Group-transfer polymerization possible mechanism

The existence of a dynamic equilibrium between dormant (covalent) and active (ionic) species in controlled carbocationic polymerizations had been debated for years. It has been argued that under certain conditions, polarized covalent species can directly react with monomer examples are the pseudocationic mechanism proposed for the polymerization of styrene initiated by perchloric acid (123,124) (Fig. 5) or the two-component group transfer polymerization proposed for the polymerization of isobutylene initiated by the dicumylacetate/BCls system (125) (Fig. 6). Recent results and theoretical considerations support the now generally accepted view that the true active species are ions, and the dormant species serve as a reservoir from which the propagating ion pairs are formed (126-131). The existence of a dynamic equilibrium between dormant and active species and the ability to suppress the formation of free ions made possible the synthesis of pol5miers with controlled molecular architecture via carbocationic polymerization. 

The term acrylic apphes to a family of copolymers of monomers that are polymerized by a chain growth mechanism. Most often, the mechanism of polymerization is by free radical initiation. Other mechanisms of polymerization, such as ionic and group transfer polymerization, are possible but will not be discussed in this publication. For a description of other polymerization mechanisms, polymer textbooks are available (5,6). Technically, acrylic monomers are derivatives of acrylic or methacrylic acid. These derivatives are nonfunctional esters (methyl methacrylate, butyl acrylate, etc.), amides (acrylamide), nitrile (acrylonitrile), and esters that contain functional groups (hydroxyethyl acrylate, glycidyl methacrylate, dimethylaminoethyl acrylate). Other monomers that are not acryhc derivatives are often included as components of acryhc resins because they are readily copolymerized with the acryhc derivatives. Styrene is often used in significant quantities in acryhc copolymers. 

ROP of p-lactones is highly prone to numerous side reactions, such as transester-fication, chain-transfer or multiple hydrogen transfer reactions (proton or hydride). Specifically, the latter often causes unwanted functionalities such as crotonate and results in loss over molecular weight control. Above all, backbiting decreases chain length, yielding macrocyclic structures. All these undesired influences are dependent on the reaction conditions such as applied initiator or catalyst, temperature, solvent, or concentration. The easiest way to suppress these side reactions is the coordination of the reactive group to a Lewis acid in conjunction with mild conditions [71]. p-BL can be polymerized cationically and enzymatically but, due to the mentioned facts, the coordinative insertion mechanism is the most favorable. Whereas cationic and enzymatic mechanisms share common mechanistic characteristics, the latter method offers not only the possibility to influence... 

Polynucleotide polymerases, or nucleotidyl transferases, are enzymes that catalyze the template-instructed polymerization of deoxyribo- or ribonu-cleoside triphosphates into polymeric nucleic acid - DNA or RNA. Depending on their substrate specificity, polymerases are classed as RNA- or DNA-dependent polymerases which copy their templates into RNA or DNA (all combinations of substrates are possible). Polymerization, or nucleotidyl transfer, involves formation of a phosphodiester bond that results from nucleophilic attack of the 3 -OH of primer-template on the a-phosphate group of the incoming nucleoside triphosphate. Although substantial diversity of sequence and function is observed for natural polymerases, there is evidence that many employ the same mechanism for DNA or RNA synthesis. On the basis of the crystal structures of polymerase replication complexes, a two-metal-ion mechanism of nucleotide addition was proposed [1] during this two divalent metal ions stabilize the structure and charge of the expected pentacovalent transition state (Figure B.16.1). 

Since the metal alkyl is only involved in initiation of chain growth while subsequent propagation and chain transfer (usually involving chain transfer to monomer via hydride abstraction ) are usually very rapid, it is generally not possible to determine the fate of the metal alkyl in such polymerizations. End-group analyses provide evidence for a cationic propagation and chain-transfer mechanism, but the initiator moiety is not detected unless the process is at least quasi-living under the conditions studied. 

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