomers. There is then a further side reaction which converts the asymmetric carbene trimer into a stable oligomer. Conversion of a diradical into an asymmetric carbene and a carbene into a stable oligomer occur only by the addition of a further monomer unit to the intermediate. These intermediates appear as the direct products of photo-polymerization below 80 K. Above K dicarbene radicals with singlet ground states and excited triplet and quintet states are observed. Some of these are sufficiently long for the interaction of the carbene radicals to... 

Diradicals longer than 6 monomer units have not been observed in TS though asymmetric carbenes and stable oligomers have been reported with ten or more repeat units. Thus, it seems that in TS the diradicals are stable only for short chain lengths and at room temperature these are rapidly converted into carbene species C33]. Although similar results have been observed for some diacetylenes, these show differences in detail, in particular the annealing processes at higher temperatures vary markedly from compound to compound. 

The results for TS are shown in Figure 6, Similar results have been reported for TS12, which exhibits longer diradical species but apparently no stable oligomers (34). For TS the results show a much less rapid variation for the longer oligomers than application of a modified free-electron model (35) suggests. 

Figure 6. Optical absorption energy versus chain length for diradicals (A), asymmetric carbenes (D) and stable oligomers (X) in TS. n is the number of chain repeat units, after (29).

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