Vinyl monomers polymerization kinetics polymeric radical

In this cormection, systematic stndies were made of the inhibiting effect of many stable mono and poly radicals on the kinetics and mechanism of vinyl monomer polymerization. The efCtiency of nitroxyls as free radical acceptors has promoted their usage to explore the mechanism of polymerization by means of the inhibition technique. Usually, nitroxyls have time to only react with a part of the radicals formed at... 

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate. By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially this includes the formation of polyesters and polyamides (nylons). Thus, there exists a large overlap between the terms stepwise kinetics and condensation polymers, and chainwise kinetics and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1. 

Eastmond, G. C., The Kinetics of Free Radical Polymerization of Vinyl Monomers in Homogeneous Solutions, Chap. 1 in Comprehensive Chemical Kinetics, Vol. 14A, C. H. Bamford and C. F. H. Tipper, eds., American Elsevier, New York, 1976a. 

Kinetics. Monomer can be converted into polymer by any chemical reaction which creates a new covalent bond. Most of this review will concern polymerization of vinyl monomers by free radical addition polymerization. However, some attention will be given to cationic polymerization of epoxy functional materials. No extensive review of polymerization processes and kinetics will be given here, but some of the fundamental notions will be described. For reviews, see (4a-d). 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

The subject of the kinetics of vinyl polymerization by radical mechanisms is treated exhaustively in a book by Bamford, et al. (4) and more briefly in many textbooks of polymer chemistry. The polymerization of vinyl monomers is a chain reaction in which the primary reactions are ... 

Sufficient experimental data from several laboratories now exist to describe the conditions under which the radiation-induced ionic propagation of many pure liquid vinyl monomers can be observed. The kinetic data and electrical conductivity measurements establish the ionic nature of the reaction scavenger studies appear to establish the preponderant role played by the carbonium ion in propagating the polymerization. On the basis of a single propagating species, it is possible to write a simple mechanism to describe the process. Limiting values of several of the kinetic rate constants can be estimated, notably the rate constant for reaction between a bare carbonium ion and a vinyl double bond. These rate constants are compared with similar constants arrived at in chemically initiated free radical, carbonium ion and carbanion polymerization. Several shortcomings of the present scheme are discussed. 

As mentioned earlier, polymerization techniques can also be used in the presence of nanotubes for preparation of polymer/CNT nanocomposite materials. In these, in-situ radical polymerization techniques of polymerization in the presence of CNT filler under or without applied ultrasound. Both new factors (presence of CNT and ultrasound) can affect reaction kinetics, stability of suspension or the size of prepared particles. For example, ultrasound waves can open C=C bond of monomer, which starts polymerization initiation. Thus vinyl monomers (styrene, methyl methacrylate or vinyl acetate) can be polymerized without addition of initiator, only by application of ultrasound. This is called sonochemical polymerization method (15,33,34). 

We pause here to note that the steady-state a.ssumption that is so helpful in simplifying the analysis of free-radical kinetics (Section 6.3.4) will not apply to many cationic polymerizations of vinyl monomers, because propagation through free carbenium ions is so much faster than any of the other reactions in the kinetic chain. 

In the chain growth free radical polymerization of a vinyl monomer (conventional polymerization), the growth reaction is the repeated reaction of a free radical with numbers of monomer molecules. According to the termination by recombination of growing chains, 2 free radicals and 1000 monomer molecules leads to a polymer with the degree of polymerization of 1000. In contrast to this situation, the growth and deposition mechanisms of plasma polymerization as well as of parylene polymerization could be represented by recombinations of 1000 free radicals (some of them are diradicals) to form the three-dimensional network deposit via 1000 kinetic... 

The absorption band (Amax 488 nm) in the transient spectrum corresponds to the pyrene radical ion (Py ) [155, 156], while the band at Amax 400 nm is assigned to the absorption of the 1-hydro-1-pyrenyl radical (Py ) [157, 158]. Steady-state photolysis of pyrene in the presence of TEA leads to its disappearance, and addition of vinyl monomers decreases the rate of pyrene photoreduction. The photobleaching process follows first-order kinetics. Encinas et al. [154] suggest that the photoinitiation of polymerization by pyrene-TEA is catalyzed by the pyrene radical ion. 

A limited number of attempts have been made to set up a general mechanistic scheme describing cationic systems in terms of fundamental reactions, in a similar manner to that used in free radical polymerizations, and to derive generally applicable kinetic equations [3—4]. Because of the individuality of each cationic system, however, this approach has met with little success, and there has been a greater tendency towards treating each polymerization in isolation for detailed kinetic analysis. It is possible, however, to postulate at least token schemes which can be used as a guide. After the pre-initiation equilibria, polymerization can be considered in terms of classical initiation, propagation, transfer and termination reactions, i.e. for vinyl monomers... 

Generalized methods of initiating the polymerization of these monomers have recently been reviewed in detail [9], and were also mentioned briefly earlier in this Chapter. As with vinyl monomers initiation can be efficient and rapid, with the production of a fixed number of active centres. Propagation appears to be much slower, however, and rates of polymerization are comparable to those in free radical addition polymerizations. Techniques such as dilatometry, spectrophotometry etc. are therefore convenient for kinetic investigation of this type of cationic reaction. 

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

To illustrate the calculation of molecular weight averages using moment equations, let us consider a free radical polymerization of vinyl monomers. Table 3 shows the kinetic equations based on the kinetic... 

Problem 6.21 A vinyl monomer of molecular weight 132 is polymerized by a free-radical initiator in the presence of dodecyl mercaptan (C12H25SH). The rate of polymerization is not depressed by the mercaptan. The purified polymer has a sulfur content of 0.02% (w/w) and its DP is 450. If 80% of the kinetic chains are terminated by coupling and 20% by disproportionation, what should be the extent of terminal unsaturation of the chains ... 

Studies of the homopolymerization kinetics that we carried out were curious and appeared to be greater than half-order in initiator and the polymerizations were sluggish under radical initiated conditions. The reason for this was cleared up by the excellent and precise homopolymerization kinetic studies of George and Hayes, who clearly demonstrated the rate was essentially first order in both initiator and monomer.39,40,41 What could cause such a rate law Could the iron center (formally Fe(II)) contribute Was a redox reaction involved that would be essentially impossible for organic monomers like styrene The observed rate law r = A M 11 [ 1111 stands in sharp contrast to the normal half-order in initiator concentration found in most vinyl addition polymerizations. Apparently, first-order chain termination had occurred rather than classic bimolecular termination. Indeed, the iron atom was playing a key role. 

In the Soviet study110, the following elementary stages were taken into account in the kinetic scheme of vinyl acetate polymerization chain transfer to the monomer, solvent, and polymer, and chain termination caused by the disproportionation of radicals. It was assumed that long-chain branches could be formed by chain transfer both to the acetate group hydrogen atoms and to the main chain hydrogen. 

As a rule, an increase in temperature in the course of polymerization is accompanied, by various kinetic effects. For example, in the radical polymerization of vinyl monomers changes can take place in the concentration of radicals and the time when the gel effect sets in. In addition a process of degradation can be superimposed on the polymerization process. The temperature and conversion non-uniformities occurring in the course of polymerization can change the thermal process itself, converting bulk polymerization into a reaction with propagating front, and vice versa. 

Case Study 2 Comparison of Mathematical Models FOR Free Radical Homopolymerization of Vinyl Monomers in scCOj In this case study, a comparison of performance of the different kinetic models proposed in the literature for dispersion polymerization of styrene and MMA in SCCO2 is presented. The models used by Quintero-Ortega et al. [43] (models 1 and 2) and those presented by the groups of Kiparissides [47] (model 3) and Morbidelli... 

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