Living polymerization processes

The character of living polymerization processes merits more detailed attention [35-39]. Their importance in the tailored synthesis of polymers is two-fold. First, the lifetime of the growing species is long relative to the time it 

These materials have been synthesized by the sequential addition of reactants or by the chemical modification of chain ends to produce end-functionalized polymers which can be used in other chain extension reactions such as step addition or step condensation polymerization processes. 

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The living polymerization process offers enormous flexibiUty in the design of polymers (40). It is possible to control terminal functional groups, pendant groups, monomer sequencing along the main chain (including the order of addition and blockiness), steric stmcture, and spatial shape. 

In this chapter, we restrict discussion to approaches based on conventional radical polymerization. Living polymerization processes offer greater scope for controlling polymerization kinetics and the composition and architecture of the resultant polymer. These processes are discussed in Chapter 9. 

Living polymerization processes lend themselves to the synthesis of end functional polymers their use in this context is described in Chapter 9. In this section we limit discussion to processes based on conventional radical polymerization,... 

Copolymers produced by living polymerization processes differ from those produced by conventional polymerization in one important aspect. Living polymerization processes produce gradient or tapered copolymers. Such copolymers are known from anionic living polymerization. 

Most reviews on living radical polymerization mention the application of these methods in the synthesis of end-lunctional polymers. In that ideally all chain ends are retained, and no new chains are formed (Section 9.1.2), living polymerization processes are particularly suited to the synthesis of end-functional polymers. Living radical processes are no exception in this regard. We distinguish two main processes for the synthesis of end-functional polymers. 

Living polymerization processes immediately lend themselves to block copolymer synthesis and the advent of techniques for living radical polymerization has lead to a massive upsurge in the availability of block copolymers. Block copolymer synthesis forms a significant part of most reviews on living polymerization processes. This section focuses on NMP,106 A TRP,265,270 and RAFT.- 07 Each of these methods has been adapted to block copolymer synthesis and a substantial part of the literature on each technique relates to block synthesis. Four processes for block copolymer synthesis can be distinguished. 

Graft copolymers made by living polymerization processes are often called polymer brushes because of the uniformity in graft length that is possible. The basic approaches to graft copolymers also have some analogies with those used in making block and star copolymers. 

Combining control over architecture with control over the stereochemistry of the propagation process remains a holy grail in the field of radical polymerization. Approaches to this end based on conventional polymerization were described in Chapter 8. The development of living polymerization processes has yet to substantially advance this cause. 

This reaction mechanism is able to account for several characteristics shown by this reaction. First of all the existence of a terminal group (e.g. -PCl3 ) that remains reactive after completing the consumption of the phosphoranimine classifies this reaction in the category of the living polymerization processes. This fact has important consequences that can be summarized below ... 

Living polymerization processes leading to linear copolymeric precursors, with either identical or complementary functional end groups (X,Y) have... 

Another very important visible light-initiated reaction of alkyl aluminum porphyrins is their 1,4-addition to alkyl methacrylates to produce ester enolate species [Eq. (4)]. This enolate then acts as the active species in the subsequent polymerization of the acrylate monomer. For example, Al(TPP)Me acts as a photocatalyst to produce polymethylmethacrylate with a narrow molecular weight distribution in a living polymerization process [Eq. (4)]. Visible light is essential for both the initiation step (addition of methylmethacrylate to Al(TPP)Me) and the propagation... 

C. Alkylaluminum Porphyrins as Initiators for Living Polymerization Processes... 

Yokozawa, T. Yokoyama, A. Chain-growth polycondensation The living polymerization process in polycondensation. Prog. Polym. Sci. 2007, 32, 147-172. 

TPX is polymerized commercially by a Ziegler-Natta polymerization. Mostly copolymers are on the market. Besides a Ziegler-Natta polymerization, also a living polymerization process has been reported. 

Figure 6.36. Schematic of the formation of gold nanoparticles coated with free-radical polymerization initiators that subsequently yield Au polymer nanostructures through a surface-controlled living polymerization process. Reproduced with permission from Ohno, K. Koh, K.-M. Tsujii, Y Fukuda, T. Macromolecules 2002, 35, 8989. Copyright 2002 American Chemical Society.

In living polymerization processes it is in fact possible to adjust the final degree of polymerization by simply tuning the initiator amount while keeping narrow the chain length distribution (CLD), i.e. with polydispersity values lower than 1.3 (this quantity reflects the broadness of the final CLD typical minimum values in FRP range from 1.5 to 2). Also, by suitable selection of chemistry and structure... 

Living polymerization processes pave the way to the macromolecular engineering, because the reactivity that persists at the chain ends allows (i) a variety of reactive groups to be attached at that position, thus (semi-)telechelic polymers to be synthesized, (ii) the polymerization of a second type of monomer to be resumed with formation of block copolymers, (iii) star-shaped (co)polymers to be prepared by addition of the living chains onto a multifunctional compound. A combination of these strategies with the use of multifunctional initiators andtor macromonomers can increase further the range of polymer architectures and properties. 

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... 

Following photochemical production of the initiator moiety from the iodonium salt, sequential reaction with monomer moieties proceeds until exhaustion of available monomer. This living polymerization process should be characterized by the following expressions ... 

Some coordination polymerizations also exhibit the character of a living polymerization process. For example, Natta32 described a heterogeneous catalyst yielding block polymers when the initially present monomer was replaced by another one. In that system, the lifetime of a growing polymer exceeds one-half hour, i.e. the termination caused by the hydride transfer to the catalytic center was very slow. Similar results were reported by Bier33 and by Kontos et al.34. ... 

The past decade has witnessed the explosive development of controlled/living polymerization processes that allow synthesis of macromolecules with precisely controlled architecture, molar mass, and functionality from a wide range of... 

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