Predetermined molecular weight

The development of anionic chemistry has placed a number of powerful tools in the hands of the polymer chemist. Polymer molecules of predetermined molecular weight, molecular weight distribution, composition, and configuration can now be synthesized nearing the purity of simple organic molecules. Controlled polymeric structures have been realized that are highly desirable as models to advance theoretical studies and indeed have vast economic values to industry as profitable consumer items. 

Thus, preparation of well-defined polymers by controlled polymerization should not be confused with living polymerization. Herein we define a controlled polymerization to be a synthetic method to make polymers with predetermined molecular weights, low polydispersity, controlled functionality, block copolymers, etc. Transfer and termination can happen in controlled polymerization but their contribution is sufficiently reduced by the proper choice of the reaction conditions. 

On the other hand, a living polymerization is a chain polymerization without transfer and termination. Living polymerization may lead to well-defined polymers with narrow molecular weight distributions and predetermined molecular weights only if some additional prerequisites are fulfilled ... 

Before going into details, let us make a brief statement that propagation in new controlled/living carbocationic systems has nearly the same mechanism as in the conventional systems discussed in Chapter 3, which consists of the electrophilic addition of carbenium ions to alkenes. The main difference is that carbenium ions are in dynamic equilibria with dormant species (covalent esters and onium ions). The correct choice of structures and concentrations of activators and nucleophilic additives as well as those of initiator allows for the preparation of polymers with predetermined molecular weights, low polydispersities, and controlled end functionality, including block copolymers (see Chapter 5). 

A similar effect was observed for trityl derivatives. Initiation of vinyl ether polymerization with trityl salts is very slow and often incomplete [257]. This precludes preparation of well-defined polymers with predetermined molecular weights and narrow MWDs. However, polymerization of vinyl ethers initiated by trityl salts in the presence of tetrahydrothiophene leads to controlled polymers [135]. The equilibrium constant for the formation of sulfonium ions is much smaller for trityl salts than for the growing species (K, < Kp), which increases the ratio of the apparent initiation to the propagation rate constants a thousand times (Scheme 14) ... 

In 1993, Georges et al. reported on the controlled radical polymerization of St initiated by benzoyl peroxide and mediated by 2,2,6,6-tetramethyl-l-piperidinyl-oxyl (TEMPO), a stable nitroxide radical [38]. TEMPO was able to bond reversibly to the polystyryl chain end and provide polystyrenes with predetermined molecular weights and low polydispersities. Nitroxides used earlier to control radical polymerizations were less successful [37, 57]. Scheme 3 illustrates the mechanism of the St polymerization, using a generalized structure of radical initiator I-I, and details the structure of TEMPO. Although several types of nitrox-... 

Reversible addition-fragmentation chain transfer (RAFT) polymerization has proven to be a powerful tool for the synthesis of polymers with predetermined molecular weight and low polydispersity [11, 12], In recent years, synthesis of polymers with complex molecular architecture, e.g. block and star copolymers, via the RAFT process have been reported [13],... 

For example, the rate of initiation may be slow relative to that of propagation, in which case the MWD will tend to become broader. If ion-generation is slow, the effects of chain transfer to initiator and slow initiation may overlap and may not even be distinguishable. Chain transfer to monomer, initiation by protic impurity ( H20 ), etc., may be present which will further complicate the synthesis of uniform predetermined molecular weight polymers. 

The development of controlled radical polymerization (CRP) methods,(1,2) including atom transfer radical polymerization (ATRP),(3-6) nitroxide-mediated radical polymerization,(7) and reversible addition fragmentation chain transfer polymerization,(8,9) has led to the synthesis of an unprecedented number of novel, previously inaccessible polymeric materials. Well-defined polymers, i.e., polymers with predetermined molecular weight, narrow molecular weight distribution, and high degree of chain end functionalization, prepared by... 

Controlled living radical polymerization methods were developed to produce polymers with predetermined molecular weights, low polydispersity index, specific... 

Autoclaving of sodium hyaluronate results in a significant depolymerization and change in viscosity (see below Kim, 1987). Responsible for the relatively high expense associated with sodium hyaluronate solution are the considerable, technological demands for the production of a non-inflammatory compound, at a predetermined molecular weight. 

CRPs produce polymers with predetermined molecular weights, well-defined architectures, low polydispersities, and functional end groups. CRPs are similar to living anionic and cationic polymerizations in that they produce polymers with a narrow molecular-weight disttihution however, they have greater tolerance for functional groups such as alcohols, esters, amides, and carboxylates. ATRP and RAFT polymeiization ° are two of the most widely used CRP techniques. 

Atom transfer radical polymerizations (ATRP) were reported simultaneously by two groups (1) Matyjaszewski et al. [218] and (2) Sawamoto and coworkers [226]. Matyjaszewski et al. utilized a Cu/bipyridine complex as a halogen transfer agent that functions between dormant and active polymer chains. Formation of polymers with predetermined molecular weight of up to ps 10 [5]... 

Macromer is a poljnnerizable macromolecular monomer, having a polymerizable end group and uniform molecular weight distribution. Macromers may be used to prepare graft copolymer containing characterized grafts with uniform and predetermined molecular weight. 

It is also possible to prepare vinylpyridine-based (co)polymers in a controlled fashion with predetermined molecular weights and narrow molecular weight distributions. For example, the controlled pol5mierization of 16C and 18C is possible via anionic polymerization (197-201). 

Atom transfer radical polymerization is one of the most commonly employed techniques for controUed/living radical polymerization due to its high efficiency in obtaining well-defined polymers characterized by a predetermined molecular weight, narrow molecular weight distribution and high degree of chain end functionality. 

For obtaining polymers with predetermined molecular weights, low PDI, specific functionalities, and diverse architecture than in conventional free-radical polymerization, controlled radical polymerization (GRP) methods were developed. There are a number of reports that enhanced rates and low polydispersity indices were observed for atom transfer radical polymerization (ATRP) under microwave conditions similarly, significant rate enhancements were reported for reversible addition-fragmentation chain transfer (RAFT) and nitroxide-mediated polymerization (NMP). 

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