Catalyzed monomer, additives effect

Additives Effect on the Catalyzed Monomer Solution. Soluble dyes can be added to the catalyzed monomer solution to color the final wood-polymer composite. Any color of the visible spectrum can be added, browns to simulate black walnut, red and blues for national colors. The color emphasizes the grain structure of the particular species and combines with the polymer to add a three-dimensional depth not present in surface-finished wood. A dense black wood-polymer, so desirable for musical instruments, is difficult to obtain because of wood s light color and the tendency of the microstructure to chromatographically separate a dye of several components into its separate colors. Dyes have an inhibiting effect on the polymerization of wood-monomer composites, some more so than others. Additional catalyst can be added to overcome this inhibition, but in the radiation process of a given geometry additional time must be allowed for complete curing. 

In the second method, activated zinc is employed for the synthesis of regioreg-ular P3ATs [241-244]. This procedure is illustrated in Scheme 58 and revolves around the selectivity of Reike zinc addition followed by Ni(0)- or Pd(0)-catalyzed cross-coupling polymerization. Catalyst selection has been shown to play a major role in this coupling scheme. For example, Pd(PPh3)4 was determined to be the most effective catalyst for the coupling of iodozinc compounds, whereas Ni(dppe)Cl2 proved superior in conjunction with bromozinc monomers. 

Sulfonyl halides, particularly arenesulfonyl halides, can afford radical species much faster than carbon halides by the assistance of a metal complex and efficiently add to olefins with little dimerization of sulfonyl radicals in comparison to carbon-centered radicals. Another feature of the compounds is that there is little effect of the substituents on the rate of addition to an olefin. These properties make sulfonyl halides an efficient and universal series of initiators for the metal-catalyzed living radical polymerizations of various monomers including methacrylates, acrylates, and styrenes (Figure g). 52.175-177... 

In recent years, catalytic asymmetric Mukaiyama aldol reactions have emerged as one of the most important C—C bond-forming reactions [35]. Among the various types of chiral Lewis acid catalysts used for the Mukaiyama aldol reactions, chirally modified boron derived from N-sulfonyl-fS)-tryptophan was effective for the reaction between aldehyde and silyl enol ether [36, 37]. By using polymer-supported N-sulfonyl-fS)-tryptophan synthesized by polymerization of the chiral monomer, the polymeric version of Yamamoto s oxazaborohdinone catalyst was prepared by treatment with 3,5-bis(trifluoromethyl)phenyl boron dichloride ]38]. The polymeric chiral Lewis acid catalyst 55 worked well in the asymmetric aldol reaction of benzaldehyde with silyl enol ether derived from acetophenone to give [i-hydroxyketone with up to 95% ee, as shown in Scheme 3.16. In addition to the Mukaiyama aldol reaction, a Mannich-type reaction and an allylation reaction of imine 58 were also asymmetrically catalyzed by the same polymeric catalyst ]38]. 

While the polymerization of optically inactive AA-BB and AB monomers under DKR conditions leads to chiral polyesters, these approaches always result in limited molecular weights since a condensation product is formed that needs to be effectively removed. A solution for this would be to use the eROP of lactones, where no condensation products are formed during polymerization. In principle, the eROP of lactones can lead to very high MW polyesters (>80kgmol 1) [57]. Addition of a methyl substituent at the ro-position of the lactone introduces a chiral center. Peeters et al. conducted a systematic study of substituted e-caprolactones which revealed that monomers with a methyl at the 3-, 4-, or 5-position could be polymerized enantioselectively while a methyl at the 6-position (a-methyl-e-caprolactone, 6-MeCL) could not [58]. The lack of reactivity of the latter monomer in a Novozym 435-catalyzed polymerization reaction was attributed to the formation of S-secondary alcohol end-groups. These cannot act as a nucleophile in the propagation reaction since the lipase-catalyzed transesterification is highly R-selective for secondary alcohols. 

Our work [26] shows that carboxylic acids have no unusual catalytic effect on gel times when gel time versus pH (pH was obtained with a glass electrode in the reaction mixture) was plotted over the range of pH = 0 — 7 for various catalysts (Figure 48.2). The reaction conditions of Mackenzie and coworkers [22,27] were used (TEOS ethanol water acid = l 4 4 var monomer and alcohol were mixed 30 min prior to water addition). The discrepancy in the catalytic effect of carboxylic acids results from Mackenzie and coworkers comparison of acids at similar concentrations, which is difficult with the wide range of p T values. Acetic acid is weaker than HCl and thus provides a higher solution pH at the same concentration. This property is accentuated in alcohol because acetic acid acts as a weaker acid in alcohol (versus water) whereas HCl maintains a more constant acid strength [28]. The sinusoidal curve obtained for gel time versus pH plots for each of the acids is consistent with the results of Pope and Mackenzie [22] for HCl-catalyzed sols and with Iler for gelation of Si(OH)4 [23]. Some differences in gel times were observed near pH 2,... 

---