Controlling Propagation

Given the important role that stcric and polar factors play in dctcmiining the rate and regiospecificity of radical additions (see 2.3), it might be anticipated tliat reagents which coordinate with the propagating radical and/or the monomer and thereby modify the effective size, polarity, or inherent stability of that species, could alter the outcome of propagation. 

The aspects of polymer structure to be controlled have already been discussed in Chapter 4. For the case of a homopolynier, these are  

For the case of copolymers, it is also possible to control the arrangement of monomer units in the chain. 

The reagents used for controlling polymer structure may be low molecular weight (e.g. the solvent - Sections 8.3.1-8.3.3, Lewis acids - Section 8.3.4) or 

For greatest effect propagation involving the complcxed or constrained species should dominate over normal propagation. For this to occur one of the following should apply  

---

An attractive feature of using the solvent as an agent to control propagation in solution polymerization is that solvents when used are usually present in very large excess in relation to any radical species. Of course, economic, solubility, toxicity, waste disposal, and other considerations limit the range of solvents that can be employed in an industrial polymerization process. 

Diffusion controlled propagation and fall in initiator efficiency as veil. 

Polymerization with Controlled Propagation. Wiley (Interscience), New York, 1959. 

Figure 6. Styrene emulsion polymerization—critical conversion for diffusion-controlled propagation as a function of temperature (---) X cr2 0. 740 + 1.846 X...

A single step of the polymerization is analogous to a diastereoselective synthesis. Thus, to achieve a certain level of chemical stereocontrol, chirality of the catalytically active species is necessary. In metallocene catalysis, chirality may be associated with the transition metal, the ligand, or the growing polymer chain (e.g., the terminal monomer unit). Therefore, two basic mechanisms of stereocontrol are possible (145,146) (i) catalytic site control (also referred to as enantiomorphic site control), which is associated with the chirality at the transition metal or the ligand and (ii) chain-end control, which is caused by the chirality of the last inserted monomer unit. These two mechanisms cause the formation of microstructures that may be described by different statistics in catalytic site control, errors are corrected by the (nature (chirality) of the catalytic site (Bernoullian statistics), but chain-end controlled propagation is not capable of correcting the subsequently inserted monomers after a monomer has been incorrectly inserted (Markovian statistics). 

In polar solvents, the cis form is generally more stable, but in the activated complex the trans conformation is preferentially formed and can be preserved [86]. Therefore data on the isomerization of ion pairs in active centres are of great importance, especially for controlled propagation to stereospecific (stereoregular) polymers. 

The second thermodynamic quantity controlling propagation is the entropy. Changes of phase transitions, of concentration, and addition itself... 

Chain propagation can be affected by variations in the mechanism or kinetics of addition. Controlled propagation in the proper sense is usually... 

---