Polymer compounds polydispersity

TSK-GEL PW type columns are commonly used for the separation of synthetic water-soluble polymers because they exhibit a much larger separation range, better linearity of calibration curves, and much lower adsorption effects than TSK-GEL SW columns (10). While TSK-GEL SW columns are suitable for separating monodisperse biopolymers, such as proteins, TSK-GEL PW columns are recommended for separating polydisperse compounds, such as polysaccharides and synthetic polymers. 

Some transition metal catalysts induce the living polymerization of various acetylenic compounds.68,69 Such polymerizations of phenylacetylene catalyzed by rhodium complexes are used in conjunction with a quantitative initiation and introduction of functional groups at the initiating chain end (Scheme 16).70 The catalyst is prepared from an [RhCl(nbd)]2/Ph2C=C(Ph)Li/PPh3 mixture and proceeds smoothly to give quantitatively the polymer 54 with a low polydispersity ratio. 

The reactive nature of compound 22 is illustrated by the series of transformations shown in Scheme 7.12, in which its Zr—C bond reacts selectively with electrophilic reagents to produce a-haloboronates 36—38. Compound 22 also catalyzes the polymerization of styrene. The polymers thus obtained had weight-average molecular masses in the range 75000—100000 with polydispersities of 1.8—2.1. An X-ray analysis of 22 confirmed it to be a four-coordinate Zr complex with two cyclopentadienyl rings, chlorine, and the aliphatic C-l carbon atom as the ligands (Fig. 7.4). 

The fundamental difficulty is that polymeric substances cannot be obtained in a structurally and molecularly uniform state, unlike low-molecular-weight compounds. Thus, macromolecular materials of the same analytical composition may differ not only in their structure and configuration (see Sect. 1.2) but also in molecular size and molecular weight distribution they are polydisperse, i.e., they consist of mixtures of molecules of different size. Hence, it is understandable that the expression identical is not, in practice, applicable to macromolecules. Up to the present time, there is no possibility of preparing macromolecules of absolutely uniform structure and size. It follows, therefore, that physical measurements on polymers can only yield average values. The afore-... 

The polydispersity of polymers prepared in this way is usually very low for example, a value MJM of 1.05 was found for a sample of poly(a-methylsty-rene). Living polymers can also be used for the preparation of block copolymers after the consumption of the first monomer, a second anionically polymerizable monomer is added which then grows onto both ends of the initially formed block. By termination of the living polymer with electrophilic compounds the polymer chains can be provided with specific end groups for example, living polystyrene reacts with carbon dioxide to give polystyrene with carboxylic end groups. 

In our opinion an adequate distinction between ideal, defect-free dendritic molecules, for which we propose the term cascadanef, and the more or less defect-free (monodisperse) dendrimers , and polydisperse, imperfect hyper-branched compounds has so far been lacking in the area between polymer chemistry and small-molecule chemistry. Other reasons for such a distinction are because highly pure compounds have al-... 

The use of Grignard compounds derived from p-chlorostyrene has already been mentioned as one of the early attempts to synthesize macromonomers 9). Vinylpyri-dine and methyl methacrylate were polymerized. However, no precise data were given on the efficiency of this unsaturated Grignard initiator nor on the polydispersity of the functional polymers obtained the possibility of side reactions (involving, for instance, the ester function of methyl methacrylate) was not discussed either. 

Table 14 shows that the form of platinum compounds display different catalytic activities. Usually, yellow colloid form is more active compared with the black one, but lower polydispersion degrees are typical of polymers, synthesized in the presence of the latter one. As a consequence, the use of black colloid for obtaining polymers of regular structure is optimal, though due to low catalytic activity of it either high temperature of the reaction or... 

The molecular weight distribution (MWD) of a PP can be controlled either during polymerization or in a compounding step. While high polydispersity (broad MWD) is achieved in the reactor by polymer design, a narrow Mw/Mn distribution can be obtained by peroxide visbreaking (which induces a concomitant reduction of Mw). The use of selective catalysts allows both possibilities. 

In some systems it is necessary to add a large amount of salts to obtain polymers with low polydispersities. This happens when salts participate in ligand/anion exchange (special salt effect) and when they enhance ionization of covalent compounds through the increase of ionic strength. The special salt effect may either reduce or enhance ionization. Strong rate increases observed in the polymerization of isobutyl vinyl ether initiated by an alkyl iodide in the presence of tetrabutylammonium perchlorate or triflate can be explained by the special salt effect [109]. The reduction in polymerization rate of cyclohexyl vinyl ether initiated by its HI adduct in the presence of ammonium bromide and chloride can be also ascribed to the special salt effect [33]. The breadth of MWD depends on the relative rate of conversion of ion pairs to covalent species and is affected by the structure of the counterions. 

The polydienes synthesized with diazo compounds have lower polydispersity than polymers synthesized using other initiators such as hydrogen peroxide. This fact can explain that, in spite of the interest in synthesis by H202, the polymerization initiated by diazo compounds is well understood. 

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