Monomeric precursors

The most common sol-gel process employs metal alkoxides of network forming elements (M(0R) where M is Si, B, Ti, Al, etc. and R is often an alkyl group) as monomeric precursors. In alcohol/water solutions the alkoxide groups are removed stepwise by hydrolysis reactions, generally employing acid or base catalysts, and are... 

Other monomeric precursors similar to 6-hexynyl-decaborane such as 6-norbornenyl-decaborane (129) and 6-cyclooctenyl-decaborane (131) (Fig. 75) underwent ROMP in the presence of either first- or second-generation Grubbs catalysts to produce the corresponding poly(norbornenyl-decaborane) (130) (Fig. 75) and poly(cyclooctenyl-decaborane) (132) (Fig. 75) with Mn > 30 kDa and polydis-persities between 1.1 and 1.8.152 Electrostatic spinning and pyrolysis of poly (norbomenyl-decaborane) was discovered to produce nanoscale, free-standing porous boron-carbide/carbon, ceramic fiber matrices.153... 

Figure 75 The conversion of the monomeric precursors, 6-norbomenyl-decaborane (129) and 6-cyclooctenyl-decaborane (131), by ROMP into poly(norbomenyl-decaborane) (130) and poly(cyclooctenyl-decaborane) (132). (Adapted from ref. 152.)...

Synthesis of Protein-Like Copolymers from Monomeric Precursors. 98... 

When discussing various methods for the synthesis of protein-like HP-copolymers from the monomeric precursors (Sect. 2.1), we pointed to the possibility of implementation of both polymerization and polycondensation processes. The studies of the potentials of the latter approach in the creation of protein-like macromolecular systems have already been started. The first published results show that using true selected reactions of the polycondensation type and appropriate synthetic conditions (structure and reactivity of comonomers, solvent, temperature, reagent concentration and comonomer ratio, the order of the reagents introduction into the feed, etc.) one has a chance to produce the polymer chains with a desirable set of monomer sequences. 

Nishimura and Yamada [10-11] introduced a water-soluble polymeric support having a linker recognized by ceramide glycanase for a synthesis of ganghoside GM3 (17). Synthesis of the polymerizable lactose derivative (14) with a ceramide glycanase sensitive linker is shown in Scheme 10.3. The lactosyl ceramide (Lac-Ger) mimetic glycopolymer (15) is obtained from the monomeric precursor (14) by co-polymerization with acrylamide. 

Polycarbonates, both aliphatic and aromatic, have been prepared by the ring opening polymerization of cyclic monomers or oligomers [22], Cyclic monomeric precursors are more common in aliphatic polycarbonates, but because of steric reasons aromatic polycarbonates can only be prepared from cyclic oligomers. Both cationic and anionic initiators have been examined and anionic initiators appear to be more efficient. 

There are principally two pathways to encapsulation of polyyne chains in the voids of the porous host, viz. the intraporous polymerization of a suitable monomeric precursor and the intercalation of soluble oligoynes followed by their propagation. The former way includes the penetration of the porous structure with a soluble monomeric precursor and the subsequent intraporous polymerization, which can be started, e.g., by the UV irradiation. The latter one... 

Most eukaryotic tRNAs are formed from monomeric precursors, each gene acting as a transcriptional unit. Processing is similar to that in bacteria (Section A, 7). Eukaryotic RNase P usually cleaves the 5 end,555-558c and another enzyme cuts at the 3 end.542 556 The 3 CCA sequence of the mature tRNA is usually not present in the primary transcript but is added.559/559a b As in bacteria (p. 1620) extensive modification of bases also occurs in the tRNA precursors of eukaryotes.235 560-562 Many tRNA genes contain introns, which must be removed by splicing (Section 5). 

Functional siloles have found applications in the elaboration of silole-containing tt-conjugated systems. Because of its unique electronic properties, the introduction of a silole component or a silole cooligomer into an unsaturated chain should be a promising route to novel --electronic materials and, in this connection, the preparation of new heterocyclopentadiene monomeric precursors remains a current challenge. 

Unlike ethylene glycol, a-hydroxycarboxylic acids, and p-mercaptoethanol, ligands such as ethanolamine, ethylenediamine, amino acids, and 2-aminoethanethiol do not form V2L2 complexes with vanadate [46], However, weakly formed products of VL stoichiometry (—546 ppm, —556 ppm) are observed with a number of a-amino acids [47], Because the amine functionality in amino acids is not suitable for formation of the cyclic [VN]2 core expected for a dimer, then one or the other of these VL compounds may correspond to the monomeric precursor to such a dimeric product. Certainly, with the a-hydroxycarboxylic acids, the monomeric complex can be a major component in solution. Similar VL complexes have not been reported for the amino alcohols. Perusal of the available reports suggests that the amine derivatives were studied under conditions where the nitrogen functionality was protonated, so the results may be somewhat misleading. 

Much effort has been devoted to the optimization of the polyesterification reaction. For instance, different types of monomeric precursors structurally related to succinic acid (e.g., dimethyl succinate or succinic anhydride) were used. Different kinds of catalysts (e.g., phenolates, titanium alkoxides, tin octanoates) at different concentrations were studied. Different reaction temperatures (130-190 °C) were reached and different procedures for water elimination (vacuum drying under different conditions or toluene distillation) were adopted. Experimental results obtained showed that the use of different catalysts and different monomer precursors (succinic acid derivatives) did not significantly alter the polymerization kinetics or yield, and for this reason, they were abandoned. The procedure finally adopted is summarized below. 

Self-assembly and grafting techniques have been used to attach both polymeric and monomeric materials to surfaces. The result of this approach is well-ordered monolayers in the case of monomeric precursors (See Section 4.3 above). Polymers... 

In this system, at least, berbamunine is the first bisbenzylisoquinoline formed [together with its diastereoisomer guattegaumerine (7)] from the monomeric precursors. The 2-norbisbenzylisoquinolines daphnoline (9) and its 12-0-methylation product daphnandrine (10) would have to be methylated at 0-7, giving 2-noroxyacanthine (141) and 2-norobaberine (169), respectively, before... 

A relatively small set of monomeric precursors can generate a wide range of polymers. This is most evident in the case of polypeptides and polynucleotides but the same principle applies to fatty acids, lipids and polysaccharides. The synthesis of these polymers is briefly sketched below [the enzymes catalysing key steps are indicated in square brackets for clarity]. 

Lewis acids based on titanium tend to aggregate and form dimers which are usually more reactive than their monomeric precursors (cf., Chapter 2). The degree of aggregation depends on the solvent, temperature, and the ligands attached to titanium no dimerization was detected by cryoscopy at -95° C in CH2CI2 [174], However, kinetic measurements of isobutene and styrene polymerizations indicate that polymerization is second order in titanium chloride [175,176], perhaps due to formation of a low concentration of the more reactive dimer or more stable Ti2Cl9-anions. However, polymerizations performed at lower [TiCl4] were reported to be first order in titanium chloride [105]. 

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