The Conversion of Monomer to Polymer

The adduct, R—CH2CH , formed from a vinyl monomer is here repre- 

The growth of polymer molecules by successive addition of monomers according to reaction (1) to the radicals Mr and to their successors may be represented by 

The same reaction rate constant kp is written for each propagation step under the assumption that the radical reactivity is independent of the chain length, in accordance with the conclusion reached in the preceding chapter. 

Bimolecular reaction between a pair of chain radicals accounts for annihilation of active centers. Two obvious processes by which this may occur are chain coupling (or combination) 

The presence of two hydroxyl groups per molecule in poly-(methyl methacrylate) and in polystyrene, each polymerized in aqueous media using the hydrogen peroxide-ferrous ion initiation system, has been established by chemical analysis and determination of the average molecular weight. Poly-(methyl methacrylate) polymerized by azo-bis-isobutyronitrile labeled with radioactive has been shown to 

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In the case of anionic polymerization (with 2-isoprOpenylthiazole) there is a chain-monomer equilibrium. Furthermore, lowering the temperature of polymerization increases the conversion of monomer to polymer (314). 

The instantaneous monomer concentration must be used. Except at the azeotrope, this changes as the conversion of monomers to polymer progresses. As in Sec. 7.2, we assume that either the initial conditions apply (little change has taken place) or that monomers are continuously being added (replacement of reacted monomer). 

Thus, the reaction is not specific for initiation of grafting. Another disadvantage is that Fe(II) ions formed—if not carefully removed—cause discoloration of the resulting product. The addition of Fe(II) sulphoxy-late is claimed to increase the rate of grafting, the yield of grafted polymer, and the conversion of monomer to polymer [60,61]. The mechanism of grafting can be represented as follows ... 

A number of authors have provided integrated forms of the Mayo equation, 2"w which have application when the conversion of monomer to polymer is non-zero. Integration of eq. 12 provides eq. 15 ... 

We shall use Rp to represent the rate of polymerization as well as the rate of propagation, therefore. According to Eq. (12), the rate of polymerization should vary as the square root of the initiator concentration. If/ is independent of the monomer concentration, which will almost certainly be true if / is near unity, the conversion of monomer to polymer will be of the first order in the monomer concentration. On the other hand, if / should be substantially less than unity, it may then depend on the concentration of monomer in the extreme case of a very low efficiency, / might be expected to vary directly as [M whereupon the chain radical concentration becomes proportional to Mand the polymerization should be three-halves order in monomer. 

The pattern presented by heats of polymerization of various monomers appears to be very well explained by two dominant factors elimination of resonance energy of conjugation, and steric interactions between substituents. The importance of the value of the heat of polymerization in determining the temperature above which reversal of the conversion of monomer to polymer occurs has been stressed by Dainton and Ivin. ... 

Initiation with only one portion of ceric amnoniiM nitrate (rather than with two portions added one hour apart) reduces both the conversion of monomer to polymer and the water absorbency of the fined product. Based on these results, starch in future reactions was gelatinized by heating for 60 min at 95 C, to more closely simulate conditions of continuous steam jet cooldng 02) r %diich would be used commercially. Also, polymerizations were initiated with two portions of ceric ammonium nitrate dissolved in IH HNQs, seponification mixtures were stirred continuously rather than intermitt Ttly (as a matter of convenience), and absorbent polymers were dried at 40 C. 

This arises, because the rubber is necessarily in the polymeric part of the reaction mixture such that removal of unreacted monomer after polymerization has reached the desired level, will result in a correspondingly higher proportion of rubber in what remains. The conversion of monomers to polymer can be run from 30 to 99%. For example, if monomer separation is carried out after only 50% conversion of the monomers, then the rubber content would... 

TABLE II. Effect of Styrene Concentration on the Conversion of Monomer to Polymer in Presence of Sulfuric Acid2. 

The radical concentration during the polymerization is not changed by reaction (6-49), but the initiator molecule involved has been wasted because its decomposition has not produced a net increase in the conversion of monomer to polymer. Induced decomposition reactions are negligible for azo initiators, but they can be very significant for some peroxides. Peroxydicarbonates, for example, have efficiencies which change greatly with reaction conditions. 

The conversions of monomer to polymer were excellent. It should be pointed out that while the reactions were typically run for an hour, it was clear that the monomer was very highly converted within the first couple of minutes (indeed, as mentioned above, the very first reaction resulted in high conversion of 2000 moles of norbornene within 3 si). 

An inhibitor is used to completely stop the conversion of monomer to polymer produced by accidental initiation during storage. To induce the inhibition, some stable radicals are mixed with the monomer. Such radicals are incapable for initiation the polymerization, but they are very effective in combining with any propagating radical. Diphenylpicryl-hydrazyl and tetramethylpiperidinyloxy (TEMPO) are two examples of radicals used to inhibit the radical polymerization. The chemical reactions of the inhibition produced by these compounds are shown in Scheme 4.8. 

As reactor temperature increases, the heat load from heating the feed becomes significant. At very high operation temperatures, the process can operate at net endothermic and require heat addition. The energy balance of the system is such that at low temperatures, the CSTR operates in exothermic mode, that is, net heat must be removed from the process jacket or sometimes internal coils. At elevated process conditions, the system becomes net endothermic as a result of the considerable heat required to raise the feed to reactor temperature. The total heat of polymerization is dependent on both the energetics of the system and the conversion of monomer to polymer. This in turn is defined by the kinetics of the system [50, 91, 92],... 

During interval III, C p monotonically diminishes according to the conversion of monomer to polymer. 

As expected, at a constant polymerization temperature and constant time, with increasing percentages of initiator, the conversion of monomer to polymer increases. In Table VII this effect of the concentration of benzoyl peroxide on the polymer formation is shown. The reaction times, given in this table vary slightly. The data are taken from a more extensive study [17]. 

The prepolymer has also been prepared in two stages at two different temperatures using two initiators which operate at two distinctly different temperatures like terr-butyl hydroperoxide and di-te/t-butyl peroxide [98]. In a strictly thermal process, diallyl o-phthalate has been polymerized at 200 -250 C. The conversion of monomer to polymer was followed by checking the change in refractive index with time. The process was short stopped before the gel point was reached by adding a solvent which separated unreacted monomer from the polymer [99]. 

Smith-Ewart Case 2 kinetics is based on the assumption that n = 0.5 which leads to volumetric particle growth independent of size or dv/dt = n., during Eiterval II. The volume growth of polymer in the particles may remain constant during Interval III if = 0.5, but the overall particle size will decrease because of the conversion of monomer to polymer which is more dense. 

Dead-End Polymerization. Dead-end polymerization refers to a polymerization process where the initiator concentration significantly decreases to a very low value during the polymerization. The measurement of the conversion of monomer to polymer, p, according to such an experiment, allows to determine the rate coefficient of initiator decomposition, kd, and the calculation of the efficiency factor f. 

The polymerization of isoprene with Ziegler-Natta catalysts is typically carried out in an aliphatic solvent such as n-butane or n-pentane at about 50 C. The conversion of monomer to polymer is restricted to 80—90% since chain scission appears to occur at higher conversions, resulting in material of low molecular wei t. The product is a slurry of polymer in the aliphatic solvent. This is agitated with aqueous methanol and then the mixture is allowed to separate into two layers. The lower layer, which contains methanol, water and catalyst residues is withdrawn. The remaining polymer slurry is washed with water and then stripped of solvent by treatment with steam to give an aqueous suspension of fine rubber crumbs. The polymer is collected, washed and dried. 

For PIPS, polymerization of the monomer or pre-polymer component in die initial mixture of liquid crystal, monomer, or pre-polymer, and photo-initiator induces fdiase separation. The conversion of monomer to polymer with thermal or photon initiated polymerization destabilizes the homogeneous mixture and induces phase separation. Photon-initiation affords the flexibility of multi-wavelen processing that has resulted in H-PDLCs and other spatially tailored LCPD morphologies (12). The morphology depends primarily on the choice of liquid crystal and monomer/pre-polymer, the weight fraction of liquid crystal in... 

We have utilized a variety of techniques for determination of the conversion of monomer to polymer, by measurement of the concentration of C=C bonds at different times during the polymerization of vinyl and allyl monomers. The NMR spectra of samples quenched and dissolved in deuterated solvent showed resonances due to the different H atoms of the monomer and polymer, and NMR was a very good method for determination of conversion. However,... 

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