Styrene polymerization tacticity

Zambelli et al. reported on the mechanism of styrene polymerization [36]. They showed that the main chain of the syndiotactic polymer has a statistically trans-trans conformation. It was established then the double-bond opening mechanism in the syndiospecific polymerization of styrene involves a cis opening. The details in the control of the monomer coordination for this polymerization mechanism were examined by Newman and Malanga using detailed, 3C NMR. It was shown through the analysis of tacticity error (rmrr) that the tacticity in the polymer is chain-end controlled and that the last monomer added directs the orientation and coordination of the incoming monomer unit prior to insertion [37]. 

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). 

Ti(0-n-Bu)4 systems, the tacticity of the obtained polystyrene strongly depends on the support material (i.e., the presence of Cl in the carrier). When MgCh-supported Ti(0-n-Bu)4 is activated with MAO, the soluble part of the catalyst in toluene can afford sPS, whereas the insoluble part of the catalyst gives iPS. This is also true for the Ti(0- -Bu)4/Mg(0H)Cl system when activated with MAO. For Ti(0-n-Bu)4/Mg(OH)2/MAO, which has no Cl in the carrier, the insoluble part gives syndiotactic polystyrene, and the soluble part is almost inactive for styrene polymerization. For the TiCls (AA)/MAO-system (AA = aluminum-activated), iPS (95% mmmm pentad, melting point 221 °C) and sPS are obtained from the insoluble and soluble part of the catalyst respectively. ... 

In the third part, tacticity considerations are expanded to other monoolefinic monomers and polymers, with chapters covering tactic a-olefin and styrene polymerizations. Ditactic polymerizations... 

The a-methyl-benzylcalcium compound 12 (Fig. 4) is less stable and showed a higher reactivity and faster initiation of styrene polymerization than the analogue a-MesSi-benzylcalcium complex 11 [33]. This was evidenced by less tailing in the low molecular weight regions of the SEC trace. The unsubstituted aminobenzyl complex 13, which exists as a methylene-bridged THF-free dimeric compound in the solid state and probably as well in benzene solution, is even more reactive towards styrene [32]. The tacticity of the PS obtained with the initiators 11,12, and 13 was, as expected, comparable. 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems... 

In polymers that exhibit tacticity, the extent of the stereoregularity determines the crystallinity and the physical properties of the polymers. The placement of the monomer units in the polymer is controlled first by the steric and electronic characteristics of the monomer. However, the presence or absence of tacticity, as well as the type of tacticity, is controlled by the catalyst employed in the polymerization reaction. Some common polymers, which can be prepared in specific configuration, include poly(olefins), poly(styrene), poly(methyl methacrylate), and poly(butadiene). 

Since the discovery of Fox et al. ( 2) that in the anionic polymerization of MMA the tacticity of the resulting polymer is fully controlled by the experimental conditions, many attempts have been made to elucidate structures of the active species responsible for the tacticities obtained under different conditions. However, this can only be achieved, if a complete analysis of the polymerization process is performed by investigating the kinetics of polymerization in different systems, as has been done for styrene. 

Experimental values are presented of the molar Kerr constants /x and dipole moments squared, lx, for the copolymers poly(styrene-co-p-bromostyrene), where x is the degree of polymerization. Some results are also presented for poly(styrene-co-p-chlorostyrene) and related polymers. The RIS model of Yoon etal. (Yoon, D. Y. Sundararajan, P. R. Flory, P. J. Macromolecules 1975, 8, 776) is used to calculate mK/x and /x values as a function of tacticity and composition. The statistical weight matrices are identical with those used by Saiz etal. (Saiz, E. Mark, J. E. Flory, P. J. Macromolecules 1977, 10, 967), with the following parameters h = 0.8 exp 397/RT), co = o = 1.3 exp - 1987/RT) and m,= 1.B exp -(2186/RT), where T = 298 K is the temperature. 

Polyethylene and polystyrene are two of the most commercially important and ubiquitous polymers, primarily because of their commercial value. Since the early days of polymer research there has been considerable interest to produce copolymers from ethylene (E) and styrene (S) because of both academic and business interests. Depending on the nature and type of polymerization chemistry, a variety of different molecular architectures can be produced. In addition to the different monomer distributions (random, alternating or blocky nature), there are possibilities for chain branching and tacticity in the chain microstructure. These molecular architectures have a profound influence on the melt and solid-state morphology and hence on the processability and material properties of the copolymers. 

Aliphatic hydrocarbons such as hexane or heptane were preferred as solvents for the stereospecific polymerization of styrene using the already mentioned initiators. Toluene can also be used as solvent, but in this case a lower polymerization temperature, about —40° C., is necessary to achieve good tacticity. Under similar conditions, considerably poorer crystalhzable polymers were produced using Alfin-type catalysts this fact was also confirmed by Williams (23) and Morton (16),... 

Diene polymerization may involve either or both of the double bonds. Geometric and structural isomers of butadiene, for example, are indicated by using appropriate prefixes — cis or irons, 1,2 or 1,4 — before poly, as in cw-l,2-poly(l,3-butadiene). Tacticity of the polymer may be indicated by using the prefix i (isotactic), s (syndiotactic), or a (atactic) before poly, such as 5-polystyrene. Copolymers are identified by separating the monomers involved within parentheses by either alt (alternating), b (block), g (graft), or co (random), as in poly(styrene-g-butadiene). 

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