Chain extension experiments polymerization

Figure 3 shows the molecular weight versus conversion plot for the polymerization of VAc mediated by Co(acac)2. The plot was hnear and passed the origin, and the polydispersity of the prepared PVAc was lower than 1.3, indicative of living features of the polymerization. The hving features were further confirmed by a chain extension experiment. " ... 

To avoid autoinitiation, the bulk polymerization was carried out at 65 °C. When DBU was used as a catalyst in the presence of benzyl alcohol, TMC was polymerized within minutes, yielding polymers with controlled molecular weight and small dyspersity index (1.09-1.15). To demonstrate the living nature of the polymerization carried out under bulk conditions, the chain extension experiment was conducted. Resultant polymer growth proceeded in a linear fashion. It was demonstrated that the intermediate chains can be employed for further growth, by the addition of more... 

Besides extensive experiments, many computer simulations have been carried out on polymer blends, primarily, including Monte Carlo (MC) [15-18], molecular dynamics (MD) [12,19-35], mesoscopic dynamics (MesoDyn) [12,24,25], and dissipative particle dynamics (DPD) [33,36,37]. In the area of theoretical polymer physics, MesoDyn and DPD have been used to treat polymeric chains in a coarse-grained (mesoscopic) level by grouping atoms together up to the persistence length of polymers. Recent trends in the use of MD simulations on bulk polymers have led to the calculations of important... 

Effects of solvent polarity, counter-anion nucleophilidty, temperature, and monomer concentration on the carbenium ion polymerization chemistry have been extensively studied29,36 38,49. Based on previous knowledge26"29 Me3Al was chosen because with this coinitiator undesired chain transfer to monomer processes are absent. Preliminary experiments showed that Et3Al coinitiator did not yield PaMeSt, possibly because the nuc-leophilicity of the counter-anion Et3AlQe is too high and thus termination by hydrida-tion is faster than propagation36. ... 

In 1988, Terry and coworkers attempted to homopolymerize ethylene, 1-octene, and 1-decene in supercritical C02 [87], The purpose of their work was to increase the viscosity of supercritical C02 for enhanced oil recovery applications. They utilized the free radical initiators benzoyl peroxide and fert-butyl-peroctoate and conducted polymerization for 24-48 h at 100-130 bar and 71 °C. In these experiments, the resulting polymers were not well studied, but solubility studies on the products confirmed that they were relatively insoluble in the continuous phase and thus were not effective as viscosity enhancing agents. In addition, a-olefins are known not to yield high polymer using free radical methods due to extensive chain transfer to monomer. 

Modification experiments and primer extension analyses were performed on RNA from all four plant species. Representative gels are shown in Fig. 4. A large number of stops occur in the native RNA without DMS treatment. This phenomenon has been observed reproducibly in multiple RNA preparations using a variety of polymerization temperatures, and it may be due to a strong secondary structure within the rRNA that cannot be sequenced by reverse transcriptase. Also, there are many naturally occurring modified base residues within the rRNA sequences that can impede reverse transcriptase.23 Another possibility is that a portion of the RNA in each preparation is degraded at specific sensitive sites, and thus chain terminations result. Similar patterns of termination have been ob-... 

Another valuable technique for study of the physical form of the active centres is viscometry. It is reasonably simple to add a vacuum viscometer to the sealed polymerization vessel and to measure the viscosity of the solution after polymerization is complete but while active centres still exist [31]. These values can be compared with the viscosity after destruction of the centres by traces of alcohol. As the solution viscosity is sensitive to the apparent total molecular weight of the polymer any association of polymer molecules via their active ends will be readily apparent. This technique was used [32] in very early experiments to show that in hydrocarbon solvents extensive association of polymer chains often occurs and has an important effect on the polymerization kinetics. Light scattering techniques can be employed in a similar manner [33] enabling the apparent molecular weight to be directly determined. 

The mechanism of the polymerization of oxetanes has been extensively investigated by means of kinetics, trapping and coupling experiments and NMR studies. It is now generally accepted that the reaction is initiated by electrophilic attack of the initiator on the ring oxygen atom and propagated by an 5n2 type chain mechanism, as shown below in equation (32) (74802699). 

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