In situ branching

PAMAM dendrimers are synthesized by the divergent approach. This methodology involves the in situ branch cell construction in stepwise, iterative stages (i.e. generation = 1, 2, 3. ..) around a desired core to produce mathemat-... 

The procedures described are based on improved modifications from original publications [4-8]. They focus on the divergent excess reagent syntheses of dendri-poly(amidoamines) using various alkylenediamine cores. Examples of both divergent in situ branch cell methods, as well as divergent preformed branch cell methods are presented. 

Poly(amidoamine)-(PAMAM-Starburst)-Monodendrons Among the first Starburst (Cascade) syntheses we performed in the early 1980s [83] involved partially masked (differentiated) initiator cores. For example dodecyl-amine, hydroxyalkyl amines or partially protected alkylene diamines were used as initiator cores and submitted to sequential (a) Michael addition with methyl acrylate followed by (b) reaction with an excess of ethylene diamine to give in situ branch cell construction in a divergent manner. The resulting products were core functionalized monodendrons as shown below ... 

The results of Figures 204 and 205 show that most of this in situ branching is introduced after the catalyst activity has peaked, which would be expected on the basis of Scheme 45, because the first increments of cocatalyst added would be consumed by the more energetic (and activity enhancing) redox reactions in steps 1 and 2. Later increments of cocatalyst would then attack the strained Cr-O-Si links, creating the mono-attached, in situ a-olefin-generating sites. The addition of still more cocatalyst then attacks the second attachment and destroys the site entirely. 

The amount of in situ branching produced is a sensitive function of many variables. Catalyst and reaction parameters have a major influence on the following (a) the a-olefin generation relative to polymer formation, (b) 1-hexene generation relative to that of other linear a-olefins, (c) how sharp or flat the Schulz-Flory distribution of linear a-olefins is, and (d) how efficiently the a-olefins are incorporated as branches. 

In another experiment, Cr/silica-titania activated at 870 °C was reduced in CO at 350 °C. Then it was further heated in N2 to 870 °C for 1 h. This treatment almost completely stopped the in situ branching response. Apparently, the rearrangement at the Cr(II) sites that occurs at... 

TABLE 63 Densities of Polymers Obtained with Cr (ll)/Silica-Titania Activated at 800 °C and then Reduced in Various Treatments, Showing Their Relative Effectiveness for In Situ Branch Generation... 

The in situ branching response is dependent on the catalyst s being completely reduced. Even a small exposure to air reverses the effect. The data of Table 64 show the results of an experiment in which a catalyst that was reduced in CO was deliberately exposed to a small amount of air. A few mL of dry air was injected into the N2 stream used to fluidize the catalyst at 25 °C. The amount injected was far less than the stoichiometric amount needed to oxidize the Cr(II) to Cr(VI). Nevertheless, only 0.016 mol 02/mol Cr decreased the degree of in situ branching by more than a factor of 2, and the addition of 0.033 mol 02/mol Cr completely stopped it. Thus the 02 seems to selectively oxidize those sites that generate olefins, or at least the active sites in general. The data shown in Table 64 indicate that the catalyst activity was not affected. 

In another experiment, the Cr(II) /silica-titania catalyst was exposed to a small amount of water vapor in N2 at 350° after reduction in CO at the same temperature. This treatment also completely stopped the in situ branching response, although it did not change the polymerization activity. 

TABLE 64 Densities of Polymers Made with Cr(ll)/Silica—Titania, Showing that Exposure of the Catalyst to Traces of 02 Before Polymerization Severely Inhibits In Situ Branch Generation... 

In contrast, the influence on polymer density becomes almost nonexistent when the support is alumina or an aluminophosphate—in part because fewer a-olefins are produced, and also because these catalysts do not incorporate comonomers (from any source) efficiently. The data of Table 66 provide a comparison of the performance of Cr/silica with the performance of three Cr/aluminophosphate catalysts in which the P/Al ratio was varied. Because all four polymers were made under the same conditions, it is clear that Cr/silica produced polymer with a much lower density (more in situ branching) than any of the Cr/aluminophosphates. Cr/alumina produced the polymer with the next lowest density. 

TABLE 65 Densities of Polymers Made with Various Catalysts, Showing that the Presence of Titania in the Catalyst Inhibits Formation of In Situ Branching... 

TABLE 67 Densities of Two Polymers Made with Cr/Silica-Titania Catalyst, Showing the Influence of Catalyst Activation Temperature on the Production of In Situ Branching in the Polymer... 

The initial temperature of catalyst activation can also influence the amount of in situ branching obtained in the polymer. This is in agreement with the olefin-generating behavior of the organochromium catalysts (Figures 185 and 192, Table 55). Table 67 shows an experiment in which Cr/silica-titania was activated at 800 °C or at 650 °C, and then it was reduced and tested for polymerization activity with 5 ppm triethylboron cocatalyst. The 800 °C catalyst resulted in significantly lower polymer density than the 650 °C catalyst. This derives from two causes. The 800 °C... 

In some commercial operations, the catalyst is made in a batch process by impregnating a metal alkyl cocatalyst onto the Cr/silica in a fixed ratio. This method is not ideal for the control of in situ branching, however, because there is no way to adjust the degree of branching once the catalyst... 

The efficiency of in situ branch generation in the polymer varies widely with the type of cocatalyst used. 

FIGURE 218 The concentration of in situ branching in the polymer, and therefore the density, can be controlled by adjustment of the cocatalyst concentration in the reactor. 

Thus, in the in situ branching process, it is important to understand how a-olefin production is dependent on ethylene concentration. Table 69 shows the results of an experiment in which the ethylene pressure in the reactor was varied about seven fold, with the reaction carried out with Cr(II)/silica-titania with 5 ppm BEt3 cocatalyst [27,238,681,682,699,700]. Surprisingly, the density of the resultant polymer was almost constant... 

Comparing the four experiments, we see that the external comonomer feed was highest with the Cr(VI) catalyst, either with or without BEt3. The third case, with Cr(II) catalyst alone, required about half as much external comonomer feed, and the last (giving in situ branching) case required none. 

During the manufacture of polyethylene when in situ derived comonomer is incorporated, the increased incorporation efficiency provides a major benefit [27,238,681,682,698-700]. It does not lessen the amount of comonomer in the polymer or the amount actually used during production. However, it does lessen the amount of comonomer in the reactor, and therefore the amount that must be recycled. This lower comonomer concentration in the reactor can affect operations significantly. Because excess comonomer in the diluent enhances polymer swelling, the increased efficiency of the in situ branching process means that lower-density polymers can be made at higher production rates without trouble. It also means that there is less comonomer residue in the polymer to be purged and recycled downstream. If volatile comonomer is left in the... 

Furthermore, the in situ branching process offers a feedstock cost advantage, because 1-hexene is more expensive than ethylene per unit mass. This differential can be significant for low- and medium-density polymers. Capital expense can also be lowered because loading and purification equipment for external 1-hexene is not required. The process is also advantageous in remote locations where 1-hexene is less easily obtained. Therefore, the in situ branching process has proven to be very useful in commercial polyethylene manufacture. 

FIGURE 224 Density of polymers made with Cr/silica in the presence of hydrosilane cocatalysts, which are extremely effective at generating in situ branching. 

Although much weaker in its effect, even H2 can be used as a cocatalyst to produce in situ branching in the polymer. Although H2 is commonly used as a means of MW control, it is less well known that H2 can also be used to... 

TABLE 73 Properties of Polymers Made with Two Catalysts in the Presence and Absence of H2, Which Was Found to be Capable of Producing In Situ Branching... 

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