Free-radical polymers tacticity

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]. 

As noted above, RTCP is supposed to involve the RT process. If RT exists, the catalyst radical (A in Scheme lb) is mediated and the propagating species is a free radical (Polymer in Scheme lb). Thus, we attempted to detect A by ESR and confirm the free radical nature from the tacticity of the product polymer in the styrene polymerizations with TI (deactivator) and BHT (precursor). 

The existence of RT (Scheme 7.3a) was examined in three ways, (a) If RT exists, A is mediated. Thus, we attempted to detect A by electron spin resonance (ESR) in the St polymerizations. The Sn, Ge, P, and N-centered A radicals could not be detected due to their low equilibrium concentrations, while TI (O deactivator), BHT (O precursor), and CHD (C precursor) derived A radicals were in fact observed (Figure 7.11). The spectrum for TI was broad, probably due to the aggregation of TI in the St medium, and that for BHT clearly split into four peaks (1 3 3 1 ratio) by the methyl group at the para position and further split into three peaks (1 2 1 ratio) by the protons at the meta positions. The spectrum for CHD was somewhat noisy due to the lower concentration of A than those for TI and BHT. The spectrum split into three peaks (1 2 1 ratio) by the protons at the meta positions and further split into two peaks (1 1 ratio) by the proton at the central position. In these experiments, a large amount (100 00 mM) of the catalyst was used to facilitate the detection of A. Nevertheless, the concentration of A was low, in the order of 10 M, in the three cases, indicating that in typical polymerization conditions (with 3-10 mM of the catalyst), it is very low, on the order of 10 -10 M (nanomolar), and the activation process effectively occurs at such a low A concentration (as indicated above). Polymer was not apparently detected (Figure 7.11) due to its much lower concentration (10 -10 M estimated from the polymerization rate) than that of A (10 M). (b) If RT exists, the propagating species is a free radical (Polymer ). This was confirmed from the tacticity of the product polymer, namely, the tacticity was virtually the same as that without the catalyst (IMP) in all studied cases (St and MMA with various... 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

Isoprene can be polymerized using free radical initiators, but a random polymer is obtained. As with butadiene, polymerization of isoprene can produce a mixture of isomers. However, because the isoprene molecule is asymmetrical, the addition can occur in 1,2-, 1,4- and 3,4- positions. Six tactic forms are possible from both 1,2- and 3,4- addition and two geometrical isomers from 1,4- addition (cis and trans) ... 

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. 

Chiral induction was observed in the cyclopolymerization of optically active dimethacrylate monomer 42 [88], Free-radical polymerization of 42 proceeds via a cycliza-tion mechanism, and the resulting polymer can be converted to PMMA. The PMMA exhibits optical activity ([ct]405 -4.3°) and the tacticity of the polymer (mm/mr/rr =12/49 / 39) is different from that of free-radical polymerization products of MMA. Free-radical polymerization of vinyl ethers with a chiral binaphthyl structure also involved chiral induction [91,92]. Optically active PMMA was also synthesized through the polymerization of methacrylic acid complexed with chitosan and conversion of the resulting polymer into methyl ester [93,94]. 

Today, the majority of all polymeric materials is produced using the free-radical polymerization technique [11-17]. Unfortunately, however, in conventional free-radical copolymerization, control of the incorporation of monomer species into a copolymer chain is practically impossible. Furthermore, in this process, the propagating macroradicals usually attach monomeric units in a random way, governed by the relative reactivities of polymerizing comonomers. This lack of control confines the versatility of the free-radical process, because the microscopic polymer properties, such as chemical composition distribution and tacticity are key parameters that determine the macroscopic behavior of the resultant product. 

For free-radical polymerization, classical results have been obtained concerning the tacticity of hydroxytelechelic poly(methyl methacrylate)109) and copolymers, 46) initiated by H202/UV. Most of the units are in a syndiotactic (64 %) or heterotactic (30 %) configuration. For poly(vinyl acetate) obtained in the presence of H202 at 120 °C 98), the polymer contains less syndiotactic (22%) and somewhat more heterotactic (38%) units with 80% of head-to-tail linkage mode. For the copolymerization of alkyl methacrylate by the H202/UV system113) quite different results, explained by the nature of the medium, especially by the solubility effect (see Table 1.1), have been obtained. 

Geomelrical isomerism of polymers made from conjugated diolefins can be regulated in some anionic polymerizations. The tacticity of vinyl polymers is, however, not always controlled in anionic reactions. The products of anionic vinyl polymerizations are usually atactic, as in free-radical syntheses. 

The tacticity (pentad distribution) of the product polymer was analyzed by NMR (data not shown). The tacticity was virtually the same with and without the catalyst (T1 and 2,4,6-Me), meaning that the propagating species is a free radical. These results strongly indicate the existence of RT. 

Free-radical polymerizations were conducted under a wide range of conditions that included photochemical, thermochemical, and low-temperature alkylborane-oxygen initiation methods. Both bulk and solution methods were used with somewhat unexpected results. Temperature appeared to have relatively little effect on the tactic order (percent syndiotacticity) of VTFA polymers. Polymerization temperatures ranged from -80 to 150 C and gave syndiotacticities between 50 and 55% by triad analysis. Polar solvents affected tacticity more. For example, polymerization in 1,2-dichloro-ethane resulted in a syndiotacticity of 43%. Presumably, such effects were related to a disruption of the association of VTFA monomer with the growing chain end. 

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