Mechanisms of polymerization

The polymerization of a vinyl monomer is initiated by the reaction of a free radical R-. This results in the formation of another radical (see Eq. 2.7). 

The propagation of the reaction occurs when the radical that is generated adds onto other monomer molecules (see Eq. 2.8). 

This mode of propagation is known as the head-to-tail propagation. This is the predominant pathway. Other modes of propagation such as head-head or tail-tail are less favored. 

How do the growing chains terminate This occurs in two ways. By (a) coupling reactions or by (b) disproportionation. 

In the coupling mechanism two free-radical polymer chains can combine to afford a larger polymer chain (see Eq. 2.9). 

Beerman and Bestian [28] offered early evidence into the existence of a Ti-Garbon bond by reacting TiGl with trimethylaluminum at 20°G to give the two alkylated products shown in Equation 2.6. 

The amoxmt of each Ti-containing species was controlled by the Al/Ti ratio employed. 

The research by Beerman also demonstrated the relative stability of the Ti-methyl bond as compared to the relatively less stable Ti-ethyl bond that contains a hydrogen on the beta carbon and can, therefore, undergo beta-hydride transfer to the titanium metal and eliminate an ethylene molecule. This early research eventually lead in the 1970s to the identification of transition metal carbene complexes (M=CH2), which when reacted with olefins provide metallacyclobutanes [29]. 

4 Second Generation Ziegler Catalyst for the Manufacture of Polyethylene 

Starting in 1959, Solvay was one of the first companies to manufacture HDPE using the Ziegler catalyst and a slurry polymerization process and had a research laboratory active in the development of improved Ziegler catalyst systems. 

FIGURE 9.1 Comparison of reactor contents at 50% conversion (a) step-growth or condensation polymerization and (b) addition (or free-radical) polymerization. M = monomer. 

The initiator molecule, represented in a generic sense by /, undergoes a first-order decomposition with a rate constant to give two free-radicals, 2 R-  

This reaction has a rate constant kj. The radical then adds a monomer by grabbing an electron from the electron-rich double bond, forming a single (R—C) bond with the monomer, but leaving an unshared electron at the other end  

The chains continue to grow as long as monomers continue to be available and accessible in the reaction mixture. The reaction ends by a termination step using one of two methods. Two chains can bump together and stick, with their unshared electrons combining to form a single bond between them (combination)  

The relative proportion of each termination mode depends on the particular polymer and the reaction temperature, but in most cases, one or the other predominates. Note that another potential method for terminating a polymerization reaction is by chain transfer, where the free-radical is transferred to another species in the reaction mixture, such as a solvent, as discussed in Section 9.6. 

We have again assumed that reactivity is independent of chain length by using the same k, for each propagation step. 

The relative proportion of each termination mode depends on the particular polymer and the reaction temperature, but in most cases, one or the other predominates. 

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Ghassemieli, E. and Nassehi, V., 2001b. rediction of failure and fracture mechanisms of polymeric composites using finite clement analysis. Part 1 particulate filled composites. Poly- Compos. 22, 528-541. 

Kinetics and mechanism of polymerization of vinyl monomers initiated by ylides. 

Among one-component polymerization catalysts subhalides of the transition metals are most similar in composition to the traditional Ziegler-Natta catalysts. In this connection, the study of the simplest one-component catalyst of this type (especially TiCl2) is of great importance for the clarification of still disputable problems of the mechanism of polymerization by two-component catalysts. 

Allen, P.E.M. Patrick, C.R. Kinetics and Mechanisms of Polymerization Reactions Ellis Horwood Chichester, 1974. 

Secondly, new techniques have been developed which allow a more detailed characterization of both polymer microstructures and the kinetics and mechanism of polymerizations. This has allowed mechanism-structure-property relationships to be more rigorously established. 

It is appropriate to differentiate between polymerizations occuring at temperatures above and below the glass transition point(Tg) of the polymer being produced. For polymerizations below Tg the diffusion coefficients of even small monomer molecules can fall appreciably and as a consequence even relatively slow reactions involving monomer molecules can become diffusion controlled complicating the mechanism of polymerization even further. For polymerizations above Tg one can reasonably assume that reactions involving small molecules are not diffusion controlled, except perhaps for extremely fast reactions such as those involving termination of small radicals. 

P. E. Allen and C. R. Patrick, Kinetics and Mechanism of Polymerization Reactions , Ellis Horwood, Chichester, 1974. 

Using the first-principles molecular-dynamics simulation, Munejiri, Shimojo and Hoshino studied the structure of liquid sulfur at 400 K, below the polymerization temperature [79]. They found that some of the Ss ring molecules homolytically open up on excitation of one electron from the HOMO to the LUMO. The chain-like diradicals S " thus generated partly recombine intramolecularly with formation of a branched Sy=S species rather than cyclo-Ss- Furthermore, the authors showed that photo-induced polymerization occurs in liquid sulfur when the Ss chains or Sy=S species are close to each other at their end. The mechanism of polymerization of sulfur remains a challenging problem for further theoretical work. 

FIGURE 2 Anionic, cationic, and coordination mechanisms of polymerization of e-caprolactone and related lactones. 

Alkyl sulfonates are very effective cationic initiators of e-caprolactone, although only the more reactive methyl triflate and methyl fluorosulfate result in a high conversion. The mechanism of polymerization in the presence of these initiators is believed to involve methylation of the exocyclic carbonyl oxygen, followed by partial ring opening of the activated lactone by the counteranion (Fig. 

In quest of an explanation for this phenomenon, one is led to conclude either that the combination of constants occurring in the rate equation (12) must undergo a large increase when autoacceleration occurs or that a totally different mechanism of polymerization must take over. We should obviously prefer the former alternative if it will lead to a satisfactory explanation of the facts. An increase in kdj seems unlikely autoacceleration is not a function of the initiator. This leaves us with the ratio which will be required to increase by... 

Allen, P. E. M Patrick, C. R., "Kinetics and Mechanisms of Polymerization Reactions", John Wiley Sons, (1974)... 

Fig. 3. Proposed mechanism of polymerization of dicarboxylic acid divinyl ester and glycol through lipase catalysis...

In this contribution, we review the mechanism of polymerization and oligomerization involving early transition metals, taking as our basis recent results in advanced organometallic chemistry. First of all, some recent examples of the previous reviews concerning the Ziegler-Natta polymerization are cited [1-10]. Then, relevant new reports are surveyed in a systematic fashion. 

Based on this observation, the mechanism of polymerization was proposed as indicated in Scheme 14. 

Fig. 2 A,B. Application of cationic polymerization for neobiopolymer synthesis. A Mechanism of polymerization. B Minoda s use of Higashimura and Sawamoto s initiating system to generate a protected glucose-substituted polymer...

It appeared to us that the only reasonable non-ionic reaction product of an acid and an olefin would be an ester, and for this reason we put forward the idea that this is the active species in the pseudo-cationic polymerizations. Of course, the idea of an ester in this role has a respectable ancestry which has been discussed in this new context [6]. The ester mechanism of polymerization will be discussed in sub-section 3.3. It must be understood that our conclusion concerning the non-ionic nature of the chain-carriers in the pseudocationic polymerizations is quite independent of our view that the chain-carriers are esters this is at present merely an hypothesis to explain our factual conclusion. 

Artificial Weathering Tests. Progress in this area has been achieved as chemists increased their understanding of the degradation mechanisms of polymeric systems. 

Tetramethylpiperidine-l-oxy (TEMPO)-containing alkoxyamine derivatives are widely used as unimolecular initiators for living radical polymerization [5], The key step of the presently accepted mechanism of polymerization is the reversible capping of the polymer chain by the nitroxide radical. In 2002, Otsuka and Takahara applied the reversible... 

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