Monomers and initiating systems

The use of polyamide solutions for the preparation of graft copolymers is restricted to monomer and initiator systems able to be dissolved and stand strongly polar media such as phenols, organic or inorganic strong adds, or aqueous salt solutions, which are common solvents for the backbone polymer. 

Since its discovery for vinyl ethers and isobutene in the 1980s, the scope of controlled/living cationic polymerization has been expanded rapidly in terms of monomers and initiating systems. Figure 18 shows a partial list of representative monomers for which controlled/living cationic polymerizations are available. They cover virtually all classes of cationically polymerizable vinyl compounds, such as vinyl ethers, isobutene, styrene and its derivatives, and A/-vinylcarbazole. A rough estimate indicates that the total number of monomers for controlled/living cationic polymerization... 

Research described in this section concerns effects of solvent polarity, temperature, monomer and initiator concentration on the polymerization of a-methylstyrene with Si-H containing initiator/Me3Al system for the synthesis of HSi-PaMeSt and of desirable molecular weight. 

Figure 2. Schematic representation of the reactor system computer-controlled pumps (PI, P2) pump controllers (fc) reactor (CSTR) reception vessel valves (S1-S4) monomer and initiator storage vessels (Tl, T2). (a) Digital input from GPC injection valve (b) analogue output from GPC (c, d) digital outputs to recorder chart drive and event marker (e, f) analogue outputs for pump set-point adjustment (g,h) reactor feeds (i) reactor output (j-m) digital outputs to reception system valves (n) manual sampling of products by GPC,...

Concentrations of the monomer, solvent, polymer, and initiator System temperature Partial pressures of the monomer, solvent, and nitrogen Total pressure... 

The thermally-initiated styrene system is considerably simpler than most industrial applications. Though these experiments provided useful guidelines, it was difficult to develop broadly applicable design criteria without carefully evaluating a broad range of monomer, polymer and initiator systems. Hence we extended our kinetic model to some other monomer systems such as styrene and methyl methacrylate using common initiators such as benzoyl peroxide (BPO) and... 

Traditional Apparatus. As indicated earlier, liquid delivery systems for controlled rate addition of monomers and initiators have tended to rely upon constant speed piston pumps (19) in which volumetric control is achieved by manual adjustment of stroke length, and monitoring is by discharge from measuring cylinders. 

This is the simplest process and is widely used for synthesis of condensation polymers. The system is homogeneous and consists of monomer/polymer. In this process the monomer and initiator are kept in a reactor and heated to suitable temperature. The chain transfer agent whenever used for controlling the Molecular weight is also dissolved in the monomer. 

If, however, these conditions are not fulfilled, for instance if the solution contains such a high level of basic impurities that they compete effectively with the monomer for the TiCI+3 then there will be no polymerisation until the concentration of impurities has been reduced sufficiently. It seems most likely now that it is these circumstances which produce the quiescent mixtures of monomer and initiator. In order to induce a polymerisation in such quiescent systems it is necessary to produce a sufficient quantity of reactive ions. This can happen either if one waits long enough for the self-ionisation to produce sufficient initiating TiCl+3 ions, or if one adds a co-initiator which reacts with the titanium tetrachloride to generate ions in a different manner and in greater numbers. 

The lowering of kp with increasing polarity (e) of the solvent is apparently in conflict with the observation that most cationic polymerisations go faster (for the same monomer and initiator concentrations), the more polar the solvent is. This effect can have several explanations - for some systems it may be completely meaningless to compare the rates in two solvents of different polarity (even at the same concentrations of reagents), unless it is proved (a) that the kinetics (i.e., the order with respect to monomer and initiator) is the same in both solvents and (b) that the total concentration of growing chains produced by a given concentration of initiator is the same. 

Hydrogen abstraction from -SH is faster than from -OH groups. It is generally of interest to increase both the yield of polymer and the grafting efficiency and decrease the formation of homopolymer. This can be achieved by proper selection of the grafting conditions, e.g. monomer concentration, initiating system and its application, reaction temperature and time. 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

Hayashi et al., 1989], involving the addition of monomer and initiator to a previously prepared emulsion of polymer particles, is especially useful for this purpose since it allows the variation of certain reaction parameters while holding N constant. Thus, h in seeded styrene polymerization drops from 0.5 to 0.2 when the initiator concentration decreases from 10-2 to 1CT5 M. At sufficiently low Ru the rate of radical absorption is not sufficiently high to counterbalance the rate of desorption. One also observes that above a particular initiation rate ([I] = lO-2 M in this case), the system maintains case 2 behavior with h constant at 0.5 and Rp independent of Ri. A change in Ri simply results in an increased rate of alternation of activity and inactivity in each polymer particle. Similar experiments show that h drops below 0.5 for styrene when the particle size becomes sufficiently small. The extent of radical desorption increases with decreasing particle size since the travel distance for radical diffusion from a particle decreases. 

The dependence of if, on monomer and initiator concentrations varied from system to system. For example, if, was proportional to [C]2-4 [M]8 for a-methyl-styrene-iodine-l,2-dichloroethane (7), [C] [M]3 for styrene-boron trifluoride... 

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