Radical polymerization syndiotactic

Tacticity % isotactic 25-29, heterotactic 47-51, and syndiotacic 22-27 in radical polymerization syndiotactic (mainiy orthorhombic), isotactic (tetragonai) ... 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

Polypropylene made by free-radical polymerization is generally atactic , that is to say, there is no pattern to the stereochemistry. On the other hand, both isotactic polypropylene (in which all the stereocenters are the same) and syndiotactic polypropylene (in which the stereocenters alternate) may be made via the Ziegler-Natta process (see Chapter 18, Problem 4). Experimentally, both isotactic and syndiotactic polypropylene generally have higher melting points than atactic polypropylene. 

Most polymers formed by radical polymerization have an excess of syndiotactic over isotactic dyads [i.e, < 0.5]. / (m) typically lies in the range... 

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

Although PS is largely commercially produced using free radical polymerization, it can be produced by all four major techniques—anionic, cationic, free radical, and coordination-type systems. All of the tactic forms can be formed employing these systems. The most important of the tactic forms is syndiotactic polystyrene (sPS). Metallocene-produced sPS is a semicrystalline material with a of 270°C. It was initially produced by Dow in 1997 under the trade name Questra. It has good chemical and solvent resistance in contrast to regular PS that has generally poor chemical and solvent resistance because of the presence of voids that are exploited by the solvents and chemicals. 

Fig. 8-9 Dependence of syndiotacticity on temperature for the radical polymerization of vinyl chloride. After Talamini and Vidotto [1967] (by permission of Huthing and Wepf Verlag, Basel and Wiley-VCH, Weinheim).

Since radical polymerizations are generally carried out at moderately high temperatures, most of the resulting polymers are highly atactic. This does not mean that there is a complete absence of syndiotacticity. There is a considerable difference in the extent of syndiotacticity from one polymer to another. Thus, methyl methacrylate has a much greater tendency toward syndiotactic placement than vinyl chloride. Whereas the poly(vinyl chloride) produced at the usual commerical polymerization temperature ( 60°C) is essential completely atactic, that is, (r) (m) 0.5, this is not the case for poly(methyl methacrylate). The polymerization of MMA, usually carried out at temperatures up to 100°C, yields polymers with appreciable syndiotacticity—(r) is 0.73 at 100°C. The difference is a consequence of the fact that MMA is a 1,1-disubstituted ethylene, leading to greater repulsions between substituents in adjacent monomer units. 

Because of acid-catalyzed hydrolysis of N-vinylpyrrolidone in water, polymerization was carried out in organic solvent - DMF. Three types of samples of poly(methacrylic acid) were used syndiotactic - obtained by radiation polymerization, atactic - obtained by radical polymerization, and isotactic - obtained by hydrolysis of isotactic poly(methyl methacrylate). It was found that in all cases the rate enhancement appeared in comparison with the blank polymerization (without template). The rate enhancement became more pronounced with increasing chain length and syndiotacticity of the template. According to the authors, the rate enhancement is connected with the stronger complex formation between poly(vinyl pyrrolidone) and syndiotactic poly(methacrylic acid) then with isotactic template. This conclusion was supported by turbimetric titration in DMF/DMSO system and by model considerations. It is worth noting, however, that... 

Many radical polymerization systems were examined hy Challa at a/. A number of papers were devoted to polymerization of methyl methacrylate, MM, in the presence of poly(methyl methacrylate). The role of tacticity of template was stressed. Isotactic PMM forms stereocomplex with syndiotactic PMM. Polymerizations of MM in the presence of isotactic PMM were carried out and the product (template + daughter polymer) was analyzed by NMR. When MM is polymerized in the polar solvent in the presence... 

Polymerization. Poly (methyl methacrylate) was obtained commercially. The polymers of other methacrylates and their copolymers were prepared in toluene with 2,2 -azobisisobutyronitrile (AIBN) at 60 °C. All the polymers prepared free radically were syndiotactic or atactic. Isotactic poly(a,a-dimethylbenzyl methacrylate) was obtained using C6H5MgBr as the initiator in toluene at 0°C. Poly(methacrylic acid) was prepared in water using potassium persulfate at as the initiator 60 °C. The molecular weights, glass transition temperatures and tacticities of the polymethacrylates are summarized in Table I. 

The formation of syndiotactic polymethylmethacrylate is the result of the prefered chain configuration in the free anionic polymerization, similar to the formation of syndiotactic structure by the free radical polymerization of methylmethacrylate. Bawn, James and North (77)... 

Although they do not crystallize, polyvinyl acetates prepared at low temperatures apparently arc more syndiotactic since they yield more than usually crystalline polyvinyl alcohols by saponification. Monomers of high polarity such as vinyl trilluoraeetate by radical polymerization can form... 

Stereocontrol of free radical polymerization is influenced by monomer constitution, solventy and temperature. Most polymerizations seem to follow at least a Markov first-order one-way mechanism. Ratios of the four possible rate constants ki/iy ki/8, k8/i, and k8/8 can be calculated from the experimentally accessible concentrations of configurational triads and diads. With increasing temperature, more heterotactic triads are formed at a syndiotactic radical whereas the monomer addition at an isotactic radical favors isotactic and not heterotactic triads. Compensation effects exist for the differences of activation enthalpies and activation entropies for each of the six possible combinations of modes of addition. The compensation temperature is independent of the mode of addition whereas the compensation enthalpies are not. 

TP he free radical polymerization of vinyl and acryl monomers normally does not lead to a true atactic polymer (I). A true atactic polymer is defined in this context as a polymer consisting of 50 isotactic and syndiotactic diads each, 25 iso- and syndiotactic triads each, and 50 heteroactic triads, etc. Furthermore all diads, triads, tetrads, etc., must be distributed at random. 

Most free radical polymerized polymers exhibit a preponderance of syndiotactic diads. The syndiotacticity normally increases with decreasing temperature. This phenomenon has lead some authors to conclude that the stereocontrol is governed by the bulkiness of the substituent. Implicit in this argument are the assumptions that only repulsive forces... 

Intuitively, the idea of an influence of the size of the substituent on the stereocontrol in free radical polymerization is very appealing. However, it is not supported by quantitative data. Table I shows the probabilities p of the formation of isotactic and syndiotactic diads at existing iso- and syndiotactic diads for the free radical polymerization of the methacryl type monomers CH2=CH(CH3)COOR in toluene at —78°C. These probabilities have been calculated from published mole fractions of iso- and syndiotactic diads (Xi, X8) and iso-, syndio-, and heterotactic triads (Xu, X88 and Xu) via... 

Table I shows some peculiarities. In contrast to what has been claimed in literature, the probabilities of forming an isotactic diad at an existing iso tactic diad (pj/j) do not always equal the probabilities of forming an isotactic diad at an existing syndiotactic diad (p8/i). The free radical polymerization of the six methacrylate monomers mentioned is thus not always Bemoullian, at least not at the temperature of — 78°C and in toluene as a solvent.

A similar result has been recently found for the free radical polymerization of methyl methacrylate in 14 solvents (32). All differences (Aff. — AHf/8) were found to be positive, but only three of the 14 differences (AH /g — AH. ). Again, isotactic triad formation is favored over heterotactic triad formation and heterotactic triad formation over syndiotactic with increasing temperature as long as the individual modes of addition are considered and not the net result. Except for meth-acrylic acid in alcohols (cf. Lando et al. (28)) no model is known which shows why a certain solvent acts differently from another one with respect to stereocontrol in free radical polymerization. 

A more detailed study has been made of the effect of temperature on the free radical polymerization of methyl methacrylate (33). Polymers were prepared at —78° and at 0° using Co80 gamma radiation and at 50° and 100° using benzoyl peroxide initiator. In Table 6 are summarized the structures of these polymers. A trend toward increasing syndiotactic character with decreasing temperature is clearly evident. 

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