Oligomers, lower-molecular-weight

Formaldehyde—Alcohol Solutions. These solutions are blends of concentrated aqueous formaldehyde, the alcohol, and the hemiacetal. Methanol decreases the average molecular weight of formaldehyde oligomers by formation of lower molecular weight hemiacetals. These solutions are used to produce urea and melamine resins the alcohol can act as the resin solvent and as a reactant. The low water content can improve reactivity and reduce waste disposal and losses. Typical specifications for commercially available products are shown in Table 7 (117). 

The lower molecular-weight thermal oligomers are crystalline compounds with Htde odor. The yields of oligomers at varying contact times and different temperatures are shown in Table 2. 

This strategy is used for the synthesis of three different exact-mass telechelic oligomers. GPC, NMR, and GC/MS evidence indicates that clean depolymerization chemistry occurs for all three samples. Poly( 1,4-butadiene) (38) is broken down into oligomeric units with two, three, and four repeat units using catalyst 23. Catalyst 14 is more efficient and produces even lower molecular weight oligomers, primarily one and two repeat units. When allylchlorodimethylsilane is used instead of ethylene with 14, telechelic dimers are the only product. 

From a practical stand point almost any of the lower-molecular weight vinyl monomers, cross-linking oligomers, and polymers can be blended with a high-molecular weight thermoplastic polymer to enhance cross-link density at lower dose rates [39]. The influence of various other additives on the efficiency of cross-linking of polymers will be highlighted in the subsequent sections. 

Simple sugars and lower-molecular-weight oligomers.248... 

A5.4.7 Simple sugars, lower-molecular weight oligomers, and carbohydrate polymers... 

When less bulky ancillary ligands are used /3-hydride elimination leads to the formation of Q-olefins. As a consequence iminopyridine complexes are typically much less active than the diimine catalysts and afford lower-molecular-weight PE.321-324 For example, MAO/(122) polymerizes ethylene to branched oligomers with Mn < 600, and 240 branches per 1,000 carbons.325 Complex (123), is highly active for ethylene polymerization (820gmmol 1 h bar ).326 As with the diimine systems, reduction in the steric bulk of the ligand substituents results in reduced activity and lower-molecular-weight products. 

Gel permeation chromatography (GPC) studies also indicated that there is always a lower-molecular-weight fraction in the final polymeric product, presumably oligomers of compounds 5 and 6, formed through the cyclization oligomerization. The cyclic oligomers from molecule 5 were isolated and the structures confirmed by NMR spectroscopy. 

In the absence of any added salts, the distribution of adducts formed was found to be variable by ESI—MS. In general, however, the lower molecular weight surfactant oligomers showed a preference for the Na+ adduct, whilst the higher oligomers showed an increase in preference for the K+ adduct. This is consistent with the metal-ion complexing abilities described for polyether derivatives [39,40], Simplification of the spectra was achieved with the addition of salt solutions to the mobile phase, as shown for NH4C1 in Fig. 2.8.4 [29], With the addition of salt... 

In one application, styrene still bottoms were chromatographed. Here styrene and the individual lower molecular weight oligomers were separated, and as the pressure was increased, the higher molecular weight polystyrene eluted. 

These lower molecular weight compounds make up only a relatively small proportion of grape proanthocyanidins, which consist mostly of higher oligomers and polymers, as in most other plant species.Heterogeneity of proanthocyanidins increases with their chain length, due to the diversity of constitutive units, linkage positions, and possible sequences. 

---