Growing chains and monomer

Transfer reaction can not only involve growing chains and impurities but can also occur between growing chains and monomer molecules as well as between initiator and monomer molecules. For example, strong bases attack acrylic compounds, i.e., via the side groups... 

The strength of adsorption of H2 on iron is known to be weaker than that of CO. If alkali increases the coverage of the surface by growing chains and monomers, there would be fewer vacant sites available for H2 adsorption making the system less hydrogenating. This could explain why alkali promotion increases the ratio of alkenes to alkanes. 

However, when diphosphines are used, in which the phosphorus donor atoms are always cis to one another (all the ligands assayed were cis coordinating), the growing chain and monomer are in cis positions as well - a prerequisite for insertion reactions. As a result, diphosphines vhth natural bite angles close to 90° (dppp) stabilize the transition state for insertion reactions (chain growth), explaining also the higher activity and polymer selectivity of dppp when compared to monophosphines. 

The stereoregulating capability of Ziegler-Natta catalysts is believed to depend on a coordination mechanism in which both the growing polymer chain and the monomer coordinate with the catalyst. The addition then occurs by insertion of the monomer between the growing chain and the catalyst by a concerted mechanism [XIX] ... 

The effect of temperature on the kinetics of the direct radiation method is quite complex. Increase in temperature increases the monomer diffusion rate but also increases transfer and termination reaction rates of the growing chains and reduces the importance of the gel effect. Solubility and radical mobility may also change as the temperature is varied [88,89]. 

Radical Desorption Rate. It is evaluated, according to the law proposed by Nonura (36), as the result of three stages in series Chain transfer of a growing chain to monomer, diffusion of the active, low molecular weight product to the particle surface and diffusion in the aqueous phase. The resulting expression has been extended to the multlconponent case as follows ... 

The immediate result of the intervention of the chain transfer process indicated in the first step is the termination of a growing chain and the reactivation of a polymer molecule, which then adds monomer to gener-... 

In the framework of the chain migratory insertion mechanism (Scheme 1.3), the stereospecific behavior of the model sites depends on the relationship between the two situations obtained by exchanging, in the coordination step, the relative positions of the growing chain and the incoming monomer. Depending on the local symmetry of the coordinated bridged n-ligand, these two situations can be as follows ... 

Propagation of the free radical take place with addition of each monomer to the growing chain and all these reactions are known as propagation. Because of propagation long chain radical is formed. 

Before continuing, it has to be noted that the energy difference between the secondary and primary propene insertion, AEK 0, can be considered composed by two main contributions, electronic and steric. The steric contribution to AE po, due to steric interaction between the monomer, the growing chain and the ligand skeleton, was modeled successfully through simple molecular mechanics calculations [78-80], and was reviewed recently [11,24], For this reason in the following we will focus only on the electronic contribution to AEKgio. 

The ratio of disappearance of monomers M1/M2 is described by Equation 7.13 from Equations 7.10 and 7.11. Remember that this is also the average composition of the growing chains and resulting polymer. 

The rate of polymerization in the fiber, Rp> is equal to kp (R )(Af/), where kp is the propagation rate constant, (R ) is the concentration of growing chains, and Af, is the concentration of monomer in the fiber averaged across its diameter. Both (R ) and (Af/) are time-dependent until a final steady state is eventually reached. 

Vogl and Bryant (25) suggested a similar mechanism in which four oxygen atoms are coordinated to a metal atom. In addition to the terminal oxygen atom of the growing chain, two monomer molecules and the penultimate monomeric unit supply oxygen atoms required for the coordination. Simultaneous coordination of two molecules of aldehyde prior to addition may justify the observed sequence of iso-... 

In this chapter, the term oligomeric compound refers to products prepared by repeatedly linking one or several types of monomer to a growing chain of monomers. Included are biopolymers, such as peptides or oligonucleotides, and non-natural products, such as peptide nucleic acids, peptoids, or other synthetic polyamides. 

The preferential polymerization of one antipode, which in this case is determined by interaction between asymmetric groups belonging to the last monomeric unit of the growing chain and the asymmetric monomer molecule, should be limited to the initial segment of each macromolecule. In fact the casual inversion of configuration of a tertiary carbon atom of the main chain during the polymerization should favour the preferential polymerization of the other antipode. 

This step is called propagation reaction it adds more and more monomer units to the growing chain and builds up the macromolecules while the fiee radical character of the chain end is maintained. Each single addition represents the reaction of a free radical with a monomer molecule—a process which requires an activation energy of 8-10 kcal. 

When polymerization occurs in a mixture of monomers there will be competition between the different kinds of monomers to add to the growing chain and produce a copolymer. Such a polymer will be expected to have physical properties quite different from those of a mixture of the separate homopolymers. Many copolymers, such as GRS, ethene-propene, Viton rubbers, and Vinyon plastics are of considerable commercial importance. 

A second element of chirality is the configuration of the stereogenic tertiary carbon atoms in the growing chain, and in particular of that in the last inserted monomer unit. 

In view of the data concerning propylene polymerisation in the presence of homogeneous vanadium-based Ziegler-Natta catalysts, the syndiospecificity of the polymerisation is believed [387,395] to arise from steric repulsions between the last inserted monomer unit of the growing chain and the methyl group of coordinated propylene molecule, i.e. chain end stereocontrol is postulated to play the essential role in the stereoregulation. 

In the model proposed for the active centre of the propylene syndiospecific polymerisation, the V(III) atom is pentacoordinated [327]. Its ligands include three chlorine atoms (two of which are bridge-bonded to the aluminium atom), the chiral carbon atom of the last monomer unit of the growing chain and the coordinated propylene molecule. Prior to its coordination and after its insertion, the vanadium atom is tetracoordinated. In the alternative similar model, two chlorine atoms are substituted by a bidentate dionate, and the chlorine atom is bridge-bonded to the aluminium atom in dimeric A1R2C1 [2]. 

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