Radical polymerization stereochemistry

The radical polymerization stereochemistry of TrMA is strongly affected by monomer concentration, reaction temperature, and solvent. A lower concentration and a higher temperature lead to a higher isotacticity of the products, and... 

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. 

The effects of increasing the concentration of initiator (i.e., increased conversion, decreased M , and broader PDi) and of reducing the reaction temperature (i.e., decreased conversion, increased M , and narrower PDi) for the polymerizations in ambient-temperature ionic liquids are the same as observed in conventional solvents. May et al. have reported similar results and in addition used NMR to investigate the stereochemistry of the PMMA produced in [BMIM][PFgj. They found that the stereochemistry was almost identical to that for PMMA produced by free radical polymerization in conventional solvents [43]. The homopolymerization and copolymerization of several other monomers were also reported. Similarly to the findings of Noda and Watanabe, the polymer was in many cases not soluble in the ionic liquid and thus phase-separated [43, 44]. 

The configuration of a center in radical polymerization is established in the transition state for addition of the next monomer unit when it is converted to a tetrahedral sp1 center. If the stereochemistry of this center is established at random (Scheme 4.1 km = k,) then a pure atactic chain is formed and the probability of finding a meso dyad, P(m), is 0.5. 

Combining control over architecture with control over the stereochemistry of the propagation process remains a holy grail in the field of radical polymerization. Approaches to this end based on conventional polymerization were described in Chapter 8. The development of living polymerization processes has yet to substantially advance this cause. 

Having established that a particular polymerization follows Bemoullian or first-order Markov or catalyst site control behavior tells us about the mechanism by which polymer stereochemistry is determined. The Bemoullian model describes those polymerizations in which the chain end determines stereochemistry, due to interactions between either the last two units in the chain or the last unit in the chain and the entering monomer. This corresponds to the generally accepted mechanism for polymerizations proceeding in a nonco-ordinated manner to give mostly atactic polymer—ionic polymerizations in polar solvents and free-radical polymerizations. Highly isoselective and syndioselective polymerizations follow the catalyst site control model as expected. Some syndioselective polymerizations follow Markov behavior, which is indicative of a more complex form of chain end control. 

Block Copolymerization Cationic Polymerization Polymer Stereochemistry Tacticity Coordination Polymerization Living Radical Polymerizations... 

Methyl methacrylate, a, can be readily converted to the polymer shown, but the stereochemistry of the product depends on the reagents used radical polymerization gives stereoisomer b, while anionic polymerization gives stereoisomer c. Deduce the stereochemistry of each isomer from the H NMR properties of their CDCI3 solutions. 

Although radical polymerization can t control stereochemistry, Ziegler-Natta catalysts can yield polymers of desired stereochemical orientation. 

The copolymerization of MMA with nBA was also studied using different catalytic systems. The calculated reactivity ratios were close to those for a conventional radical polymerization and similar for different Cu-based catalytic systems (bpy, PMDETA, and Me6TREN) [92]. The distribution of triads and the polymer stereochemistry was as in any other free radical system [93]. 

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