Maximum molecular weight

The maximum molecular weight M at which disentanglement can occur is determined when strain hardening occurs at Xc 4 such that... 

Figure 7 shows the limiting maximum molecular weight of products from a reactor of fixed size varies directly with the frequency factor of the initiator at a fixed activation energy, while the limiting conversion varies inversely with the frequency factor. In addition, the length of the chain-transfer controlled zone is increased inversely with the frequency factor. 

Example 11-2 Suppose 2 mole % hexylamine is added to caprolactam, which is allowed to polymerize. What is the maximum molecular weight of the polymer if aU end groups are equally reactive ... 

Na+ > K+ > Cs+. The amount of unsaturation also increases with number-average molecular weight (AT) suggesting that the rate of polymerization decreases relative to the rate of isomerization (chain transfer). The maximum molecular weight of base-catalyzed PPO, limited to 6000 by the ratio of polymerization, to transfer, k, is about kpf ktr 100. A theoretical upper limit to the molecular weight of PPO has been calculated. 

Data quoted in Table 5 illustrate also the effect of some impurities on the molecular weight of the product. The presence of 0.1 % of hexalde-hyde in the dioxane solution was sufficient to reduce the molecular weight of the product by a factor of 2, and only a low molecular-weight oligomer, not precipitated by ethanol, was produced when the proportion of the aldehyde increased to 1 %. The presence of acetone had no influence on the product. It is possible that the impurities present in the reacting mixture, or formed by some side reactions, impose an upper limit on the maximum molecular weight of the polypeptide formed in this system. 

As in the ethylene oxide system kinetics are complex and do not lend themselves to exact interpretation (19). The boron fluoride — water catalyst system appears to be most effective at a boron fluoride/water ratio of about three, a surprising and probably fortuitous similarity to the efficiency of this catalyst in the isomerization of some hydrocarbons (20). At low water concentrations the number of polymer molecules formed equals the number of water molecules added and chain transfer may be assumed, though it has not actually been demonstrated. There is some indication of a maximum molecular weight of 15,000—20,000 at — 20° C but the present data are inadequate to establish this point. The order in monomer appears to be first at low water concentrations rising to second at higher water levels, but it seems quite possible that this apparent change in order is due to some factor such as catalyst destruction. 

If polymerization is carried to say 20% conversion at 25° and the reaction mixture is transferred to a 60° bath, there is a sudden fast reaction followed by the normal 60° rate (7, 9). The fast reaction is attributed to radicals trapped at 25° and liberated at 60°. Successive fast reactions can be observed by preparing polymer at 25, 60, and 70°. No further reaction occurs at 70°. From this fact, from the much greater rate at 60° than at 40° and from the fact that maximum molecular weight polymer is made at 50 to 60°, it is concluded that polymer undergoes... 

Essential oils extracted by water or steam distillation need further purification, especially drying to remove water. Essential oils produced by distillation are limited to compounds with a maximum molecular weight of 225-250. 

Experiments with aqueous solutions of DNA and of other polymers suggest that polymers having high molecular weights — and hence large viscoelastic relaxation times — do not take part in the acoustic moticm and thus do not contribute to the wave dissipation [72], For operation at 5 MHz, the maximum molecular weight for which reliable viscosity measurements can be made acoustically is about 15,000 daltons. 

This sequence of monomer addition and molecular weight determination by GPC was continued until the maximum molecular weight was observed. At this point, stirring was stopped, and the reaction mixture was filtered through a sintered glass funnel to separate the polymer solution from the urea byproduct. The solution was then added dropwise to 1 L of well-stirred methanol, from which a white, rubbery polymer precipitated. After decantation, the polymer was dried at 80 °C in a vacuum oven. Table III lists the yield and elemental analysis. 

The common crystal form of polyoxymethylene is the hexagonal form. Mortillaro et al. [56—58] found that another crystalline form of polyoxymethylene was produced when the polymerization of aqueous formaldehyde was carried out at 20° to 35° C at pH > 10 in high salt concentrations (>20%). It is shown in Table 12 that high molecular weight polymer is only achieved below 35°C. The rate of polymerization is slow it takes about 10 days to obtain maximum molecular weight. The formation of the proper seed crystal is important almost any crystalline polyoxymethylene regardless of the crystal structure can be used as seed. The type of salt used as catalyst is critical to obtain orthorhombic polyoxymethylene of reasonable molecular weight (Table 13). 

The solvent power of the organic phase can be varied by including in it an organic liquid that is a solvent for the polyamide. Sokolov and Turetskii [107] showed that for mixtures of tricresol (polymer solvent) and dibutyl ether (non-solvent) there was an optimum ratio of the two solvents for maximum polymer viscosity and therefore for maximum molecular weight (Fig. 7). The content of tricresol in those experiments also influenced the yield of polymer [107] (Fig. 8). 

The allyl alcohol then yields vinyl-terminated polyether monols and the presence of monols results in many problems. Hence, the maximum molecular weight available as commercial products is limited to less than 5,000. 

Perhaps one of the most important effects of a low reaction rate and premature phase separation is the decrease in molecular weight of the material. This clearly must be avoided as it seriously decreases the tensile properties of the polymer. There exists therefore a balance between the degree of phase separation and organization that can be achieved and the maximum molecular weight. 

A linear polyester is to be synthesized from phthalic anhydride and propylene glycol. While the glycol is of high purity, the available phthalic anhydride sample is impure, containing 5% (w/w) phthalic acid. In what proportions should the impure anhydride and glycol sample be reacted to obtain maximum molecular weight ... 

Variations in temperature above and below 65 °C showed decreased yields. A maximum molecular weight of 37 600 Da with a PDI of 1.6 was achieved with a yield of up to 95%. Figure 13.11 shows a plot of yield and molecular weight for a typical reaction at 2.0MPa and 65 °C. 

Usually an equimolecular mixture diamine, TDI is used (a very small excess of TDI is frequently used to compensate for the consumption of TDI in reaction with hydroxyl groups), which normally leads to maximum molecular weight in the resultant polyurea. As a general observation, the molecular weight of solid polyurea from PHD polyols has a lower molecular weight than the copoly [ACN - styrene] obtained as filler in graft polyether polyols. 

Figure 3, which shows the effect of nonfunctional end groups on the molecular weight of nylon 66.with equal amine and carboxyl end-group concentrations at 50 eq/10° g, illustrates the necessity for the absence of monofunctional impurities in condensation polymer intermediates if maximum molecular weight is to be obtained. 

---