Activated monomer mechanism Initiation step

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

The polymerization occurs by the activated monomer mechanism, which supposes a two-step mechanism involving the acylation of the lactam anion (NaL) by the AT-acyllactam end-group followed by a fast proton-exchange with the monomer. In bulk polymerization, the preformed AT-acyllactams or their precursors (so-called CIs) are introduced into the system in order to avoid the slow initiation step due to the absence of AT-acyllactam groups at the beginning of polymerization ... 

The "activated monomer mechanism. The "a.m." mechanism eiq>hasizes the basic rather than the nucleophilic properties of aptotic initiators. The principal nucleophile considered in the mechanism is the anion obtained by abstracting a proton from a NCA, this anion being termed an "activated monomer, VI. If an aptotic base Is represented by B (B=%CHa, K3N, etc.), the initiation step can be represented as follows ... 

Figure 4.2 Initiation and growing steps of a primary amine-initiated ROP of NCAs (a) amine mechanism (b) activated monomer mechanism.

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

More evidence has been accumulated [see e. g. ref. (55)] to show that the polymerisation yielding high molecular weight polypeptides proceeds in two steps — initial self-accelerated reaction followed by an apparently first order reaction. It seems that the growing species slowly reach their stationary concentration and in this period the reaction appears to be auto-catalytic. In the terms of Bamford s mechanism this behaviour is easily explained by postulating slow initiation and rapid propagation. The initiation results from an attack of an activated monomer on a non-aetivated NCA. The propagation results from a... 

The polymerization undergoes a coordination-insertion mechanism. The initiation step involves nucleophilic attack of the active group, such as a hydride, alkyl, amide or alkox-ide group, on the carbonyl carbon atom of a lactide or lactone to form a new lanthanide alkoxide species via acyl-oxygen cleavage. The continued monomer coordination and insertion into the active metal-alkoxo bond formed completes the propagation step as shown in Figure 8.50. 

Within the scope of this review we shall only consider those compounds possessing one or more alkenyl functions susceptible to activation by electrc hilic attack. Included in this family is a vast array of monomers varying in basicity from ethylene, which is so resistant to protonation that the ethyl carbenium ion has hitherto eluded observations even under the most drastic conditions (see below), and which in fact is equally resistant to cationic polymerisation, to N-vinylcarbazole, whose susceptibility to this type of activation is so pronounced that it can be polymerised by almost any acidic initiator, however weak. We shall also deal with olefins which, because of steric hindrance, can only dimerise (e.g., 1,1-diphenylethylene) or cannot go beyond the stage of protonated or esterified monomeric species (e.g., 1,1-diphenylpropene). The interest of such model compounds is obvious they allow clean and detailed studies to be conducted on the kinetics and mechanism of the initiation steps and on the properties of the resulting products which simulate the active species in cationic polymerisation. The achievements and shortcomings of the latter studies will be discussed below. 

The initiation step is normally fast in polar solvents and an initiator-free living polymer of low molecular weight can be produced for study of the propagation reaction. The propagation step may proceed at both ends of the polymer chain (initiation by alkali metals, sodium naphthalene, or sodium biphenyl) or at a single chain end (initiation by lithium alkyls or cumyl salts of the alkali metals). The concentration of active centres is either twice the number of polymer chains present or equal to their number respectively. In either case the rates are normalized to the concentration of bound alkali metal present, described variously as concentration of active centres, living ends or sometimes polystyryllithium, potassium, etc. Much of the elucidation of reaction mechanism has occurred with styrene as monomer which will now be used to illustrate the principles involved. The solvents commonly used are dioxane (D = 2.25), oxepane (D = 5.06), tetrahydropyran D = 5.61), 2-methyl-tetrahydrofuran (D = 6.24), tetrahydrofuran (D = 7.39) or dimethoxy-ethane D = 7.20) where D denotes the dielectric constant at 25°C. 

Example 4-8 An ideal continuous stirred-tank reactor is used for the homogeneous polymerization of monomer M. The volumetric flow rate is O, the volume of the reactor is V, and the density of the reaction solution is invariant with composition. The concentration of monomer in the feed is [M]o. The polymer product is produced by an initiation step and a consecutive series of propagation reactions. The reaction mechanism and rate equations may be described as follows, where is the activated monomer and P2, . . , P are polymer molecules containing n monomer units ... 

According to these experimental results, the proposed reaction mechanism for the formation of poly(styrene oxide) with a regular chain structure by anionic polymerization involves the oxirane ring opening exclusively at the 3 position. However, two kinds of active centers, A and B in the reactions above, occur in the initiation step. The active center A, formed by a-ring opening, adds to a monomer molecule in the next step, but in the second step the oxirane ring is opened at the 3 position. 

Chain-growth polymerization proceeds by one of three mechanisms radical polymerization, cationic polymerization, or anionic polymerization. Each mechanism has three distinct phases an initiation step that starts the polymerization, propagation steps that allow the chain to grow, and termination steps that stop the growth of the chain. We will see that the choice of mechanism depends on the structure of the monomer and the initiator used to activate the monomer. 

Figure 5.9 outlines the steps for the chain polyaddition mechanism involved in the coordination polymerizations for any kind of active species initiated through different cocatalysts. The counteranion species was suppressed for practical representation of the active site. Once the cationic species is created, it starts the growth of the polymeric chain through continuous addition of monomer. The propagation step is forward described in Figure 5.9 according to the most accepted reaction cycle proposed by Cossee and Arlman, which is known as the Cossee-Arlman mechanism [51]. 

A comparison of Ni- and Pd-based Brookhart systems leads to the conclusion that the analysis of the polar comonomer binding mode can be used as a screening test to select the best prospective catalytic candidates for the copolymerization the complexes with preference of the 0-complexes can be excluded from further studies. Here, the use of theoretical methods can be very useful. Obviously, the monomer binding is only an initial step in the complex mechanism of the copolymerization processes. Therefore, the studies on the polar comonomer binding mode can be used only for a negative selection. To get more definitive answers about the catalyst activities, a full mechanistic study is needed, involving evaluation of the barriers of all the elementary reactions and the relative stabilities of the reaction intermediates. 

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