Molecular weight polyethene

Whereas, as mentioned above, aluminum trialkyls oligomerize ethene under pressure, cationic compounds of the type [ AlMe X- and [(L2AlMe)2(/r-Me)]+X [L = benzamidinate anion, X = MeB(C6F5)3] give high molecular weight polyethene under mild conditions.32... 

Anderson, J C. "High Density and Uirra-High Molecular Weight Polyethenes", Trib. Int. Vol No 1, 43-47, 1982. 

Solvent polymerization. For the synthesis of low-molecular-weight polyethene, the solvent proeess ean be used [77,78]. Cyclohexane or another appropriate solvent is heated to 140 to 150 °C. After addition of the catalyst, very rapid polymerization starts. The vessel must be cooled indirectly by water. Temperature control is also achieved via the ethene pressure, which can be varied between 0.7 and 7 MPa. 

Apparently, soluble catalysts are obtained by reaction of Ti(OR)4 with AIR3 [144]. High-molecular-weight polyethene is obtained in variable amounts, with Al/Ti ratios ranging between 10 and 50. Similar results are attained by replacing titanium alkoxide by Ti(NR2)4 [145]. Soluble catalytic systems are also obtained by reaction of Ti(acac)3 [146] and Cr(acac)3 [147] with AlEts as well as by reaction of Cr(acac)3 and VO(acac)2 with AlEt2Cl in the presence of triethyl phosphite [121]. With vanadium catalysts the activity reaches its maximum at Al/V ratio = 50. Under these conditions up to 67% vanadium is in the bivalent oxidation state. Bivalent and trivalent compounds will be active. 

The inhibitory effectiveness of a polyanion increases markedly with molecular weight between 5,000 and 12,000 as shown for polyethene sulfonic acid 453). This might mean that a weight of RNA comparable to that of the enzyme was involved in the interaction. There is no direct evidence from chemical or X-ray binding studies for any such large number of sites as this would imply. 

The polyethenes prepared with catalyst 2 (Fig. 3a) have greatly elevated elastic modulus G values due to LCB compared to the linear polymers shown in Fig. 3b. LCB also shifts the crossover point to lower frequencies and modulus values. The measured complex viscosities of branched polymers (see also Table 2) are more than an order of magnitude higher than calculated zero shear viscosities of polymers having the same molecular weight but a linear structure. The linear polymers have, in turn, t] (0.02 radvs)... 

Polyethenes with comparatively low molecular weights were synthesized by the use of Ni and Pd systems with bis(di-t-butylphosphinojmethane ligands a) Lippert, F., Hohn, a., Schauss, E. (BASF AG), WO 96/37522 and WO 96/37523 b) Feldmann, j., Hauptmann, E., McCord,... 

Elliott (1978) in a brief communication reported that more than 70% of the aluminium in blood was present in the plasma compartment. In normal subjects, they reported that a very small proportion of the plasma aluminium was ultrafiltrable. These findings were not consistent with thos of Lundin et al. (1978). In the study of (1978) the tendency was for the ultrafiltrable fraction to decrease as the total plasma aluminium concentration fell below 200 fiQlL. At high plasma aluminium levels, studies using polyethene glycol and direct ultrafiltration indicated that 60-70% of the aluminium was bound to high-molecular weight proteins, 10-20% was bound to albumin, and 10-30% was ultrafiltrable. 

An alternative method of obtaining water solubility, without salt or ion formation, is to incorporate short lengths of naturally water-soluble polymer into the main polymer structure. Often polyethene glycol is used. This is a polyether (Chapter 15) of molecular weight about 1000, which is made from ethylene oxide and ethylene glycol. Between 5 and 20% by... 

Different plastics present different painting problems but, on the whole, once clean they do not often present problems of paint adhesion. Exceptions are the polyolefin plastics, e.g. polyethene and polypropylene. Because these polymers lack polar groups, they have low energy surfaces which are difficult to wet and are not readily penetrated by solvents. In addition to this, the surface layer of the plastic, a few nanometres thick, is often different to the bulk, being low in molecular weight and weak. Paint adhering to this layer will pull the layer away and peel off, thus appearing to have poor adhesion. 

The termination of the polymerization reaction by the addition of carbon monoxide is used to determine the active centers (sites) of the catalyst. Hydrogen is known to slightly reduce the catalyst s activity. Yet it is commonly used as an important regulator to lower the molecular weights of the polyethene or polypropene produced. 

Table 2 shows polymerization conditions for the high-pressure process and density, molecular weight, and weight distribution of the polyethene (LDPE). Bunn [54] was the first to study the structure of polyethene by x-ray. At a time when there was still considerable debate about the character of macromolecules, the demonstration that wholly synthetic and crystalline polyethene has a simple close-packed structure in which the bond angles and bond lengths are identical to those found in small molecules such... 

FIGURE 6.2 Correlation of properties of polyethene with crystallinity and molecular weight. Adapted from Richards (1951). 

Polymers that are amenable to MALDI must have some degree of polarity -polyethene and polypropene are still essentially incompatible with the technique. Another essential requirement is that the polymers be narrowly distributed in terms of molecular weight, chemical composition, and functionality. If the molecular weight distribution is not very narrow, then the smallest molecules will be over-represented in the resulting mass spectrum ( discrimination ). If one type of molecule is abundant, it may dominate the entire mass spectrum, completely suppressing the ionization of polymeric components present in lower concentrations ( ion suppression ). Selective ionization may also be observed in the case of variations in functional groups or end groups. 

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