Activated monomer mechanisms

Anionic polymerization of lactams was shown to proceed according to what is called the activated monomer mechanism. With bischloroformates of hydroxy-terminated poly(tetramethyleneglycol) and poly(styrene glycol) as precursors for a polymeric initiator containing N-acyl lactam ends, block copolymers with n-pyrrol-idone and e-caprolactam were obtained by bulk polymerizations in vacuum at 30 and 80 °C, respectively361. ... 

An interesting cationic activated monomer mechanism was reported by Penczek. 

Activated magnesium, 72 835 Activated monomer mechanism, for lactide polymerization, 20 300 Activated sludge, 72 24... 

So it is obvious that mechanical activation of monomers brings about two competitive processes the growth of polymer chain and the chain destruction. A number of studies were undertaken to find optimal conditions regarding duration of the polymerization reaction, its temperature, allowed loading, and so on. Examples were presented in a review by Mit et al. (2003). 

Carboxylated polymers can be prepared by mechanical treatment of frozen polymer solutions in acrylic acid (Heinicke 1984). The reaction mechanism is based on the initiation of polymerization of the frozen monomer by free macroradicals formed during mechanolysis of the starting polymer. Depending on the type of polymer, mixed, grafted, and block polymers with a linear or spatial structure are obtained. What is important is that the solid-phase reaction runs with a relatively high rate. For instance, in the polyamide reactive system with acrylic acid, the tribochemical reaction leading to the copolymer is completed after a treatment time of 60 s. As a rule, the mechanical activation of polymers is mainly carried out in a dry state, because the structural imperfections appear most likely here. 

Basko M, Kubisa P (2006) Cationic copolymerization of E-caprolactone and L,L-lactide by an activated monomer mechanism. J Polym Sci A Polym Chem 44 7071-7081... 

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 anionic polymerization of lactams proceeds by a mechanism analogous to the activated monomer mechanism for anionic polymerization of acrylamide (Sec. 5-7b) and some cationic polymerizations of epoxides (Sec. 7-2b-3-b). The propagating center is the cyclic amide linkage of the IV-acyllactam. Monomer does not add to the propagating chain it is the monomer anion (lactam anion), often referred to as activated monomer, which adds to the propagating chain [Szwarc, 1965, 1966]. The propagation rate depends on the concentrations of lactam anion and W-acy I lactam, both of which are determined by the concentrations of lactam and base. 

Polymerizations initiated by strong bases (R-, IIO, RO-) and tertiary amines (which are poor nucleophiles) proceed at much faster rates than do polymerizations initiated hy primary amines. Also, unlike the latter, where each polymer chain contains one initiator fragment (i.e., RNH—), these polymerizations do not result in incorporation of the initiator into the polymer chain. Polymerization proceeds by an activated monomer mechanism similar to that in the anionic polymerization of lactams. The reacting monomer is the NCA anion XLIV... 

NCA polymerization by secondary amines may involve the amine or activated monomer mechanisms or both mechanisms simultaneously. Unhindered secondary amines such as dimethylamine and piperidine react like primary amines, and polymerization occurs by the amine mechanism. Polymerization by slightly hindered amines such as diethylamine, N-methylbenzylamine, and di-n-propylamine involves a combination of the amine and activated monomer mechanisms. More hindered secondary amines, such as di-n-isopropylamine and dicyclohexylamine, react almost exclusively via the activated monomer mechanism. 

Cationic ROP of lactones in the presence of an alcohol proceeds by an activated monomer mechanism similar to that for cyclic ethers (Sec. 7-2b-3-b) [Endo et al., 2002 Lou et al., 2002]. Propagation proceeds by nucleophilic attack of the hydroxyl end group of a propagating chain on protonated (activated) monomer ... 

For copolymerizations proceeding by the activated monomer mechanism (e.g., cyclic ethers, lactams, /V-carboxy-a-amino acid anhydrides), the actual monomers are the activated monomers. The concentrations of the two activated monomers (e.g., the lactam anions in anionic lactam copolymerization) may be different from the comonomer feed. Calculations of monomer reactivity ratios using the feed composition will then be incorrect. 

Some ROPs proceed with the simultaneous operation of two different mechanisms, for example, NCA copolymerizations initiated by some secondary amines proceed with both the amine and activated monomer mechanisms. The monomer reactivity ratios for any comonomer pair are unlikely to be the same for the two different propagations. Any experimentally determined r values are each composites of two different r values. 

A second route (route B in Fig. 1) relies on an initiation process with an (meth)acryl hydroxyl compound and is adopted from the chemical ROP of lactones. The controlled character of these polymerizations ensures a virtually quantitative initiation and thus incorporation of hydroxy-functional initiator (e.g., acrylate) into the polymer chain. However, this is not automatically the case for lipase-catalyzed ROP due to the different mechanism. The latter follows an activated monomer mechanism in which the lipase activates any carbonyl group of a carboxylic acid derivative present in the system. It has recently been shown that acrylation using hydroxy-functional acrylate initiators like hydroxy ethyl(meth)acrylate (HEMA or... 

Before getting into the subject, classifying in accordance with their process and mechanism the electrolytic initiation reactions which have appeared in the literature will afford the reader a better understanding. The reactions are classified firstly into two types cathodic and anodic. For the cathodic reaction, generation of free-radical and radical-anion, and formation of unstable monomer and active catalyst are visualized from the corresponding references. 

Another misconception arises from the statement that a strong base, e. g., C(Ph)3-, Na+ will be converted into an extremely weak acid, i. e. Ph3CH, and will not provide a proton for the decarboxylation of carbamate ions. This again is true, but the proton need not be provided by the conjugated acid — it is given by the non-activated NCA which is converted in this process into an activated NCA. This reaction cannot take place with the N-substituted NCA s and therefore the activated monomer mechanism is not operative for these monomers. 

Blout observed that the rate of polymerization of y-benzyl glutamic acid NCA was approximately 100-times faster when initiated with sodium methoxide rather than a primary or secondary amine.19,101 He proposed the active monomer mechanism of NCA polymerization previously formulated by Ballard and Bamford,1111 which involves the extraction of the proton from the NCA nitrogen to give anion 3, followed by nucleophilic attack of this anion on the amino acid carbonyl of a second NCA (Scheme 2). The active monomer mechanism was further studied and substantiated by Goodman.112-141... 

Additionally, an NCA may simultaneously polymerize by more than one mechanism. The active monomer mechanism provides polymers whose molecular weight is not dependent on initiator concentration, and high molecular weight polymers are readily obtained. The normal or carbamate mechanism should provide one polymer molecule per initiator molecule and the initiator is incorporated into the polymer. 

The polymerization of an NCA may be initiated by any moderately strong base or by nucleophiles. Weak nucleophiles, such as water, alcohols, or primary amines, generally initiate polymerization by the normal (nucleophilic) mechanism or by the carbamate mechanism. Tertiary amines and strong bases, such as methoxide, initiate polymerization by the active monomer mechanism. Secondary or primary amines may initiate polymerization by any one or all of these mechanisms. More than one mechanism may be active at any one time and frequently a polymerization may begin by the active monomer mechanism and then, at a later stage, propagate by the normal mechanism. 16 ... 

The fundamental task, in our opinion, is to correlate the principles and methods of the proposed synthesis with those of mechanochemical synthesis. Thus, besides the destruction processes and mechanochemical synthesis discussed in the literature, other lands of transformations sometimes occur as side reactions, or even as major processes. These include chemical fixation of small molecules (methyl chloride or butyl alcohol) on mechanically activated macromolecular backbones grafting of inorganic surfaces (quartz, metals, metallic oxides, inorganic salts, etc.) dispersed by vibratory milling on polymerized fragments synthesized from monomers present in the reaction medium, and activated by centers on the inorganic surface (14) and the possibility of some reactions (such as nitration), achieved so far on macromolecular supports and only as side reactions. 

We recently investigated a different route for the synthesis of poly(IB-h-f-CL) diblock and poly( -CL-fo-IB-fo- -CL) triblock copolymers by site-transformation of living cationic polymerization of IB to cationic ring-opening polymerization of -CL via the activated monomer mechanism [95]. 

Anionic polymerization of 67 by the activated monomer mechanism should occur with the selective cleavage of the CO—NH bond of the monomer to give a polyamide composed of kinetically controlled cis units (68c). However, the cis units isomerize to the thermodynamically more stable trans units (68t) through the proton abstraction from the methine group adjacent to the carbonyl group. This was ascertained by the isomerization experiment in which a polymer consisting of 92% cis unit and 8% trans unit was converted to one containing 40% cis unit and 60% trans unit when heated in dimethyl sulfoxide at 80 °C for 6 hours in the presence of 15 mol% potassium pyrrolidonate. 

The lack of anionic polymerizability of 69 can be ascribed to the instability of 71 due to a stereoelectronic effect. [9] One of the lone pair orbitals on the nitrogen atom of 71 is perfectly antiperiplanar to the C(l) —0(2) bond, which causes this bond to cleave fadly. Thus, as soon as 71 is formed, it is transformed to 72, so that anionic polymerization by the activated monomer mechanism does not take place. 

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