Bulk Monomer

The irradiation of some monomers results in the formation of an excited state M by the absorption of light photons (quanta)  

The excited species undergoes homolysis to produce radicals 

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In mass polymerization bulk monomer is converted to polymers. In solution polymerization the reaction is completed in the presence of a solvent. In suspension, dispersed mass, pearl or granular polymerization the monomer, containing dissolved initiator, is polymerized while dispersed in the form of fine droplets in a second non-reactive liquid (usually water). In emulsion polymerization an aqueous emulsion of the monomer in the presence of a water-soluble initiator Is converted to a polymer latex (colloidal dispersion of polymer in water). 

These acids can be used alone or as mixtures. It is especially advantageous to use a mixture of liquid and gaseous acids. The gaseous acid will stabilize free monomer in the headspace of a container, while the liquid acid will prevent premature polymerization of the bulk monomer or adhesive. However, it is important to use only a minimum amount of acid, because excess acid will slow initiation and the formation of a strong adhesive bond. It can also accelerate the hydrolysis of the alkyl cyanoacrylate monomer to 2-cyanoacrylic acid, which inhibits the polymerization of the monomer and reduces molecular weight of the adhesive polymer. While carboxylic acids inhibit the polymerization of cyanoacrylate monomer, they do not prevent it completely [15]. Therefore, they cannot be utilized as stabilizers, but are used more for modifying the reactivity of instant adhesives. 

Alternatively, liquid phase polymerization (in bulk monomer at a temperature of 20° C) furnishes an isomer (II) characterized by a cis-transoid (or trans-cisoid) configuration of the main chain, with carboxyl groups located on both sides of it. These isomers will be shown later to differ in chemical and physicochemical properties. 

Note that the value of Te is dependent on the monomer concentration. In the literature, values of Tc may be quoted for [M] = 1.0 M, for [M] = [ M et or for bulk monomer. Tints care must be taken to note the monomer concentration when comparing values of TK. One problem with using the above method to calculate fc. c or l c, is the paucity of data on ASp. A further complication is that literature values of AHP show variation of +2 kJ mol1 which may in part reflect medium effects. 77 This "error" in A//p corresponds to a significant uncertainty in Tc. 

Table 4.11 Kinetic Parameters for Propagation in Selected Radical Polymerizations in Bulk Monomer...

The behaviour patterns ensuing when bulk monomers are diluted by solvents are very varied. The most detailed information concerns the VE. My re-examination of the results shows that, contrary to current belief, no one kinetic scheme will fit all the systems over the whole range of m. My interpretations were facilitated considerably by the availability of the dependence of c on m, which for most systems can be expressed by a linear equation of the form c = Am + B, where in some systems A is positive, in others negative. By making this substitution in the kinetic equations it becomes obvious why for most systems the external kinetic order with respect to m is greater than unity, an effect noted, but hitherto not explained convincingly. 

This means that the rate constants derived from ionizing radiation experiments with bulk monomers are not the second-order k+p given by (2.5), but first-order rate constants, pj, given by (3.1). A comparison of these equations shows that the putative k+p reported... 

My idea that in bulk monomer and in some solutions the propagation is a unimolecular reaction is supported strongly by the way in which the DP depends on the monomer concentration. At low m, 1/DP increases rectilinearly with 1/m, as demanded by the conventional Mayo equation for bimolecular propagation but at high m, 1/DP increases rectilinearly with m, as is required for unimolecular propagation (see Section 5). 

When considering the polymerizations by ionizing radiations in solution, I adopt a point of view opposite to that customary in conventional reaction kinetics. In these it is normal practice to progress from dilute to more concentrated solutions, usually up to no more than ca. 2 mol dm"3. In the present context, the actual experimental practice determines that we think in terms of a gradual dilution of the bulk monomer this also happens to be heuristically fruitful. 

In the mid-1960s the first measurements of propagation rate-constants for unsaturated monomers became available, from polymerisations initiated by y-radiation [5]. The circumstances of these experiments were such that it was immediately clear that these very high rate constants (106 to 108 1 mol"1 s 1) were those of unpaired cations, kp. All these reactions were carried out with bulk monomer, i.e., the polymerisations occurred in a medium of very low polarity (e c. 2 for hydrocarbons and 5 to 6 for alkylvinylethers). Unfortunately, the y-radiation method is not applicable to polymerisations in solution, especially in polar (usually alkyl halide) solvents. The methods which have been used to... 

Part of ca neutralised by impurities, whose concentration is [Imp]. Concentration of monomer, i.e., [M], m in bulk monomer i.e., (molar volume) 1. 

Polymerisations of undiluted, bulk monomer are rare except for those initiated by ionising radiations and they require a special treatment which will be given later. The most common situation is to have the propagating ions in a mixture of monomer and solvent, and as the solvation by the solvent is ubiquitous and may dominate over that by other components of the reaction mixture, mainly because of the mass-action effect, it will not be noted by any special symbol, except in a few instances. This means that we adopt the convention that the symbol Pn+ denotes a growing cation solvated mainly by the solvent correspondingly kp+ denotes the propagation constant of this species, subject to the proviso at the end of Section 2.3. Its relative abundance depends upon the abundance of the various other species in which the role of the solvent as the primary solvator has been taken over by any or all of the anion or the monomer or the polymer. The extent to which this happens depends on the ionic strength (essentially the concentration of the ions), and the polarity of the solvent, the monomer and the polymer, and their concentrations. 

The theory of unimolecular propagation for the cationic polymerisation of a bulk monomer was first developed by Szwarc (1978), and Equation (44) introduced by him has been named Szwarc s equation (Plesch, 1993). 

Options 1 and 2 can be dismissed as highly unlikely. Option 3a can be set aside just because the kp+ in question which should be, but are not, the same as the k2, were selected as those (very few) which had been derived from reactions for which the concentration of propagating carbenium ions was deemed to have been well-established and which were clearly first-order with respect to the monomer. The much-quoted rate constants for radiation-induced polymerisations of bulk monomers, which are internally of zero order, were not included (see Plesch [1] for details). 

The ternary complex consisting of the carbenium ion with an anion and a monomer molecule can isomerise with incorporation of the previously complexed monomer molecule into the chain and a shift of the positive charge to the new chain end. This is a unimolecular propagation reaction of zero order with respect to the monomer concentration. It occurs in polymerisations of bulk monomer and in nonpolar solvents, and at relatively high monomer concentrations in polar solvents. 

Any carbenium ions which are not paired have both their complexation sites occupied by the most polar or polarisable species available, which can be the solvent or the monomer, according to their relative polarities, polarisabilities, and concentrations for paired cations, the picture applies to their other, still vacant, site. Such a situation will generally prevail in nonpolar solvents because in these the concentration of paired cations is dominant. In a polar solvent, both sites at an unpaired cation can be occupied by solvent, or one by solvent and one by monomer, or both by monomer. In the radiation polymerisations, one sees clearly that as the monomer concentration is reduced from bulk monomer, the kinetics change and they eventually become first order in monomer, whatever the solvent the critical monomer concentration at which this happens depends on the polarity of the solvent [12]. 

Theoretical considerations indicate that kc would be very large, about 8 x 109 L mol-1 s 1, in low-viscosity media (such as bulk monomer) for the reaction between two radicals. The rate constants for reactions of small radicals (e.g., methyl, ethyl, propyl) are close to this value (being about 2 x 109 L mol s 1) [Ingold, 1973]. Experimentally determined kt values for radical polymerizations, however, are considerably lower, usually by two orders of magnitude or more (see Table 3-11). Thus diffusion is the rate-determining process for termination, kc 3> fct, and one obtains... 

Polymerization of the monomer in solution undoubtedly takes place but does not contribute significantly, since the monomer concentration is low and propagating radicals would precipitate out of aqueous solution at very small (oligomeric) size. The micelles act as a meeting place for the organic (oil-soluble) monomer and the water-soluble initiator. The micelles are favored as the reaction site because of their high monomer concentration (similar to bulk monomer concentration) compared to the monomer in solution. As polymerization proceeds, the micelles grow by the addition of monomer from the aqueous solution whose concentration is replenished by dissolution of monomer from the monomer droplets. A simplified schematic representation of an emulsion polymerization system is shown in Fig. 4-1. The system consists of three types of particles monomer droplets, inactive micelles in which... 

Let us first consider the irradiation of bulk monomer the ionization of monomer molecules by radiation results in the formation of cations and electrons. The former are formed through the removal of one electron from neutral molecules, so that they have an unpaired electron as well as positive excess charge and therefore are termed cation radicals. Most of the cation radicals and the electrons recombine immediately with each other, and only a fraction of them have a life-time long enough to enable themselves to act as primary active intermediates to bring about ionic reactions. However, both primary intermediates may react with monomer molecules, so that both cationic and anionic reactions of monomer may proceed and the whole reaction scheme is too complex to be studied distinctly. 

Polyacrylamides in which all bulk monomers bore a site of attachment have also been prepared [80]. /V-Acryloyl-/V-2-(4-acctoxyphcnyl)ethylamine (Figure 2.5) has been polymerized in the presence of l,4-bis(acryloyl)piperazine to yield a support with an initial loading of 5.0 mmol/g. The successful synthesis of a decapeptide on this support demonstrated that the use of highly loaded supports is feasible and offers a cost-efficient alternative to standard supports [80]. 

Lindfors, L., Forssen, S., Skantze, P, Skantze, U., Zackrisson, A., and Olsson, U. (2006a) Amorphous drug nanosuspensions. 2. Experimental determination of bulk monomer concentrbtiogajuir, 22 911-916. 

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