Homogeneous polymerization propagation

When initiator is first added the reaction medium remains clear while particles 10 to 20 nm in diameter are formed. As the reaction proceeds the particle size increases, giving the reaction medium a white milky appearance. When a thermal initiator, such as AIBN or benzoyl peroxide, is used the reaction is autocatalytic. This contrasts sharply with normal homogeneous polymerizations in which the rate of polymerization decreases monotonicaHy with time. Studies show that three propagation reactions occur simultaneously to account for the anomalous auto acceleration (17). These are chain growth in the continuous monomer phase chain growth of radicals that have precipitated from solution onto the particle surface and chain growth of radicals within the polymer particles (13,18). 

The heat of an emulsion polymerization is the same as that for the corresponding bulk or solution polymerization, since AH is essentially the enthalpy change of the propagation step. Thus, the heats of emulsion polymerization for acrylic acid, methyl acrylate, and methyl methacrylate are —67, —77, and —58 kJ mol-1, respectively [McCurdy and Laidler, 1964], in excellent agreement with the AH values for the corresponding homogeneous polymerizations (Table 3-14). 

In a few instances, poly(methylmethacrylate) has been prepared exceeding the syndiotactic content attainable through a free ion pair or free radical mechanism at the same temperature [20]. A possible mechanism for homogeneous syndiotactic propagation has been proposed. However, none of these highly syndiotactic systems has been reproducible [10], and it appears to be no real need for such a mechanism. Coordination-directed stereospecific polymerization of methyl methacrylate seems to be limited to isotactic propagation. 

The polymers possess one sulfonyl group per chain, which can be utilized as end-functional polymers as discussed later (section III.B.l). Narrower MWDs (MJMn = 1.2—1.4) were obtained in MMA polymerization with 1-32 as well as 1-33 and 1-34 in conjunction with CuCl/L-1 in />xylene at 90 °C.175 In a homogeneous system with CuCl/L-4,1-32 can afford narrow MWDs MJMn = 1.1—1.3) for styrene, MMA, and nBA.176 The fast addition of the sulfonyl radical to these monomers was evidenced by H NMR analysis of the reactions, where the apparent rate constants of initiation are 4 (for styrene and MMA), 3 (nBA), and 2 (MA) orders of magnitude higher than those of propagation. A similar controlled and homogeneous polymerization of MMA with 1-32 (X = CH3)/ CuBr/L-4 was reported in diphenyl ether at 90 °C.178 A better control of molecular weights and MWDs with 1-32 (X = CH3)/CuBr/L-9 in diphenyl ether was also... 

Example 4-8 An ideal continuous stirred-tank reactor is used for the homogeneous polymerization of monomer M. The volumetric flow rate is O, the volume of the reactor is V, and the density of the reaction solution is invariant with composition. The concentration of monomer in the feed is [M]o. The polymer product is produced by an initiation step and a consecutive series of propagation reactions. The reaction mechanism and rate equations may be described as follows, where is the activated monomer and P2, . . , P are polymer molecules containing n monomer units ... 

The propagating radical of dialkyl itaconate in the homogeneous polymerization was foimd by Sato et al. to be stable enough to be observed by ESR even above 60 °C. Fig. 62 a displays the 5-line spectra obtained when dimethyl 2,2 -azobis-isobutyrate, benzoylperoxide, and di-tert-butylperoxide were used as initiators. On the other hand, the polymerization of DBI initiated with azonitriles such as 2,2 -azobisisobutyronitrile, 2,2 -azobis(2,4-dimethylvaleronitrile), and cyclohexanecarbo-... 

It follows from this equation that the [M]e observed in the homogeneous polymerization may be greatly reduced by performing polymerization under conditions when the propagation step proceeds at the crystalline sites of a solid polymer. 

In eqn [2], kp is the rate constant of propagation, n the average number of radicals per particle, Na the Avogadro s number, Ri the initiation rate in the aqueous phase, and MMm the monomer molar mass. Propagation thus obeys a zeroth order with respect to monomer concentration and not a first order as observed in homogeneous polymerizations and suspension polymerization. 

This shows the reaction to be pseudo-first-order since for each particular reaction /Cp[I] is a constant. Equation (2.67) is usually satisfactory for homogeneous polymerizations performed in polar solvents. However, for reactions carried out in non-polar solvents the effects of slow initiation and of aggregation of both the initiator species and the carbanionic active centres must be taken into account when evaluating the concentration of propagating carbanionic active centres. 

The polymerization of olefins and di-olefins is one of the most important targets in polymer science. This review article describes recent progress in this field and deals with organo-transition metal complexes as polymerization catalysts. Recent developments in organometallic chemistry have prompted us to find a precise description of the mechanism of propagation, chain transfer, and termination steps in the homogeneously metal-assisted polymerization of olefins and diolefins. Thus, this development provides an idea for designing any catalyst systems that are of interest in industry. 

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