Polymerization efficiency

Cagho, S Righetti, PG, On the pH Dependence of Polymerization Efficiency, as Investigated by Capillary Zone Electrophoresis, Electrophoresis 14, 554, 1993. 

As described above, the enzymatic polymerization of phenols was often carried out in a mixture of a water-miscible organic solvent and a buffer. By adding 2,6-di-0-methyl-(3-cyclodextrin (DM-(3-CD), the enzymatic polymerization of water-insoluble m-substituted phenols proceeded in buffer. The water-soluble complex of the monomer and DM-(3-CD was formed and was polymerized by HRP to give a soluble polymer. In the case of phenol, the polymerization took place in the presence of 2,6-di-O-methyl-a-cyclodextrin (DM-a-CD) in a buffer. Only a catalytic amount of DM-a-CD was necessary to induce the polymerization efficiently. Coniferyl alcohol was oxidatively polymerized in the presence of a-CD in an aqueous solution. ... 

However, there is no indication that the presence of the observed signals correlates with the polymerization efficiency of the catalyst. In fact, systems which exhibit these signals are less effective catalysts and in some cases do not even polymerize ethylene under the chosen conditions. In contrast, systems without EPR signals correlated to Ti species are foimd to be catalytically active. It has to be emphasized at this point that the lack of an ESR signal associated to Ti + ions, in cases where no additional argon or electron bombardment has been applied, cannot be interpreted as an indication of the absence of Ti + centers at the surface. It has been discussed in the literature that small spin-lattice-relaxation times, dipole coupling, and super exchange may leave a very small fraction of Ti " that is detectable in an EPR experiment [115,116]. From a combination of XPS and EPR results it unhkely that Ti " centers play an important role in the catalytic activity of the catalysts. 

Substituents in the allyl group of a catalyst have a marked effect on the polymerization efficiency (9,12). This is shown in Table IV for the polymerization of ethylene with chromium and zirconium allyls and for the polymerization of methyl methacrylate with chromium allyls. Introducing a methyl group into the allyl ligand increases the activity by a factor of 2 to 7. In some polymerizations of ethylene Cr(2-Me-allyl)3 compounds are ten times more effective than the simple allyl derivatives. The introduction of... 

Although benzoyl peroxide will initiate the polymerization (by a radical chain reaction) of either styrene or acrylonitrile, -methoxy- -nitrobenzoyl peroxide will not initiate polymerization efficiently in the latter monomer because it is too rapidly destroyed by the polar decomposition. Acrylonitrile, but not styrene, causes the polar decomposition to predominate, and the intermediates of the polar decomposition are not catalysts for the polymerization of acrylonitrile. 

Free radicals should initiate polymerization efficiently. Some peroxides such as dialkyl peroxides and peresters tend to abstract hydrogen from the monomer more readily than they react to initiate polymerizations. Consequently, their efficiency as initiators is reduced. 

The latter mechanism is met in amine-vinyl monomer systems [41-46] (see Scheme 4). Due to the small n-acceptor ability of normal substituted vinyl monomers, an interaction in the ground-state level does not take place. The exciplexes assumed are detectable in aromatic amine-acrylonitrile (AN) systems by their emission spectra, as is shown in Fig. 1 for typical examples. The emission bands at 350 nm (by JV,JV-dimethyl-p-toluidine (DMT)) and 370 nm (by p-phenylene diamine (TMPD) result from the normal fluorescence of the isolated amine. As can be seen, the intensity of the exciplex emission is much higher in the DMT-AN system. This corresponds to the higher polymerization efficiency of that system (<)>[, by A. = 313 nm and 80 K 0.6 for DMT 0.15 for TMPD [46]). Mainly, the much higher dipole moment of DMT (1.1 D) is responsible for this result. The cation radicals [46] or neutral radicals [42] of the amines formed after PET and proton transfer have been detected by ESR measurements. As expected, the rate of photopolymerization of the systems discussed increases with increasing... 

Also, coordination compounds and metal carbonyls are able to undergo a PET, resulting in initiating radicals [63]. Recently investigated examples are iron chloride based ammonium salts [149], vanadium(V) organo-metallic complexes [150], and metal sulfoxide complexes [151]. However, the polymerization efficiency of some systems is only low due to redox reactions between the central metal ion and the growing polymer radical, and the low quantum yields of PET. 

The comparison of the kinetic data reported in Tables 26 and 28 clearly shows that poly(VBPO-co-MMA)s are more active in the photoinitiated polymerization of the HDDA/BA equimolar mixture than poly(MAPO-co-MMA)s, notwithstanding their uncomplete solubility in the acrylic formulation. In particular, the remarkable shortening of the induction period (to) causes a sharp increase of the overall polymerization efficiency in poly(VBPO-co-MMA)s, as indicated by ti/2 values which are about one third of those for poly(MAK)-co-MMA)s. 

Figure 6.26 gives information on the effect of carbon black loading on the polymerization efficiency of pyrrole. The polymerization rate was reduced as the loading of carbon black was increased. The reduced rate is caused by the oligomer coupling on the surface of the carbon black and by the absorption of the chemical oxidant needed for polymerization."" ... 

Figure 6.26. Effect of carbon black on polymerization efficiency of pyrrole. [Data from Wampler W A, Rajeshwar K. Pethe R G, Hyer R C, Sharma S C, J. Mat. Res., 10, No.7, 1995, 1811-22.]...

The reaction R5 in Fig.6, apart from its validity, is the most favorable one in view of polymerization efficiency. A thermochemical basis for R3 is the exothermicity of the process generally expressed as (19) ... 

The hydrodynamics of the electrochemical system and temperature are also important because these control the rate of transport of reactants and products to and from the electrochemical reaction zone. This in turn determines the polymerization efficiency. Hydrodynamics are also important in determining the form of the PPy produced. For example, using a flow-through cell and in appropriate chemical envi-... 

Under such conditions the rate of polymerization varies directly with fcj which is a function of the radical cation or Bronsted acid derived from rhe sensitizer on electron transfer quenching by onium salt. Accordingly, dependence of polymerization efficiency on chemical nature of the sensitizer, independent of the efficiency of the photoinduced electron transfer, can be understood. 

This approach shows that a deep knowledge of the photophysics and photochemistry of the photoinitiators in solution can thus serve as a good basis for the prediction of the polymerization efficiency in film experiments. Several recent approaches also suggest that a direct investigation on the excited-state processes involved in bulk... 

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