Radical chain polymerization molecular weight

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

However, upon terminating chains with thiols, sulphur centered low-molecular weight radicals are formed that are able to start a polymerization of the remaining monomer B. Therefore, formation of homopolymer consisting of B is inevitable if thiols are used. A suitable alternative to the classical transfer additives are degra-dative chain transfer agents, such as allylmalonic acid... 

The molecular weight and chain-end structure of polymers can be modified using the chain transfer reaction [65-68]. When an appropriate chain transfer agent, X-Y, is used in radical polymerization, two types of oligomers or telomers having different end groups, 4 and 5, are formed depending on the value of the chain transfer constant, Ctr> of X-Y used. 

Polymer Synthesis. General Procedure—All polymers were prepared by free-radical-initiated solution polymerization. Typical quantities utilized were as follows 5.0 g total monomer and 0.02 g AIBN or Vazo 33 in 30-60 mL solvent. More dilute solutions were employed in some cases to eliminate gel formation. In addition, a chain transfer agent, dodecanethiol, was used to control molecular weight in some polymerizations. 

The molecular weight distribution in radical chain polymerizations is more complex than those in step polymerization. Radical chain polymerization involves several possible modes... 

Emulsion polymerization refers to a unique process employed for some radical chain polymerizations. It involves the polymerization of monomers in the form of emulsions (i.e., colloidal dispersions). The process bears a superficial resemblance to suspension polymerization (Sec. 3-13c) but is quite different in mechanism and reaction characteristics. Emulsion polymerization differs from suspension polymerization in the type and smaller size of the particles in which polymerization occurs, in the kind of initiator employed, and in the dependence of polymer molecular weight on reaction parameters. 

The theoretical molecular weight distributions for cationic chain polymerizations are the same as those described in Sec. 3-11 for radical chain polymerizations terminating by reactions in which each propagating chain is converted to one dead polymer molecule, that is, not including the formation of a dead polymer molecule by bimolecular coupling of two propagating chains. Equations 2-86 through 2-89, 2-27, 2-96, and 2-97 withp defined by Eq. 3-185... 

Solution Polymerization. By adding a solvent to the monomer-polymer mixtnre, heat removal can be improved dramatically over bnlk reactions. The solvent mnst be removed after the polymerization is completed, however, which leads to a primary disadvantage of solution polymerization. Another problem associated with radical chain polymerizations carried out in solution is associated with chain transfer to the solvent. As we saw in Section 3.3.1.2, chain transfer can significantly affect the molecular weight of the final polymer. This is particnlarly trne in solntion polymerization, where there are many solvent molecules present. In fact, chain transfer to solvent often dominates over chain transfer to other types of molecnles, so that Eq. (3.79) reduces to... 

The initiation efficiency depends on the relationship between the rate of macroradical formation and that of initiation. In Table 1 are compiled results on certain redox systems employed for the initiation of cellulose grafting either by direct oxidation of cellulose (or its derivative) or chain transfer from active low molecular weight radicals. Table 1 indicates that in systems where the matrix acts as a reducing agent (1st group), the initiation efficiency does not exceed 15%, i.e. only a minor portion of macroradicals formed at the first stage of oxidation initiates graft polymerization while the rest is oxidized to stable... 

The propagation reaction in free-radical polymerizations is rapid.1 One important feature of the polymerization is that high molecular weight polymer is formed even at very low levels of monomer conversion. Thus, each propagating radical or its progeny lives for well under a minute. To control molecular weights in these polymerizations, the use of chain... 

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 ... 

The theoretical molecular weight distributions for cationic chain polymerizations (see Problem 8.30) are the same as those described in Chapter 6 for radical chain polymerizations terminating by disproportionation, i.e., where each propagating chain yields one dead polymer molecule. The poly-dispersity index (PDI = DP /DPn) has a limit of 2. Many cationic polymerizations proceed with rapid initiation, which narrows the molecular weight distribution (MDI). In the extreme case where termination and transfer reactions are very slow or nonexistent, this would yield a very narrow MDI with PDI close to one (p. 681). 

Figure 1.2 Variation of molecular weight with conversion in (a) step polymerization, (b) free-radical chain polymerization, and (c) ionic chain polymerization.. (Adapted from Odian, 1991.)...

It is often observed that the measured molecular, weight of a polymer product made by free-radical chain polymerization is lower than the molecular weights predicted from Eq. (6.102) for termination by either coupling [Eq. (6.103)] or disproportionation [Eq. (6.104)]. Such an effect, when the mode of termination is known to be disproportionation, can be due to a growing polymer chain terminating prematurely by transfer of its radical center to other species, present in the reaction mixture. These are referred to as chain transfer reactions and may be generally written as... 

The emulsion polymerization in the third step is carried out in the presence of a water soluble monomer, such as acrylic acid. The radicals formed by the photolysis of HMEM (Fig. 6.18b) on the surface start radical chain polymerization by a grafting-from technique (see Section 6.13.3) thus generating chains of poly(acrylic acid). The polymer chains remain bound to the surface by an ester bond which can be cleaved by hydrolysis to obtain the polymer for analysis. Thus the molecular weight of the bound polymer chains can be determined which gives their contour length Lc. The thiclcness L of the brush (Fig. 6.17) attached to the surface of the particles can be deduced from the hydrodynamic radius as measured by dynamic light scattering. 

The theoretical molecular weight distributions for cationic chain polymerizations (see Problem 8.25) are the same as those described in Chapter 6 for radical chain polymerizations terminating by disproportionation, i.e., where each propagating... 

Oxidation, which can occur by autoxidation or photo-oxidation (initiation by light), is a complex chain reaction. The initial step of oxidation is the formation of hydroperoxides. This initial step is followed by secondary reactions in which species, such as aldehydes, acids, alcohols and hydrocarbons, are formed. Since it is a mechanism based on the formation of radicals, dimerization of some intermediates can occur, leading to formation of higher molecular weight products. Oxidative polymerization can also occur. In addition to these mechanisms, fuel deterioration can also occur hydrolytically through the presence of water. A detailed book on oxidation has been published by Frankel (2005). 

Comparison of the performance of 6, 21, and 22 which yield mono, di, and tetra carbon radicals respectively, clearly shows a relationship between functionality and molecular weight during styrene polymerization (Fig. 20). Because of the predominance of termination by chain coupling, polymerizations using the tetrafunctional initiator 22 gave crosslinked PS when taken to high conversion. 

Ttp 4 Chain microstructure and propagation reactions. Propagation reactions are mainly responsible for the development of polymer chain microstructure (and control chain composition and sequence length distribution in copolymerizations). In free radical polymerization, the stereoregularity of a high molecular weight homopolymer chain depends on polymerization temperature almost exclusively. It is usually independent of initiator type and monomer concentration. Calculations on stereoregularity... 

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