Oligomers production

By Naylor and Gilliam62, d (pT)6 was condensed in the presence of Poly A with carbodiimide to give d-(pT)12 with a yield of 5%. In the experiment of Shabarova and Prokofiev67, d-(pA)2 preactivated in the form of an amino add amidate was condensed on Poly U to give products with a yield of 10%. Uesugi and Ts o68 studied the condensation of (2 MeIp)6 or (2 MeIp)s in the presence of a Poly C template. The relative overall yield of the oligomer products was 43 to 71%. 

TABLE 8 Composition of the Ethylene Oligomer Product of the Shell Process ... 

Comparable polymer yields are obtained for the four samples however, the Phosnic 390 samples IJ 3 and IL-22 have appreciably iK)re high and intermediate MW oligomer products. Nearly half the trin r in the Phosnic 390 samples undergoes conversion compared to 35% for the purified trimer. Finally, it is noted that the Phosnic 390 samples achieve similar polymer yields in about one-fifth to one-half the time as the pure PN trimers. These observations suggest that the Phosnic 390 sanqjles may contain component (s) that behave as catalysts or accelerators and that also tend to increase the high and intermediate MW oligomer yields. 

Scheme 1. Catalytic cycle of the [Ni°L]-catalyzed cyclo-oligomerization of 1,3-butadiene affording Cg-cyclo-oligomer products (according to Wilke et al.) 14...

The cyclo-oligomer products are formed in final reductive elimination steps commencing from the octadienediyl-Ni11 and dodecatrienediyl-Ni11 complexes for the C8- and Ci2-cyclo-oligomer production channels, respectively. Reductive elimination is accompanied with a formal electron redistribution between the nickel and the organyl moieties, which will be analyzed in Section 5.4. 

On the basis of the free-energy profiles presented so far for the refined catalytic reaction cycles for the C8- and Ci2-cyclo-oligomer production cycles, we now rationalize the critical factors that are decisive for the regulation of the product selectivity for the two reaction channels. 

A. Selectivity Control for the Reaction Channel Affording C8-Cycio-Oligomer Products... 

For PR3/P(OR)3-stabilized nickel complexes, there are two borderline cases known from the experimental investigation of Heimbach et al. 1 which, unlike the usual behavior, redirect the cyclo-oligomerization reaction into the Ci2-cyclo-oligomer production channel. Catalysts bearing either strong a-donor ligands that must also introduce severe steric pressure (e.g., PBu Pr2) or sterically compact n-acceptors (like P(OMe)3) are known to yield CDT as the predominant product. From a statistical analysis it was concluded,8a,8c that the C8 Ci2-cyclo-oligomer product ratio is mainly determined by steric factors (75%) with electronic factors are less important. 

A fourth mechanism for chain transfer involves the transfer of the polymer chain to the large excess of aluminium alkyls present. This has also been observed. Since the reaction rate for this process depends on the concentration of the aluminium compound, the molecular weight distribution now also depends on the latter concentration. For oligomer production this is only relevant when a very large excess of aluminium alkyls is present. 

Tris [Pg.218]

Figure 4. Effect of oligomer production on experimental and simulated initial and end-of-run isomerization rates...

Figures 14 and 15 show the relations between the amount of iron arene initiator, the reaction enthalpy (AHj and the glass transition temperature Tg of the polymerized Bisphenol-A diglycidylether (cf. Table 2, structure I, x = 0.15) and the oligomer product based on the former compound (cf. Table 2, structure I, x = 11.8). The maximum polymerization heat per mole of epoxide is observed ivith an initiator concentration of 1.5-2.5% (w/w). At this concentration, Tg of the crosslinked resin is about 115 °C for the polymerized low-molecular-weight expoxide and about 80 "C for the polymerized high-molecular-weight epoxide resin.

Oligomer products of diamine condensation with carbonyl compounds, using excess of one of bifunctional compounds have been used lately. These products are used as stabilizers of thermooxidative destruction for different polymers, moreover oligomer stabilizers are more interesting because they are washed out from the polymer at photofading and sweated out, while in use least of all [294]. 

The t-Boc/Z chemistry provides oligomers of the highest quality and allows the preparation of PNA by manual synthesis. The shortcomings of this method, however, include the use of extremely harsh chemicals and long cycle times. The Fmoc/Bhoc protection method provides PNA with a shortened cycle time and has been adapted to a DNA synthesis platform. Custom PNA suppliers have adopted this method for more rapid oligomer production. 

---