Formation of polymers with

A number of mechanisms for thermal decomposition of persulfate in neutral aqueous solution have been proposed.232 They include unimolccular decomposition (Scheme 3.40) and various bimolecular pathways for the disappearance of persulfate involving a water molecule and concomitant formation of hydroxy radicals (Scheme 3.41). The formation of polymers with negligible hydroxy end groups is evidence that the unimolecular process dominates in neutral solution. Heterolytic pathways for persulfate decomposition can he important in acidic media. 

Molecular model studies have shown that PMVE, PEVE, and PnPVE chains are capable to penetrate y -CD cavities. Model studies further indicate that the single cavity can accommodate three monomer units. The inclusion complex formation of polymers with cyclodextrins is entropically unfavorable. However, formation of the complexes is thought to be promoted by hydrogen bond formation between cyclodextrins. Therefore, the head - to - head and tail - to - tail arrangement, which results in a more effective formation of hydrogen bonds between cyclodextrins, is thought to be the most probable structure. This structure was proved by X-ray studies on a single crystal of the complex between y -CD and 1-propanol. 

During the 1980s, catalyst and process developments focused mostly on the formation of polymers with a controlled molecular weight distribution and... 

BR with narrow MMDs (Mw/Mn > 3.5) and a low solution viscosity can also be obtained by the use of a multi-component catalyst system which comprises the following six components (1) Nd-salt, (2) additive for the improvement of Nd-solubility, (3) aluminum-based halide donor, (4) alumoxane, (5) aluminum (hydrido) alkyl, and (6) diene. The solubility of the Nd-salt is improved by acetylacetone, tetrahydrofuran, pyridine, N,N-dimethylformamide, thiophene, diphenylether, triethylamine, organo-phosphoric compounds and mono- or bivalent alcohols (component 2). The catalyst components are prereacted for at least 30 seconds at 20 - 80 °C. Catalyst aging is preferably performed in the presence of a small amount of diene [397,398 ]. As the additives employed for the increase of the solubility of Nd salts exhibit electron-donating properties it can be equally well speculated that poisoning of selective catalyst sites favors the formation of polymers with a low PDI. 

The formation of polymers with terminal LM during chain termination in free-radical polymerization is based on the ability of anthracene and some of its derivatives to participate in homolytical reactions It was established that anthracene-containing compounds interact with macroradicals which are generated in free-radical... 

The use of bis (2-chloroethyl) formal as the predominant monomer in the manufacture of polysulphides results in the formation of polymers with very good low-temperature properties. The glass transition temperature of these disulphide polymers is -59°C. Furthermore, there is no tendency in these polymers to crystallise at a temperature above the glass transition point. A glass transition temperature of -59°C applies to all the commercial liquid polysulphide (LP) polymers. 

The change from one chemical class to another, as in Table I, will produce changes in polymer properties such as solubility and melting point. The same type of change within a polymer class can be produced by varying the structures connecting the functional groups. Low-temperature polycondensation procedures are especially applicable to the formation of polymers with wide property differences, as indicated in Table II. 

This reaction could be extended to bichromophoric systems, resulting in the formation of polymers with oxetane rings in the main chain. 

More evidence In favor of polymer chain growth on both sides of a metalloporphyrln plane was obtained In the copolymerization of propylene oxide and phthallc anhydride. The catalyst was the combination of EtPh3PBr and TPPAl-(-0-CHMe-CH2-)-Cl (TPPAlPPO), which can be obtained by the polymerization of propylene oxide with TPPAICI (Equation 1). If the copolymerization proceeds on both sides of a metalloporphyrln plane, a block copolymer, polyetherpolyester. Is expected to be formed on one side, and a polyester Is expected on the other side. In fact, GPC of the reaction product (Figure 8) showed two narrow peaks and clearly Indicated the formation of polymers with different chain lengths. Thus, this reaction provides the first example of a catalytic reaction occurring on both sides of a metalloporphyrln pleme. 

Two general mechanisms have been proposed to explain the formation of polymers with precipitated catalysts (a) the bound-ion-radical mechanism and (b) the bound-ion-coordinate mechanism. The bound-ion-radical mechanism involves chain growth in a chemisorbed layer of monomer molecules initiated by radicals or ion-radicals bound to the surface of the catalyst, while the coordinate mechanism involves chain growth from a complex ionic center in the catalyst. 

Optically active oligomers of methyl methacrylate were obtained by the asymmetric polymerization of triphenylmethyl methacrylate, followed by the substitution of methyl for triphenylmethyl, and subsequent GPC separation275 and optical resolution.276 Detailed study on the stereostructure of the oligomer made it possible to discuss precisely the mechanism of polymerization leading to the formation of polymer with one-handed helical conformation. 

Figure 69 shows (filled triangles and filled squares) that different structural parts of Kapton decompose at different times. Intermediates are formed at the same time (filled circles in Fig. 69) and subsequently pyrolysed again. This suggests that Kapton decomposes completely in air via a multistep process, without the formation of polymers with low molecular weight. 

Formation of polymers with relatively narrow molecular weight distribution provides a fair evidence for the cleaness of the system. This was achieved in polymerization of meticulously purified methyl methacrylate initiated by biphenylide salts of alkali metals and carried out in polar solvents, THF or DME, at a low temperature, - 78°C360). Under... 

There is practically no difference in the polydispersity between PMMA samples prepared in the presence of the diamagnetic (18-electron) and paramagnetic (17-electron) closo complexes 4 and 5, respectively. On the contrary, steric factors have a quite noticeable effect on the chain propagation step and the macromolecular characteristics of the samples. Thus, if the phosphine groups at the metal center are linked via methylene bridges (complexes 4 and 5), favorable steric conditions for controlling the polymer chain growth probably occur that lead to the formation of polymers with MWDs much narrow than in the case of complex 2. 

Poly(methylene-1,3 -cyclopentane) synthesized in the presence of (-)-(R,R)-3 and methylalumoxane revealed a molar optical rotation of [0] 4O5 = +51.0° (c 8.0, CHCI3), whereas the polymer analogously prepared using the enantiomer (+)-(S,S)-3 showed a value of [0]284O5 = -51.2° (c 8.0, CHCI3). These findings are consistent with the formation of polymers with main-chain chirality. [9a]... 

Copolymers of vinyl fluoride with such monomers as vinylidene fluoride or l-chloro-2-fluoroethylene were prepared in the presence of trichloroacetyl peroxide at 0 C in sealed tubes. The chlorine-containing copolymers were then reductively dechlorinated at 60 C in tetrahydrofuran with tri- -butyltin hydride in the presence of 2,2 -azobis(isobutyronitrile) for up to 40 hr. This general procedure led to the formation of polymers with a reasonable control of the level of head-to-head, tail-to-tail linkages in the product [27]. 

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