Linear oligomers, physical

These anionic ring opening polymerizations are usually carried out either in bulk or in solution. A host of catalyst types are active. For synthetic references using specific catalysts, the reader is referred to several excellent sources (4,7,31,32). Representative catalysts include hydroxides, alcoholates, phenolates, silanolates, siloxanolates, mercaptides of the alkali metals, organolithium and potassium compounds, and quaternary ammonium and phosphonium bases and their silanolates and siloxanolates. Some physical characteristics of linear oligomers are given in Table 5 (10). 

TABLE 5. SOME PHYSICAL CHARACTERISTICS OF LINEAR OLIGOMERS... 

Dendrimers are a class of macromolecules with a precisely controllable branched structure, consisting of three structural units a core, a hyperbranched scaffold and an external surface [16]. Dendrimers have been shown to possess unusual physical and chemical properties that differ significantly from those of linear oligomers and polymers. By using a fluorescent chromophore as the core of a dendrimer, one can apply fluorescence spectroscopy to study stmctural aspects and the conformational mobility of dendrimers in solution [17, 18]. At the same time, the dendritic shell provides a unique nanometer-sized environment for the spatial isolation of the chromophore, making them interesting materials for investigations by SMS. The synthesis of dendrimers with fluorescent chromophores attached to the rim serves as an efficient way to obtain a weU-defined number of chromophores in a confined volume [19-25]. Not only can the number of chromophores be easily controlled. 

Monomers are usually low-MW, monofunctional materials that chemically incorporate into the cured coating rather than volatilize into the atmosphere, as is common with solvent diluents. Monomer diluents are chosen on the basis of providing good solvency, effectively reducing the viscosity of the oligomer without excessively retarding the cure rate. Certain diluents will contribute to the physical properties of the adhesive. However, generally they provide soft, thermoplastic films because of their linear and uncrosslinked nature. 

Urea-formaldehyde resin solutions are shown to be dominated by physical associations rather than primary chemical bonding. These physical associations, or colloidal dispersions, are directly related to the thermodynamic balance of secondary bond formation between resin and solvent systems. Steric and entripic evaluations of molecule configuration have shown that linear urea-formaldehyde oligomers resemble polypeptides, and have the potential to form both 3-sheets and n-helixs, While the exact configuration of the associations is not known, their presence has been confirmed by x-ray analysis, which shows that urea-formaldehyde resins are crystalline in solid form. 

If the linear fraction of siloxane oligomers is used direcdy in the manufacture of silicone polymers, extremely pure (greater than 99.99%) dimethyldichlorosilane is required. A higher content of methyltrichlorosilane can produce significant amounts of trifunctional units and considerably affect the physical properties of the final products. If such high purity dimethyldichlorosilane is not achieved, an additional step, called cracking, must be included in the production scheme (66). 

Results of Calculations of NLO Properties and Their Discussion. - 5.2.1 Solid State Physical Calculations. - The calculation of static linear polarizabilities < zz/N = oll/N for the polymers poly(H2), and the H-bonded systems poly(H20) and poly(LiH) using the above described solid state physical formalism has given much larger values116 than those obtained by extrapolating oligomer values with the aid of different procedures. Further computations for... 

Table 6.11 shows the results of elementary analysis, physical chemical properties and the yield of ethylsiloxane oligomers containing cyclic fragments in the side chain. In the IR spectra of oligomers with cyclic fragments in the side chain one can observe the absorption bands characteristic of asymmetric valence oscillations of the =Si-0-Sis bonds in the region 1020 and 1080 cm characteristic of both linear siloxane links and cyclosiloxane rings. In the IR spectra one can also observe absorption bands for unreacted =Si-H bonds at 2160 cm. ... 

Calixarenes, coined by C. D. Gutsche, are the cyclic oligomers produced by condensation of phenols and formaldehyde (1). In other words, they are cyclic phenol resins, but their physical and chemical properties are much different from those of linear phenol resins. Many of the calixarenes are crystalline and generally have poor solubilities in either water or in organic solvents. Their melting points are generally high, while ordinary phenol resins, novolaks, soften below 150°C. 

Figure 17. Convergence of the linear absorption (the imaginary part of a (eq 1.5)) with the number of modes used for N= AO atom oligomer. The line width is T = 0.2 eV. Note, that the fundamental band at 2.57 eV with strength 109 eAW (1.57 x 10 esu) remains basically the same in all panels. Reprinted with permission from ref 300. Copyright 1996 American Institute of Physics.

Figure 20. Linear absorption spectrum for PPV-4 oligomer calculated using the DSMA (top panel) and the Lanczos algorithm (bottom panel). Reprinted with permission from ref 219. Copyright 1996 American Institute of Physics.

The cyclic polymers or oligomers are distinct in their physical properties from the corresponding linear chains. There has been considerable interest in the synthesis, isolation, characterization, and utilization of polymeric cyclics, although in a number of cases, they are at best oligomeric. A coUectiOTi of reviews on various aspects of cyclic polymers has been published and an introduction to this area has been given [72—74a-c]. 

As pointed out by Hajji et al. [32], polymer blend nanocomposite systems can be prepared by various synthesis routes because of their ability to combine in different ways to introduce each phase. The organic component can be introduced as (1) a precursor, which can be a monomer or an oligomer (2) a preformed linear polymer (in molten, solution, or emulsion states) or (3) a polymer network, physically (eg, semicrystalline linear polymer) or chemically (eg, thermosets, elastomers) cross-linked. The mineral part can be introduced as (1) a precursor (eg, tetraethyl orthosilicate) or (2) preformed nanoparticles. Organic or inorganic polymerization generally becomes necessary if at least one of the starting moieties is a precursor. 

Unlike the linear analogues, cyclic dimethylsiloxanes are low melting solids and the Tm generally decreases with increasing ring size. Physical properties for a series of M-stopped oligomers, MDj M, and cyclic compounds, D , are listed in Table 3. 

---