Polymerization, activation spontaneous

In polymerization by one-component catalysts [chromium oxide catalyst (75), titanium dichloride 159) at ethylene concentrations higher than 1 mole/liter and temperatures below 90°C the transfer with the monomer is a prevailing process. The spontaneous transfer, having a higher activation energy, plays an essential role at higher temperatures and lower concentrations of the monomer. 

The activation energy for the spontaneous decomposition of benzoyl peroxide is 30 ( 1) kcal. per mole, and the same value applies also within experimental error to the azo nitrile.The apparent activation energy for the polymerization of styrene initiated by either is about 23 kcal. per mole, therefore. 

Figure 1 Schematic structures of micelle and liposome, their formation and loading with a contrast agent, (a) A micelle is formed spontaneously in aqueous media from an amphiphilic compound (1) that consists of distinct hydrophilic (2) and hydrophobic (3) moieties. Hydrophobic moieties form the micelle core (4). Contrast agent (asterisk gamma- or MR-active metal-loaded chelating group, or heavy element, such as iodine or bromine) can be directly coupled to the hydrophobic moiety within the micelle core (5), or incorporated into the micelle as an individual monomeric (6) or polymeric (7) amphiphilic unit, (b) A liposome can be prepared from individual phospholipid molecules (1) that consists of a bilayered membrane (2) and internal aqueous compartment (3). Contrast agent (asterisk) can be entrapped in the inner water space of the liposome as a soluble entity (4) or incorporated into the liposome membrane as a part of monomeric (5) or polymeric (6) amphiphilic unit (similar to that in case of micelle). Additionally, liposomes can be sterically protected by amphiphilic derivatization with PEG or PEG-like polymer (7) [1].

Spontaneous polymerization of 4-vinyl pyridine in the presence of polyacids was one of the earliest cases of template polymerization studied. Vinyl pyridine polymerizes without an additional initiator in the presence of both low molecular weight acids and polyacids such as poly(acrylic acid), poly(methacrylic acid), polyCvinyl phosphonic acid), or poly(styrene sulfonic acid). The polyacids, in comparison with low molecular weight acids, support much higher initial rates of polymerization and lead to different kinetic equations. The authors suggested that the reaction was initiated by zwitterions. The chain reaction mechanism includes anion addition to activated double bonds of quaternary salt molecules of 4-vinylpyridine, then propagation in the activated center, and termination of the growing center by protonization. The proposed structure of the product, obtained in the case of poly(acrylic acid), used as a template is ... 

Higher thioaldehydes seem to have been overlooked. Thioketones, however, have received much attention. Work in this area has involved principally thio-acetone. This compound polymerizes spontaneously and rapidly, though it is not as active as thioformaldehyde. Some work has also appeared on higher... 

Whereas five- and six-membered systems are usually easy to prepare and often form spontaneously from suitable intermediates, seven-membered heterocycles can be quite difficult to synthesize. Only if the precursor has little conformational flexibility and a conformation suitable for ring-closure can readily be attained, will the cycli-zation proceed smoothly (as, e.g., in the formation of benzodiazepines from 2-(ami-noacetylamino)benzophenones). Otherwise, ring-closure must be forced by chemical activation or high reaction temperatures, and a low loading might be required to suppress polymerization. 

Higginson and Wooding 277) also reported a transfer reaction to solvent for the case of the polymerization styrene in ammonia initiated by potassium amide. There was no termination event in their kinetic scheme, i.e., active center deactivation via a spontaneous termination event was not considered to be a significant event. 

Dilution with toluene slowed the copolymerization rate, and kinetic measurements were carried out in toluene at 0°-30°C. As reported previously (II), the over-all activation energy of the spontaneous copolymerization of CPT and S02 was calculated to be 16.5 kcal/mole from the Arrhenius plot of the initial rate vs. polymerization temperature. Dependence of the intial rate of copolymerization upon monomer concentration was checked at various monomer concentrations and found to be quite high (II) this could not be explained without participation of the monomer in the initiation step. 

It isn t difficult to form addition polymers from monomers containing C=C double bonds many of these compounds polymerize spontaneously unless polymerization is actively inhibited. 

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