Limiting molecular weight

The compound R X is a chain-transfer agent, with X usually H or Cl. The net effect of chain transfer is to kill a growing chain and start a new one in its place, thus shortening the chains. Mercaptan chain-transfer agents ate often used to limit molecular weight, but under appropriate conditions, almost anything in the reaction mass (solvent, dead polymer, initiator) can act as a chain-transfer agent to a certain extent. 

The selective separation range of P-6/S-200 was determined with Blue Dextran (Vexdi exclusion limit) and fructose (V,o total permeation limit). Molecular weight (degree of polymerization) calibration (Fig. 16.22) was established with dextran standards and low dp pullulans (dp 3, 6, 9, 12, 15, 18) formed by the controlled hydrolysis of high dp pullulan. 

Low-molecular weight chains do not experience enough shear force to induce scission. Watson et al." demonstrated (by the intrinsic viscosity characterization of masticated NR) that the limiting molecular weight for the shear-induced degradation is in the order of 0.7-1.0 X 10 Frenkel independently speculated that shear-induced cleavage occurs near the midpoint of the polymer chain. 

The third class of polymers contains one or more nitrogen atoms on a pendant sidechain in the polymer repeat unit (13,14). The nitrogen may or may not be quaternary. In addition to being swelling clay stabilizers, these polymers also stabilize nonswelling mineral fine particles. Limited molecular weight data ig available but molecular weight values from 50,000 to 1 X 10 daltons have been cited for various polymers. 

Some proteins, generally with limited molecular weight, are expressed at high levels as a response to thermal shocks. 

Finally, chain transfer is undesirable except when it is used intentionally to limit molecular weight by adding good chain transfer agents such as carbon tetrachloride. Here transfer of a chlorine atom limits the size of one chain and at the same time initiates formation of a new chain by the trichloromethyl radical. Instead of (3), (3), (3), etc., we get (3), (3), (7), (8), (3), (3), (7), (8), etc., with a lower average chain length. 

Very large macromolecules cannot penetrate into the pores of the gel. Hence, such large molecules cannot be separated from one another. The so-called exclusion limit gives an approximate indication of the limiting molecular weight up to which the macromolecules of the polymer to be fractionated can penetrate the network and therefore be separated. Network structure and exclusion limit are closely related the tighter the network, the smaller the exclusion limit. 

Higher silicon halides Si X +2 with n > 2 but of limited molecular weight have been known for a long time, above all from chlorine, but a few from bromine, and none from iodine. In the case of fluorine, some higher silicon fluorides derived from difluorosilylene have been recently discovered (see p. 60). 

Small (much less than stoichiometric) amounts of diamines sometimes are added to the hydroxyl portion of the adhesive to provide a rapid but limited molecular weight increase as soon as the components are mixed.61,62 In this way a two-package adhesive can be made that will flow easily before mixing but will not readily flow after mixing. Such adhesives can be applied to vertical surfaces or overhead, and will remain in place until the bonds are closed and cured. 

Chemical Transformations. Operations that modify a mer, such as saponifying poly (vinyl acetate) to poly (vinyl alcohol), should be considered. The introduction of chain transfer agents, 1, which limit molecular weight, need consideration for many cases. The symbol i for initiator might be introduced. However, details of these operations are beyond the scope of the present work. 

For example, styryl cations react with perchlorate to form covalent esters that are relatively stable at low temperatures [81], whereas the more basic triflate anion tends to abstract j3-protons from carbenium ions in a transfer reaction [56]. The basicity of the counteranion determines the contribution of /3-proton elimination relative to propagation, and therefore the limiting molecular weight in a polymerization. 

These developments in cationic polymerization of 1,3,5-trioxane are discussed in more detail, because in this system the problems related to the mechanism of cyclization are now well understood. Cyclic oligomers were identified, isolated, their molecular weight distribution was determined, and the plausible explanation for observed distribution was given. From the synthetic point of view, the cationic polymerization of 1,3,5-trioxane offers the possibility of preparing macrocyclic polymers with relatively narrow molecular weight distribution and predictable (within discussed limits) molecular weights. The cyclic polymers can be prepared easily in relatively large quantities and conveniently separated from linear polymer by alkaline hydrolysis of the latter. 

Transfer agents that lead to production of unreactive radicals may be added to limit molecular weight [48-51]. Best suited are agents whose radicals are stabilized by adjacent groups or by resonance. The effectiveness of a transfer agent is characterized by its transfer constant, defined as the ratio of the rate coefficients of chain transfer and propagation ... 

The most important processes which limit molecular weights in cationic vinyl polymerizations are transfer reactions to monomer (9-41) or residual water (9-42) ... 

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