Anion free radical polymers

Meyerhoff and Cantow (118) compared the relationships between [ /] and MW for polystyrenes prepared in various ways they found that for given Mw isotactic polystyrenes produced with Ziegler catalysts had the highest [ij], followed by low-conversion free-radical polymers both high-conversion (80%) and anionic (Szwarc) polymers had lower [ij]. These differences were all attributed to differences in LCB, though in principle differences in tacticity such as those between Ziegler and free-radical or anionic polymers could produce differences in the coil size in solution and hence in [iy]. 

A macromonomer is a macromolecule with a reactive end group that can be homopolymerized or copolymerized with a small monomer by cationic, anionic, free-radical, or coordination polymerization (macromonomers for step-growth polymerization will not be considered here). The resulting species may be a star-like polymer (homopolymerization of the macromonomer), a comblike polymer (copolymerization with the same monomer), or a graft polymer (copolymerization with a different monomer) in which the branches are the macromonomer chains. 

T his paper presents a polymerization reaction, as yet unreported in the literature, wherein block polymerization of a free radical type can be caused to take place onto an actively growing chain which had proceeded by an anionic mechanism. Specifically, a Ziegler type of polymerization, such as that of propylene or ethylene, can be interrupted by adding vinylic monomers and an organic peroxide, and a vinyl polymer grown on the end of the polyolefin. For simplicity we will refer to these types as anionic free radical (AFR) polymerizations. 

Effect of Substrate. Again, polyethylene and ethylene-propylene copolymers are better substrates for block formation than polypropylene (Table XI). Polyethylene is better than polypropylene, and a polyethylene-polypropylene-polyethylene type of block polymer is better than polyethylene. This agrees with what has been found for AFR polymers containing methylvinylpyridine and acrylonitrile. It also supports our belief that AFR polymers are formed by the growing of a free radical polymer onto active ends of anionic polymer chains. If it were a random grafting reaction, it would be hard to explain why a propylene polymer with a more vulnerable tertiary hydrogen should give a lower... 

Nitroxide-Mediated Controlled Radical Polymerization (NMCRP) was first discovered by Solomon et al., who patented their discovery in 1985 [205]. This opened up new pathways in the field of free-radical polymerization. Polymer architectures, which were the domain of the anionic polymer chemist, became accessible to the free-radical polymer chemist. However, it was not until the work of Georges et al. [206] was published in 1993, that the world of polymer chemistry became aware of the possibihties of this new class of free-radical polymerization. This was the beginning of what is today one of the leading topics in free-radical polymer chemistry Controlled or Living Free Radical Polymerization. This initiated the search for new Controlled or Living Free Radical Polymerization techniques, and soon afterwards other methods (which will be discussed later) were developed. 

Fig. 29. Comparison of TVA thermograms for high and low molecular weight polymethylmethacrylate samples prepared by an anionic mechanism with the thermogram for a low molecular weight free radical sample [87], Dotted line, polymer D,M = 1,500,000 full line, polymer E, M = 60,000 dashed line, free radical polymer C, M = 20,000.

Stereoregular polymers of butyl and propyl acrylates have been made by anionic methods. For example, a crystalline isotactic poly(isopropyl acrylate) with a T of 162 C was prepared using phenyl magnesium bromide as an initiator whereas the free radical polymer was amorphous and rubbery at room temperature with a T of about -4 C. Isotactic polymer was found to be capable of... 

The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical or an anion or cation reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation, and in the other from the termination-can be neglected. Accordingly, the overall reaction can be written... 

The kind of reaction which produces a dead polymer from a growing chain depends on the nature of the reactive intermediate. These intermediates may be free radicals, anions, or cations. We shall devote most of this chapter to a discussion of the free-radical mechanism, since it readily lends itself to a very general treatment. The discussion of ionic intermediates is not as easily generalized. 

The active centers that characterize addition polymerization are of two types free radicals and ions. Throughout most of this chapter we shall focus attention on the free-radical species, since these lend themselves most readily to generalization. Ionic polymerizations not only proceed through different kinds of intermediates but, as a consequence, yield quite different polymers. Depending on the charge of the intermediate, ionic polymerizations are classified as anionic or cationic. These two types of polymerization are discussed in Secs. 6.10 and 6.11, respectively. 

Figure 6.11 Comparison of the number distribution of n-mers for polymers prepared from anionic and free-radical active centers, both with f = 50.

Polymerization. In the absence of inhibitors, acrolein polymerizes readily in the presence of anionic, cationic, or free-radical agents. The resulting polymer is an insoluble, highly cross-linked soHd with no known commercial use. 

The vast majority of commercial apphcations of methacryhc acid and its esters stem from their facile free-radical polymerizabiUty (see Initiators, FREE-RADICAl). Solution, suspension, emulsion, and bulk polymerizations have been used to advantage. Although of much less commercial importance, anionic polymerizations of methacrylates have also been extensively studied. Strictiy anhydrous reaction conditions at low temperatures are required to yield high molecular weight polymers in anionic polymerization. Side reactions of the propagating anion at the ester carbonyl are difficult to avoid and lead to polymer branching and inactivation (38—44). 

Anionic polymerization offers fast polymerization rates on account of the long life-time of polystyryl carbanions. Early studies have focused on this attribute, most of which were conducted at short reactor residence times (< 1 h), at relatively low temperatures (10—50°C), and in low chain-transfer solvents (typically benzene) to ensure that premature termination did not take place. Also, relatively low degrees of polymerization (DP) were typically studied. Continuous commercial free-radical solution polymerization processes to make PS, on the other hand, operate at relatively high temperatures (>100° C), at long residence times (>1.5 h), utilize a chain-transfer solvent (ethylbenzene), and produce polymer in the range of 1000—1500 DP. 

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

VEs such as MVE polymerize slowly in the presence of free-radical initiators to form low mol wt products of no commercial importance (9). Examples of anionic polymerization are unknown, whereas cationic initiation promotes rapid polymerization to high mol wt polymers in excellent yield and has been extensively studied (10). 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

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