Poly preferentially

The selectivity of pervaporation membranes varies considerably and has a critical effect on the overall separation obtained. The range of results that can be obtained for the same solutions and different membranes is illustrated in Figure 41 for the separation of acetone from water using two types of membrane (89). The figure shows the concentration of acetone in the permeate as a function of the concentration in the feed. The two membranes shown have dramatically different properties. The siUcone mbber membrane removes acetone selectively, whereas the cross-linked poly(vinyl alcohol) (PVA) membrane removes water selectively. This difference occurs because siUcone mbber is hydrophobic and mbbery, thus permeates the acetone preferentially. PVA, on the other hand, is hydrophilic and glassy, thus permeates the small hydrophilic water molecules preferentially. 

The strategy of Kaeriyama represents a so-called precursor route and was developed to overcome the characteristic shortcomings (insolubility, lack of process-ability) of previous PPP syntheses. The condensation reaction is carried out with solubilized monomers, leading to a soluble polymeric intermediate. In the final reaction step this intermediate is then converted, preferentially in the solid state allowing the formation of homogeneous PPP films or layers, into PPP (or other poly(arylene)s). 

The fraction of head-to-head linkages in the poly(fluoro-olefms) increases in the series PVF2 < PVF PVF3 (Tabic 4.2). This can be rationalized in terms of the propensity of electrophilic radicals to add preferentially to the more electron rich end of monomers (i.e, that with the lowest number of fluorines). This trend is also seen in the reactions of trifluoromethyl radicals wilh the fluoro-olefins (see 2.3). 

Poly(isoprene) can also he prepared by radical polymerization/ 17 Although the ratio of l,4- l,2- 4,3- units is stated to be ca 90 5 5 irrespective of the polymerization temperature (range -20-50 °C), the proportion of cis-1,4-addition increases from 0 at -20 °C to 17.6% at 50 °C. EPR studies indicate that radicals add preferentially to the 1-position.87... 

Poly(DL-lactide) was used as the excipient in microspheres of CCNU, a nitrosourea, prepared by a solvent evaporation procedure (96,97). PLA-CCNU microspheres 3.0 pm in diameter were injected i.v. and leukemia cell survival was determined by spleen colony assay. A 100-fold decrease in leukemia cell survival was observed with the microspheres in both spleen and liver compared to untreated controls. Promising results were also obtained with Lewis lung carcinoma in mice. These studies showed that 2- to 4-ym microspheres were preferentially targeted to the lungs. 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

Successive 1,4 units in the synthetic polyisoprene chain evidently are preponderantly arranged in head-to-tail sequence, although an appreciable proportion of head-to-head and tail-to-tail junctions appears to be present as well. Apparently the growing radical adds preferentially to one of the two ends of the monomer. Which of the reactions (6) or (7) is the preferred process cannot be decided from these results alone, however. Positive identification of both 1,2 and 3,4 units in the infrared spectrum shows that both addition reactions take place during the polymerization of isoprene. The relative contributions of the alternative addition processes cannot be ascertained from the proportions of these two units, however, inasmuch as the product radicals formed in reactions (6) and (7), may differ markedly in their preference for addition in one or the other of the two resonance forms available to each. We may conclude merely that structural evidence indicates a preference for oriented (i.e., head-to-tail) additions but that the 1,4 units of synthetic polyisoprene are by no means as consistently arranged in head-to-tail sequence as in the naturally occurring poly-isoprenes. 

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

Early soil-release agents, applied particularly to resin-finished cellulosic goods, were water-soluble polymers, many being related to thickeners (section 10.8) such as starch, hydroxypropyl starch, sodium carboxymethylcellulose, methylcellulose, hydroxyethyl-cellulose, alginates, poly(vinyl alcohol) and poly(vinylpyrrolidone). These functioned essentially as temporary barriers and preferential reservoirs for soil, which was thus easily removed along with the finish in subsequent washing, when they then helped to minimise... 

Philippova and Starodubtzev have also extensively studied the complex-ation behavior of polyacids and PEG, especially, the system of crosslinked of poly(methacrylic acid) and linear poly(ethylene glycol) (Philippova and Starodubtzev, 1995 Philippova et al., 1994). They observed that decreasing the molecular weight of PEG from 6000 to 1500 resulted in its slower diffusion into the swollen network of PMAA, and a drastic decrease in both the stability and equilibrium composition of the intermacromolecular complex. Analysis of dried polymer networks of PMAA with absorbed PEG chains by FT-IR spectroscopy revealed the presence of two types of hydrogen bonded structures (1) dimers of methacrylic acid at absorption frequency of 1700 cm-1 and (2) interpolymer complexes of PMAA and PEG at 1733 cm-1. In addition, they also suggested as a result of their studies, that the hydrogen bonded dimer of PMAA forms preferentially to the intermacromolecular complex between the PMAA network and PEG chains. 

These are two possibilities for the polymerization of MA deviated from the ideal living radical polymerization (i) the chain end of poly(MA) formed primary radical termination with a DC radical does not dissociate or dissociates at an unfavorable position like 41 (ii) bimolecular termination leading to the deactivation of the iniferter sites occurs preferentially to the primary radical termination with the DC radical which reproduces the iniferter site. 

Another significant cooperativity effect in preferential helical screw sense optically active copolymers is the majority rule phenomenon.18bl8q In this case, the screw sense of a helical main chain with unequal proportions of opposite chirality enantiopure chiral side groups is controlled by the enantiomeric excess only. Since this phenomenon was first reported from poly-a-olefins made of vinyl co-monomers bearing nonenantiopure chiral moieties by Green et al.8b and Pino et al.,16b this majority rule has been established in... 

We also reported an intriguing chiral termini effect in rodlike poly(n-hexyl-(2-methylpropyl)silane) with (5)-2-me(hylbu(yl termini (15).28g Although the value of gabs is only +1 x 10-5, which indicates a very weak preferential screw sense in 15, it is noted that the preferential screw sense of 15 was opposite to that of 4, bearing the same (5)-2-me(hylbutyl side groups.28d,28f,28k... 

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