Polymerization, activation mechanisms

The value of Kp as a measure of the reactivity of the active centers in the propagation reaction is the most fundamental characteristic of polymerization catalysts. The conclusions on the polymerization mechanism based on the correct values of N and Kp are much more unambiguous than those made when considering only the data on the polymerization activity and molecular weight of a polymer. 

The term catalyst is often misused in anionic polymerization. These mechanisms require the use of initiators that differ from catalysts in that they are not regenerated at the end of the reaction. The similarity between initiators and catalysts is that they both create a situation that permits polymerization via a reduction in the activation energy of the process. 

The first mechanism was proposed by Baird et al. [189]. The carbocationic species is schematically shown in Scheme IX. The attack of monomer, well known on the carbocationic center of a metal-ion-activated olefin, proceeded in the normal manner for carbocationic polymerization. This mechanism is based on the following two evidences. Alcoholysis of the polymerization system, TiMe3Cp /B(C6F5)3, resulted in the presence of an alkoxy group at an end group, and vinyl ethers and iV-vinylcarbazole were polymerized by using the same system. 

Parrinello studies, Meier suggested the possibility that in dimeric complexes dynamic effects might favour insertion (at a single metal centre, e.g., in 5) over chain transfer, but no free-energy data were provided to support this hypothesis [35], Insertion over two metal centres (as in 6), as an alternative to the standard Cossee mechanism, was shown to have a comparable barrier to the standard mechanism [36, 37]. However, this alternative mechanism does not have an improved propagation/chain transfer balance and hence does not offer a better explanation for the observed polymerization activity. It appears... 

The template polymerization of methacrylic acid at 60 C in DMF was studied with atactic poly(vinyl acetate) M =66,400 used as a template. The effect of template, monomer, and initiator (AIBN) concentration on the kinetics of polymerization was studied dilatometrically. Viscometric measurements showed that complexation between poly(vinyl acetate) and poly(methacrylic acid) was maximized when the template to polymer ratio was 1 1, and for the same ratio of the monomer to the template, the rate of template polymerization also reached the maximum. The overall energy of activation was the same (115 kJ/mol) in the presence and absence of the template. The polymerization follows mechanism II ( pick up mechanism ). 

The so-called Klenow fragment of DNA polymerase 1 of E. coli (Chapter 14, section Al) contains the 5 -3 -polymerization and the 3 -5 -exonuclease domains. Detailed pre-steady state kinetics have been made of the polymerization and exonuclease activities.39-43 The editing site is 35 A away from the polymerization site.32 The mechanism of the polymerization activity (Figure 13.7) is very similar to that for hydrolysis (Figure 13.8). The key to both is the presence of two metal ions, 3.9 A apart, that stabilize the developing charges on the transition state and metal-bound HO- or RO ions (see Chapter 2, section B7).44,45... 

Scheme48 Butadiene polymerization with Nd[N(SiMe3)2]3/[NHMe2Ph] [B(C6F5)4]/R3A1 and proposed activation mechanism [202]...

Mechanization and automation are possible in the synthesis of peptides using solid polymeric active esters also. By passing a solution of the amine component in a suitable solvent through a column packed with the solid polymeric activated carboxyl component, mechanization could be effected. The product, the protected peptide, which is in the eluent, is then N-deprotected, and the product in solution is passed... 

Stem et al. developed a batch-processing approach to mechanized peptide synthesis using polymeric active esters105). The procedure consisted of the following three consecutive steps without isolation of the intermediate compounds ... 

The traditional cocatalyst, diethylaluminumchloride or triethylaluminum, shows only a pure polymerization activity and was used as a homogeneous system to understand the polymerization, which is simpler in soluble than in heterogeneous systems. Kinetic studies and applications of various methods have helped to define the nature of the active centers, to explain aging effects, to establish the mechanism of the interaction with olefins, and to obtain quantitative evidence of some elementary steps [12,13]. 

Relative to the initiator/activator mechanism shown in Scheme 5, it is interesting to compare vinyl ether polymerizations initiated with the HI/I2 system and with iodine alone. The former system provides living polymers of controlled molecular weights and very narrow MWD [58], whereas the latter has been known for more than a century but fails to give such controlled polymerizations (cf., Sections IV.A) [49,55]. In the iodine-mediated polymerization, iodine serves as both the initiator and activator one molecule of iodine first slowly adds across the vinyl ether double bond to give an adduct. The a-carbon-iodine bond is activated by another molecule of iodine [34,95]. Thus, both systems would in fact form the identical growing chain end [ CH2CH(OR)+.I3 ], and the ob-... 

Chemical modification of polymers by the addition of various flame retardants may have different effects on the ignitability of polymeric materials. Information about such effects helps to explore certain features of the activity mechanism of flame retardants... 

It has been generally accepted that macromolecular stabilizers protect polymers by the same physical or chemical mechanism as conventional stabilizers containing a comparable functional moiety. (For details dealing with activity mechanisms of macromolecular stabilizers see Sect. 4.2.) Some differences must be however taken into consideration due to the macromolecular character of polymeric stabilizers. 

The nature of the active sites is open to discussion. Toby et al. chose to follow a possible earlier suggestion and used the addition of formaldehyde to a neutral polymer chain as the propagation mechanism. Because of a slight inhibition of the polymerization by oxygen, a radical mechanism was not completely ruled out. Looking at the formaldehyde polymerization as a whole and accepting some of the observations of Toby et al., it must be concluded that their formaldehyde polymerization was a cationic polymerization. Active centres, or active sites were actually oxonium... 

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