Molecular weight of anionics

Table 4.1. Molecular weights of anionic polystyrenes used for Fig. 4.4...

Complications resulting from kjk > 1 may be responsible for the observations of Lyssy (34) who found deviations in the molecular weights of anionically polymerized styrene from the theoretically expected value of Mtotaljl0 when the degree of polymerization was very low. In the discussion of his results he implies that the initiation is slow, and indeed, he was able to demonstrate the presence of naphthalene- radical-ions in the solution of living polystyrene. The work of Levy and Szwarc (35) leads to similar conclusions. 

Figure 5-53. Skin irritation and molecular weight of anionic surfactants. Note the decrease in irritation with increasing molecular weight (size).

Ma, M M molecular weights of anion, cation, and water, respectively... 

Only relative mass changes are needed, and thus calibration of the balance is not required. The main source of error is the molecular weight of anion, M. Generally it is not known how many solvate molecules the anion is carrying into and out of the polymer phase. Also, the polymer may be swollen by the solvent, whose mass adds to the mass of the polymer. 

Table 6.5 Molecular weights of anionically synthesized polybutadienes...

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

Molecular weights of polymers that function as bridging agents between particles are ca 10 —10. Ionic copolymers of acrylamide are the most significant commercially (see Acrylamide POLYMERS). Cationic comonomers include (2-methacryloyloxyethyl)trimethylammonium salts, diethyl aminoethyl methacrylate [105-16-8], and dimethyldiallylammonium chloride [7398-69-8], anionic comonomers include acryUc acid [79-10-7] and its salts. Both types of polyacrylamides, but especially the anionic, can be more effective in the presence of alum (10,11). Polyetbylenimine and vinylpyridine polymers, eg, po1y(1,2-dimethy1-5-viny1pyridiniiim methyl sulfa te) [27056-62-8] are effective but are used less frequentiy. 

Many fluids of natural origin contain detectable quantities of high molecular weight organic anions, such as those from humic, fulvic, and tannic acids, which can be carried to and deposited on AX membranes. Such deposits can behave as thin films partially selective to cations (6). The iaterfaces between such films and the undedyiag AX membranes then act as very thin stagnant depletion compartments and the AX membranes may exhibit polarization at current densities that are much lower than would be expected for new membranes ia the abseace of such anioas. 

Preparation and Reactions of S-b-MM. As mentioned in the introduction, we were interested in block copolymers of styrene and alkali metal methacrylates with overall molecular weights of about 20,000 and methacrylate contents on the order of 10 mol%. The preparation of such copolymers by the usual anionic techniques is not feasible. An alternative is to prepare block copolymers of styrene and methacrylic esters by sequential anionic polymerization, followed by a post-polymerization reaction to produce the desired block copolymers. The obvious first choice of methacrylic esters is methyl methacrylate. It is inexpensive, readily available, and its block copolymers with styrene are well-known. In fact, Brown and White have reported the preparation and hydrolyses of a series of S-b-MM copolymers of varying MM content using p-toluenesulfonic acid (TsOH) (6). The resulting methacrylic acid copolymers were easily converted to their sodium carboxylates by neutralization with sodium hydroxide. 

In order to avoid the SET process, we chose diphenylmethylsilyl anions (PI MeSiM 8a, M = K 8b, M = Na 8c, M = Li) as initiators for 7 instead of alkyllithium and benzene as a solvent. The polymerization did not take place in benzene with silyl anions alone. However, in the presence of an equimolar amount of suitable cryptands, the silyl anions initiated the polymerization. The results are summarized in Table 2. The molecular weights of polysilylenes thus obtained were in good agreement with the calculated values within experimental error. 

S-Methacryloyloxy-ljl -biadamantane, MBA, (Scheme 4) was efficiently polymerized anionically using [ l,l-bis(4/-trimethylsilylphenyl)-3-methylpen-lyl]lithium as the initiator, prepared in situ by the reaction of s-BuLi with l,l-bis(4-trimethylsilylphenyl)ethylene [17]. The polymerization took place at - 50 °C in order to avoid the solubility problems of the monomer, observed at - 78 °C. Narrow molecular weight distribution block copolymers of rather low molecular weights of PMBA with fBuMA and (2,2-dimethyl-... 

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