Dilute solution polymerizations

If polymer concentrations are held to about 15% by weight or less, viscosities remain low to moderate. (Gelatin and similar polymers that form a structured liquid phase are wonderful exceptions to this rule-of-thumb, but they are indeed exceptions.) Some commercial processes for HOPE and many processes for special polymers satisfy this criterion. The solutions are not truly dilute since there are entanglements between polymer chains, but they are adequately dilute so that viscosities remain low, say 100 times that of water. 

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R denotes any monosubstituted COT, R = polymer tail or f-Bu Figure 2. Cycloextrusion in dilute solution polymerization of cyclooctatetraenes. 

Incidentally, bulk polymerization is also the chief method used for commercial polycondensations. Polycondensations are not as exothermic as free radical catalyzed vinyl-type polymerizations, so thermal control is less of a problem. Bulk polycondensation also favors formation of linear polymer rather than the cyclic products that are favored by dilute solution polymerization, particularly if AB-type monomers are being used. Finally, since a high degree of polymerization (i.e., high DP, or high molecular weight) is only... 

These generalizations may be illustrated by example (61). Consider the experiments outlined in Table 1. PHEMA, PMMA and copolymers of the two were prepared by dilute solution polymerization. Copolymer compositions (Table 1A [parentheses] ), obtained by application of proton Nuclear Magnetic Resonance spectroscopy (as detailed later in the discussion), are almost identical to the monomer feed ratios, despite high conversions. The latter increase through series 3 to 1, due to the square root relationship between initiator concentration and instantaneous rate of polymerization (63). Close to random co-polymerizations have been observed also in monomer mixtures containing... 

The kinetics of dilute solution polymerization of butyl acrylate were studied by Melville and Bickel (15). They measured the propagation and termination coefficients at 25 c kp 13(i/mole sec) and kt 1.8 x 10 ( /mole sec). Melville and Bickel also concluded that the termination reaction at 25 C occurred mostly throu combination, thus theoretically increasing the probability to produce a carboxyl terminated PnBA with an ideal ftuiction-allty of f=2 when the polymerization reaction is initiated with ABCVA. 

Efficient toughening by the rubber modifier can be obtained, as discussed earlier, only if the RLP is capable of sufficient grafting to the epoxy matrix. Theoretically, the optimal linear rubber chain should possess two terminal reactive groups. Since this configuration could not be synthesized in dilute solution polymerization, a branched copolymer of nBA and ethylene dlacry-late (EDA), structure I, with a potentially higher functionality, was studied. Ethylene diacrylate is used primarily as a cross-... 

The results of the dilute solution polymerization suggested that solvent chain transfer was a governing factor In the termination step. By the same token, the mechanism of solvent chain transfer termination forms free radicals which do not carry carboxyl groups, and serve as potent initiator species In the polymerization media. It was concluded that in order to obtain higher functionality of the PnBA the solvent had to be eliminated from the polymerization reactor. 

Functionality of the CTPnBA rubber dramatically increased in the bulk polymerization experiments, suggesting that chain transfer to solvent was a significant reason for the poor functionality in dilute solution polymerization. 

The study of the toughening of DGEBA resin with a new RLP, Carboxyl Terminated n-Poly Butyl Acrylate (CTPnBA), is reported here. The CTPnBA rubber was synthesized by dilute solution polymerization and bulk polymerization techniques (20,21). The carboxyl functionality of the polymers varied from f = 1.3 (in solution polymerization) to f = 1.8-2.0 (in bulk polymerization). 

Haward et al.t have reported some research in which a copolymer of styrene and hydroxyethylmethacrylate was cross-linked by hexamethylene diisocyanate. Draw the structural formula for a portion of this cross-linked polymer and indicate what part of the molecule is the result of a condensation reaction and what part results from addition polymerization. These authors indicate that the crosslinking reaction is carried out in sufficiently dilute solutions of copolymer that the crosslinking is primarily intramolecular rather than intermolecular. Explain the distinction between these two terms and why concentration affects the relative amounts of each. 

In the concluding chapters we again consider assemblies of molecules—this time, polymers surrounded by solvent molecules which are comparable in size to the repeat units of the polymer. Generally speaking, our efforts are directed toward solutions which are relatively dilute with respect to the polymeric solute. The reason for this is the same reason that dilute solutions are widely considered in discussions of ionic or low molecular weight solutes, namely, solute-solute interactions are either negligible or at least minimal under these conditions. 

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

C2S2, is a red Hquid (mp —0.5° C, bp 60—70°C at 1.6 kPa (12 mm Hg)) produced by the action of an electric arc on carbon disulfide (1 4). The stmcture has been shown to be S=C=C=C=S on the basis of its reactions to form malonic acid derivatives and on the basis of physical measurements. It is unstable and decomposes ia a few weeks at room temperature it decomposes explosively when heated rapidly at 100—120°C with formation of a black polymeric substance (C2S2) (5,6). Dilute solutions ia CS2 are fairly stable, but photochemical polymerisation to (C2S2) occurs. 

Molecular weights of PVDC can be determined directly by dilute solution measurements in good solvents (62). Viscosity studies indicate that polymers having degrees of polymerization from 100 to more than 10,000 are easily obtained. Dimers and polymers having DP < 100 can be prepared by special procedures (40). Copolymers can be more easily studied because of thek solubiUty in common solvents. Gel-permeation chromatography studies indicate that molecular weight distributions are typical of vinyl copolymers. 

Polyborates and pH Behavior. Whereas bode acid is essentiaHy monomeric ia dilute aqueous solutions, polymeric species may form at concentrations above 0.1 M. The conjugate base of bode acid in aqueous systems is the tetrahydroxyborate [15390-83-7] anion sometimes caHed the metaborate anion, B(OH) 4. This species is also the principal anion in solutions of alkaH metal (1 1) borates such as sodium metaborate,... 

Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials Corrosive, particularly when diluted. Attacks most common metals, including most stainless steels. Excellent solvent for many synthetic resins or rubber Stability During Transport Stable Neutralizing Agents for Acids and Caustics Dilute with water, rinse with sodium bicarbonate solution Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Chemical Reactivity - Reactivity with Water Reacts violently forming flammable hydrogen gas and a strong caustic solution Reactivity with Common Materials May ignite combustible materials if they are damp or moist Stability During Transport Stable if protected from air and moisture Neutralizing Agents for Acids and Caustics Caustic that is formed by the reaction with water should be flushed with water and then can be rinsed with dilute acetic acid solution Polymerization Not pertinent Inhibitor of Pofymerization Not pertinent. 

Chemical Reactivity - Reactivity with Water A slow, non-hazardous reaction occurs, forming propanolamine Reactivity with Common Materials No reactions Stability During Transport The product is stable if it is kept in contact with solid caustic soda (sodium hydroxide) Neutralizing Agents for Acids and Caustics Dilute with water and rinse with vinegar solution Polymerization This material will polymerize explosively when in contact with any acid Inhibitor of Potymerization Solid sodium hydroxide (caustic soda). 

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