Controlled/living polymerizations attributes

An extremely favorable consequence of both strategies is the presence of significant amounts of covalent, or inactive, chain ends. This substantially lowers the overall concentration of reactive chain ends which results in a decrease in the occurrence of unwanted side reactions such as termination, disproportionation, or combination. This enables the polymer chain to grow in a controlled fashion, exhibiting many of the attributes typically associated with a living polymerization. However, it should be pointed out that the occurrence of these side reactions is not eliminated and in the strictest sense, the polymerizations are not truly living. 

The ROP of lactide affords high molecular weight PLA polymers with better control of the polymerization process relative to polycondensation. These advantages can be directly attributed to the fact that ROP can be a living polymerization process. Living polymerization is a chain-growth polymerization where chain termination is absent and is characterized by a linear relationship between the monomer to initiator ratio and the experimental molecular weight, and narrow dispersity indicates the... 

The first demonstration of living polymerization and the current definition of the process can be attributed to Swarc. Living polymerization mechanisms offer polymers of controlled composition, architecture, and molecular weight distribution. They provide routes to low-dispersity end-functional polymers, to high-purity block copolymers, and to stars and other more complex architectures. Traditional methods of living polymerization are based on ionic, coordination, or group transfer mechanisms. 

To study the living nature of this surface initiated polymerization, several groups have performed kinetic studies. " They reported that the nonlinear growth of the polymer brushes as a funetion of irradiation time was mainly attributed to bimolecular termination reaetions, rather than chain transfer to monomer. To avoid irreversible termination reaetions, a strategy to increase the amount of deactivating species by adding tetraethylthiuram disulfide to the polymerization mixture, which is mandatory to provide a controlled radical polymerization behavior, was introduced. ... 

Polymerization with reversible addition-fragmentation chain transfer (RAFT) is a reversible deactivation radical polymerization (RDRP) a process which, with appropriate attention to reagents and reaction conditions, can possess the attributes usually associated with living polymerization. These processes are also often called hving radical polymerizations or controlled radical polymerizations. However, the use of these terms in this context is now discouraged. ... 

Dithiocarbamatc 16 has been used to prepare low dispersity PMAA ( Mw 1 Mn-1.2).52 Photopolymerization of S in the presence of dithiocarbamate 16 also displays some living characteristics (molecular weights that increase with conversion, ability to make block copolymer). However, 17 appears to behave as a conventional initiator in S polymerization.53 The difference in behavior was attributed to the relatively poor leaving group ability of the 2-carboxyprop-2-yI radical. This hypothesis is supported by MO calculations. Dithiocarbamatc 17 was used to control polymerizations of MMA,54 HEMA54 and NIPAM.5... 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

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