Coordination polymerization polymer microstructural

In anionic and coordination polymerizations, reaction conditions can be chosen to yield polymers of specific microstructurc. However, in radical polymerization while some sensitivity to reaction conditions has been reported, the product is typically a mixture of microstructures in which 1,4-addition is favored. Substitution at the 2-position (e.g. isoprene or chloroprene - Section 4.3.2.2) favors 1,4-addition and is attributed to the influence of steric factors. The reaction temperature does not affect the ratio of 1,2 1,4-addition but does influence the configuration of the double bond formed in 1,4-addition. Lower reaction temperatures favor tram-I,4-addition (Sections 4.3.2.1 and 4.3.2.2). 

Stereospecific emulsion polymerization of butadiene has been achieved in the presence of soluble transition metal salts 350, 351). Polymer microstructure was controlled by varying the transition metal ion and its ligands. Although the initiation mechanism has not been determined, it is most likely to be of the coordinated radical type with steric control arising from the transition metal-diene complexes. 

Since butadiene can also undergo coordinated anionic polymerizations, some of the differences in polymer microstructure are attributable to changes in mechanism. Based on the catalysts reported to date, the isotactic and syndiotactic 1,2-polybutadienes appear to arise from coordinated anionic mechanisms. Qs and trans 1,4-polybutadienes can probably be made by all mechanisms, with cis arising from soluble catalysts which are capable of multi-coordination at one metal site. Trans structure is favored by cationic mechanism and by anionic mechanism involving coordination at two metal centers. 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

Thirdly, the established correlation between the microstructure of the polymer and the paramagnetism of the active center in anionic-coordination polymerization is both unexpected and not trivial. The predictions of results of this kind cannot be fortuitous. The correct predictions suggest that these postulates actually anticipate the real physical bases of the elementary act of catalytic polymerization. 

Alternatively, the complete population balance can be solved dynamically using efficient ODE solvers [70, 71], The versatile commercial software PREDICI can solve population balances that describe polymerizations with coordination catalysts and many other polymerization mechanisms [72]. In this approach, the complete microstructural distributions are modeled, leading to a detailed description of the polymer microstructure. 

Monte Carlo modeling has also been used extensively to describe LCB formation in coordination polymerization. Monte Carlo methods are very powerfiil because polymer chains are generated individually and the model keeps track of as many microstructural details as required. Monte Carlo methods will not be discussed here, but some references provided at the end of the chapter illustrate some interesting applications of this technique [51-53]. 

What is really key for the SSC family based on metallocenes and CGCs is the opportunity to have a catalyst precursor of defined chemical structure. Eor example, in the case of metallocenes, the n-bound cyclopentadienyl-type ligands remain coordinated to the transition metal atom during the course of polymerization. This allows for the following (1) the control of polymerization behavior and polymer microsttucture by modifying the structure of the ligands, and, as a consequence, (2) establishment of a correlation between ligand structure and polymer microstructure. 

In the present case, the electron hopping chemistry in the polymeric porphyrins is an especially rich topic because we can manipulate the axial coordination of the porphyrin, to learn how electron self exchange rates respond to axial coordination, and because we can compare the self exchange rates of the different redox couples of a given metallotetraphenylporphyrin polymer. To measure these chemical effects, and avoid potentially competing kinetic phenomena associated with mobilities of the electroneutrality-required counterions in the polymers, we chose a steady state measurement technique based on the sandwich electrode microstructure (19). 

It should be mentioned that donor substitution of the phenylene backbone of the salphen ligand was shown to have a decreasing effect on activity [103], which explains the overall lower productivity compared with halogen-substituted chromium salphens. However, experiments clearly proved an increased activity upon dimerization. Whereas the monomeric complex m = 4) converts about 30% of p-BL in 24 h, producing a molecular weight of 25,000 g/mol, the corresponding dimer yields up to 99% conversion with > 100,000 g/mol. Moreover, the smaller polydispersity (PD < 2) shows the better polymerization control, which is attributed to the decreased rate of polymer chain termination. This behavior is caused by the stabilization of the coordinated chain end by the neighboring metal center, as recently reported for dual-site copolymerizations of CO2 with epoxides [104-106]. The polymeric products feature an atactic microstructure since the... 

Slcreospecific solution polymerization has been emphasized since the discovery of the complex coordination catalyses that yield polymers or butadiene and isoprene having highly ordered microstructures. The catalysts used are usually mixtures of organometallic and transition metal compounds. An example of one of these polymers is cis- 1.4-polybutadiene. 

The mechanism is complicated by the possibility of anti-syn-isomerization and by n - a-rearrangements (it - r 3-allyl Act - r 1 -allyl). In the case of C2-unsubstituted dienes such as BD the syn-form is thermodynamically favored [646,647] whereas the anti-isomer is kinetically favored [648]. If monomer insertion is faster than the anti-syn-rearrangement the formation of the czs- 1,4-polymer is favored. A higher trans- 1,4-content is obtained if monomer insertion is slow compared to anti-syn-isomerization. Thus, the microstructure of the polymer (czs-1,4- and frazzs-1,4-structures) is a result of the ratio of the relative rates of monomer insertion and anti-syn-isomerization. As a consequence of these considerations an influence of monomer concentration on cis/trans-content of BR can be predicted as demonstrated by Sabirov et al. [649]. A reduction of monomer concentration results in a lower rate of monomer insertion and yields a higher trans-1,4-content. On the other hand the czs-1,4-content increases with increasing monomer concentration. These theoretical considerations were experimentally verified by Dolgoplosk et al. and Iovu et al. [133,650,651]. Furthermore, an increase of the polymerization temperature favors the formation of the kinetically controlled product and results in a higher cis- 1,4-content [486]. l,2-poly(butadiene) can be formed from the anti- as well as from the syn-isomer. In both cases 2,1-insertion occurs [486]. By the addition of electron donors the number of vacant coordination sites at the metal center is reduced. The reduction of coordination sites for BD results in the formation of the 1,2-polymer. In summary, the microstructure of poly(diene) depends on steric factors on the metal site, monomer concentration and temperature. 

Thermodynamics determines whether or not a monomer will polymerize, to what extent it polymerizes, and what conditions such as solvent, temperature, and concentrations are required. As discussed in Chapter 1, the thermodynamic polymerizability of a monomer is independent of the mechanism and is therefore identical for radical, anionic, cationic, and coordinative mechanisms if structurally identical polymers are obtained. Although this requires that both the end groups and the microstructure are the same, the influence of regioselectivity and stereoselectivity on the enthalpy and/or entropy of polymerization has not been confirmed experimentally yet. 

A variety of Lewis bases have been used to control microstructure in anionic polymerization, the main requirement being that the Lewis base is sufficiently stable in the presence of the propagating anion to allow living polymerization. The most commonly used modifiers are ethers and tertiary amines. Since amines are poisons for many hydrogenation catalysts, ethers are used more frequently in the production of hydrogenated polymers. A further distinction can be made between monobasic species such as dialkyl ethers and bidentate species that have the potential to coordinate with lithium, such as glyme ethers and TMEDA (N, V, N V -tetramethylethylenediamine). The former must... 

This monomer is usually obtained as a mixture of the cis and trans isomers both of which have been polymerized with coordination type catalysts. Polymerization of the cis form is considered to be preceded by isomerization, since those catalysts which do not isomerize the cis monomer (e.g. cobalt salt—organo aluminium halide) selectively polymerize the trans isomer. A kinetic study of the polymerization of cis 1,3-pentadiene using Ti(OBu-n)4/AlEt3 (Al/Ti = 1.3—6) as catalyst has been published [267]. This gives a polymer containing ca. 73% cis 1,4 15—16% trans 1,4 and 11—12% 3,4 microstructure. 

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