Depolymerization crosslinking

Hie hydrolytic depolymerization of nylon-6 was followed by gel permeation chromatography (GPC), viscometry, and gravimetry. GPC determinations were performed on a Waters 150C chromatography system using benzyl alcohol as die eluant, two Plgel 10-p.m crosslinked polystyrene columns, and a differential refractometer detector. The flow rate was 1 mL/min. The concentration of the polymer solutions was 0.5 wt% and dissolution was accomplished at 130°C. 

A breaker an enzyme (at T<140°F), strong oxidizing agent, or an acid, is used to depolymerize polysaccharides and break crosslinks such that viscosity declines at a controlled rate so that the proppant may be deposited in the fracture. Too rapid proppant dropout would cause a premature "sand-out" which prevents future extension of the fracture. Peroxydisulfates are the most frequently used breakers. Less reactive organic peroxides may be preferred for high temperature formations (85). 

The polyester domains of suberized walls can also be depolymerized using chemical and/or enzymatic approaches similar to those used for cutin. The aromatic domains are far more difficult to depolymerize as C-C and C-O-C crosslinks are probably present in such domains. Therefore, more drastic degradation procedures such as nitrobenzene, CuO oxidation, or thioglycolic... 

Actin-binding proteins ABP-50, purification, 196, 78 ABP-120, purification, 196, 79 ABP-240 purification, 196, 76 effect on actin depolymerization, 215, 74 extraction, 196, 311 isolation from Dictyostelium discoideum, 196, 70 platelet-derived actin binding proteins [characterization, 215, 58 purification, 215, 58, 64 recombination with actin, 215, 73] 30-kDa Dictyostelium discoideum actin-crosslinking protein [assays, 196, 91 preparation, 196, 84] actin-depolymerizing factor [assay, 196, 132[ DNase assay, 196, 136 platelet-derived a-actinin [characterization, 215, 58 purification, 215, 58, 70 recombination with actin, 215, 73]. 

High-resolution 13C NMR studies have been conducted on intact cuticles from limes, suberized cell walls from potatoes, and insoluble residues that remain after chemical depolymerization treatments of these materials. Identification and quantitation of the major functional moieties in cutin and suberin have been accomplished with cross-polarization magic-angle spinning as well as direct polarization methods. Evidence for polyester crosslinks and details of the interactions among polyester, wax, and cell-wall components have come from a variety of spin-relaxation measurements. Structural models for these protective plant biopolymers have been evaluated in light of the NMR results. 

Preliminary structural studies of cutin and suberin breakdown involved examination of 13C NMR spectra for insoluble residues that were resistant to chemical depolymerization. In cutin samples, flexible CH2 moieties in particular were removed by such treatments, but CHOCOR crosslinks and polysaccharide impurities were retained preferentially. A concomitant narrowing of NMR spectral lines suggested that the treatments produced more homogeneous polyester structures in both cases. Our current studies of cu-ticular breakdown also employ selective depolymerization strategies with appropriate enzymes (1,28). 

The second soluble starch-g-PAN (No. 3) was prepared by subjecting another starch graft copolymer to a mild acid hydrolysis to partially depolymerize the starch moiety. The ethanol-water solvent system apparently promotes a greater number of starch PAN crosslinks than water, as evidenced by the lower solubilities (45 vs. 71% and 42 vs. 65%) and the higher viscosities for water dispersions (2400 vs. 920 cp and 690 vs. 

The second significant stage in the direct production of polyimide structures involves the thermal conversion of the patterned crosslinked film to the patterned polyimide film. It is important to understand how and under what condition the photo-crosslinked polyimide precursor is converted into polyimide as well as how completely. Mechanistically it is intriguing to determine wether the crosslinking fractures are split into small pieces or escape as pure hydroxyethylmethacrylate comparable to the zip-off depolymerization of polymethylmethacrylate. 

With regard to thermal conversion to polyimide, we evaluated the suitable reaction conditions for thermal conversion of the photo-crosslinked patterns into completely polyimide patterns and found, that the crosslinked bridges are split off and depolymerization takes place. More than 95 % of the volatile product is the monomer hydroxyethylmethacrylate. 

In the process of heating, polyorganosiloxane rubbers undergo a series of chemical transformations where, along with destruction and depolymerization reactions, intramolecular conversions, polymerization-type, exchange, and crosslinking reactions proceed. 

Remarkably, ceramic yields were not influenced by the reaction pathway applied. They are mainly a function of the molecular structure of the precursors, i.e. the nature of the silicon-bonded substituents R. The methyl group in 3M (T2-1 [2]) and 3P is responsible for low ceramic yields (ca. 50%). It does not contribute to cross-linking reactions and is split off at 500 °C. In contrast, 2M and 2P (ca. 84% ceramic yield) are highly cross-linked consequently depolymerization reactions are inhibited. Ceramic yields are highest in IM and IP. This is because of the possibility to crosslink during thermolysis by dehydrocoupling of Si-H and N-H units, as mentioned above. 

Silicon-containing polymers, which are used as starting materials, should be highly crosslinked in order to avoid thermal depolymerization during MP formation. 

This can give a comparative measure of the ceiling temperature, T, in polymerization (which is important for the onset of thermal depolymerization as discussed in Section 1.4.1). The thermodynamic ceiling temperature is rarely achieved in practice, both because of the requirement for a closed system and also owing to the onset of other degradation reactions such as crosslinking. 

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