General Features of the Polymerization

TXN polymerizes easily in the presence of cationic initiators, and polymerization does not occur in the presence of anionic or radical initiators 48). Claims of radical-cationic polymerization have been made but the evidence is not convincing. 

A typical initiator for TXN polymerization is boron trifluoride, usually in the form of its complexes with ethers, esters or amines. Such stable, easily available complexes are conveniently handled and can be purified by distillation 49). 

Melting point Vapor pressure at 25 °C Boiling point Heat of vaporization Heat of sublimation Specific gravity at 65 °C (liquid TXN) 

The patent literature discloses a large variety of other initiators including different Lewis acids, stable carbenium, oxonium or diazonium salts, and derivatives (esters or anhydrides) of strong protonic acids. However, BF3 seems to maintain its superior position in industrial applications. 

Recently low concentrations of triflic acid and its derivatives (particularly the anhydride) were shown to give high molecular weight homo- and copolymers (with DXL) in high yield. It is remarkable that in order to obtain comparable yields it was sufficient to use almost 102 times less anhydride than BF3 0(C2H5)2 50). 

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General features of the polymerization kinetics for polymerizations with deactivation by reversible coupling have already been mentioned. Detailed treatments appear in reviews by Fischer," Fukuda et ai and Goto and I vikuda" and will not be repeated here. 

General features of the polymerization kinetits for polymerizations with deactivation by reversible coupling have aheady... 

The general features of bulk polymerization are retained in systems to which non-solvents have been added. Microscopical studies (130) demonstrate that the structure of the particles can be varied from very... 

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

In polymerizing systems, the resonance-stabilised fragments could act mainly as terminating species, but it remains a fact nonetheless, that their formation, simultaneous with a reactive, initiating radical, is a general feature of the known, efficient, photoinitiator systems presumably because their relative stability facilitates dissociation. This may help to explain why benzil... 

The general kinetic features of the polymerization show the rate to be first order in monomer and vanadium oxychloride concentrations, and independent of Al/V ratio above 5/1. The rate was increased in the presence of the polar solvent up to THF/V = 6 but then became independent of THF concentration. The catalyst underwent a rapid decomposition, accounting for declining polymerization rates with time, which was shown to be second order. The rate coefficient for decompo-... 

Definitions, chemical and physical properties, and general features of the most relevant catalyst types used in coordination polymerizations are described in Section 5.3, after the classification by type of monomers most frequently used and studied in this kind of polymerization reactions. 

Two general features of emulsion polymerization reaction systems for the production of carboxylated rubber latexes are of special interest The first b that the polymerization usually takes place under acidic conditions (c. pH 3-4) and not under the alkaline conditions which are usual for the production of non-functionalized synthetic rubber latexes. Polymerization is carried out under acidic conditions in order to encourage the carboxylic acid monomer to become copolymerized into the molecxiles of rubber being prcxluced. If the reaction is carried oui under alkaline conditions, then the carboxylic acid monomer is present mainly as a carboxylate salt which partitions strongly in favour of the aqueous phase If it polymerizes at all under these conditions, polymerization occurs mainly ir the aqueous phase of the reaction system, and the polymer molecxiles in whicl it becomes incorporated are far more hydrophilic than are the majority of the polymer molecxiles, which are produced in the latex particles. The requiremen... 

For the determination of the key process parameters and their respective values, fluorescence spectra from five 96-microreactor arrays were collected and processed. The normalized spectra are presented in Figure 5.7. The spectral features of the polymeric materials in the microreactors contain a wealth of information about the chemical properties of the materials that were extracted using PCA. According to the PCA results, the first two principal components (PCs) accounted for more than 95% of the spectral variation among all spectra. Thus the first two PCs were used for an adequate description of the fluorescence spectra. Results of the principal components analysis of the spectra from all 96-microreactor arrays as a function of catalyst concentration C are presented in Figure 5.8. The plot demonstrates the existence of the major general trend in the spectral descriptors where the variation in scores of both PCs strongly depends on concentration of component C for all screened process parameters. 

Polymerization reactions are complicated somewhat because they involve very large numbers of molecules. But we can demonstrate the general features of the thermodynamics of polymerization by considering a much smaller model system. Instead of considering the formation of polyethylene, for example, we can begin with the following reaction, in which two ethylene molecules combine with hydrogen to form butane ... 

In a solid state polymerization reaction monomer diacetylene crystals are transformed to polymer crystals in successive reaction steps. Nearly perfect polymer single crystals are obtained thermally (kT) or by UV- or X-ray irradiation (hv) of the monomer crystals [1-3]. Within the class of diacetylene molecules (R-C=C-C=C-R) which show this unusual chemical reaction, the TSHD (with side groups R = -CH2SO2-0-CH2) is the best known representative, which is characterized by a variety of reaction intermediates [4-19]. The unconventional reactivity and the unusual properties of the polymer crystal have attracted the interest of both, physicists and chemists. The general feature of the low temperature photopolymerization reaction is shown schematically in Fig. 1 by example of the diradical DR-intermediates. 

Knowledge of the order of basicities of cyclic and linear ethers is important for understanding certain phenomena in cyclic ether polymerization. As indicated earlier, chain transfer to polymer is a general feature of the cationic polymerization of cyclic ethers because the nucleophilic site of the monomer molecule (oxygen atom) is transferred to the polymer unit. To what extent chain transfer to polymer competes with propagation depends on the relative nucleophilicity of monomer and polymer unit. Thus, for five-membered THF, the polymer unit is a weaker base than the monomer. This makes the polymer less reactive than the monomer in nucleophilic substitution type reactions. Consequently, for this monomer, chain transfer to polymer is slow as compared to propagation. In contrast, in the polymerization of three-membered EO, the polymer unit is more basic than monomer. Therefore, reactions involving the polymer chain are important in this system. Practical consequences will be discussed in the subsequent sections devoted to polymerization of different classes of cyclic ethers. 

General Features of Heterogeneous Polymerization of 1,3,5-Trioxane The Induction Period in the Homo- and Copolymerization of 1,3,5-Trioxane... 

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