Control of molar mass and MMD

The requirement for water-soluble initiator incited other groups to start the polymerisation with a bicomponent initiating system using a water-soluble radical initiator in conjunction with TEMPO or a derivative (Marestin et al, 1998 Gao et al, 2001). For instance, Marestin et al (1998) studied the ab initio emulsion polymerisation of styrene 

In a similar work, Cao etal. (2001) also assessed the effect of hydrophiUdty of TEMPO-based nitroxides for batch emulsion polymerisations of styrene conducted at 120°C in the presence of a water-soluble initiator. Among the studied nitroxides, only the acetoxy derivative (N7) achieved living polymerisation. Like the previous example, this result was ascribed to an optimised balance between the hydrophilic and the hydrophobic character of the nitroxide. In contrast to the previous study, stable latexes with small particle size (below 100 nm) and good regularity were obtained. This was despite the possibility of self-initiation in the monomer droplets, which unfortunately was not discussed by the authors. 

Ab initio emulsion polymerisations of styrene were also conducted at 90°C, using the stable acyclic phosphonylated nitroxide radical SGI (NIO) as a mediator together with a water-soluble redox initiator (Lansalot et al, 2000). A long induction period was observed, assigned to the formation of water-soluble alkoxyamines before nucleation. In this system, molar mass of the polymer increased with conversion following the theoretical line, but the MMD was rather broad (PDI between 2.0 and 2.5). Rather small particles were obtained (average diameter was 120 nm) with a broad particle size distribution. It was also found that a few per cent of coagulum formed usually. 

From these few reports, it can be concluded that nitroxide-mediated LRP is still a difficult process to achieve in db initio emulsion polymerisation. This is the direct consequence of the complex nucleation step and of an extensive droplet nucleation (Cunningham, 2003). Thus, the various groups decided to focus on miniemulsion polymerisation, in which such difficulties are naturally overcome. 

Monomer-soluble radical initiator in conjunction with free nitroxide 

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As in Nd-catalyzed solution processes in gas-phase polymerization of BD regulation of molar mass is a serious problem as there are no agents for the control of molar mass readily available. Vinyl chloride and toluene are no viable options. Vinyl chloride is ruled out due to ecological reasons and toluene is not applicable due to low transfer efficiencies and the required low concentrations if applied in a gas-phase process. For the control of molar mass and MMD in the polymerization of dienes a combination of different methods is recommended [457,458] (1) temperature of polymerization, (2) partial pressure of BD, (3) concentration of cocatalyst (or molar ratio of Al/MNd)> (4) type of cocatalyst, (5) residence time of the rare earth catalyst in the polymerization reactor. 

As variations in Ai/ Nlj-ratios are used for the control of molar mass and the MMD this aspect is addressed in separate sections on molar mass regulation (Sect 2.2.8 and 4.5). 

RAFT in emulsion polymerization The development of RAFT in true emulsion polymerization processes was more challenging than in miniemulsion. A general difficulty of RAFT in aqueous dispersed systems, and particularly emulsion polymerization, is related to the need for a radical initiator in conjunction with the RAFT agent. Consequendy, it is not always easy to control the locus where reversible transfer will take place, and this may have important and sometimes deleterious consequences on the control over molar mass and MMD. Again, the most important parameters to consider are both the water solubility and the reactivity of the chain transfer agent. 

When using a water-soluble initiator in conjunction with a monomer-soluble nitroxide, the polymerisation starts in the aqueous phase and should be transported towards the monomer droplets in the early stage of the polymerisation, via entry of the oligoradicals or the oligomeric alkoxyamines generated in the aqueous phase. This might affect the polymerisation kinetics, the initiator efficiency and hence the control over molar mass and MMD. 

Variations of the amount of cocatalyst which are usually expressed by the molar ratio W Nd have a significant influence on polymerization rates, molar masses, MMDs and on the microstructures of the resulting polymers. These aspects are addressed in the following sections with a special emphasis on ternary catalyst systems. For ternary systems it has to be emphasized, however, that in many reports the ratio Ai/ Nd only accounts for the amount of aluminum alkyl cocatalyst and not for other Al-sources such as alkyl aluminum halides. Variations of the Ai/ Nd-ratios are also used for defined control of molar mass. This aspect is addressed in separate sections (Sects. 2.2.8 and 4.5). 

In a scientific paper on the control of molar mass in gas-phase polymerization, the importance of Ai/ Nd is also emphasized, hi addition, a combination of the cocatalysts TIBA and DIBAH is recommended. By the use of two aluminum alkyl compounds the concentration ratio of two different active Nd-species is adjusted. As these two species produce different molar masses and MMDs the combination of TIBA and DIBAH allows for the control of these two parameters [229,230]. 

The MMD of a polymer is of prime importance in its application. In most instances, there is a molar-mass (MM) range for which a given polymer property will be optimal for a particular application. The control of molar mass (expressed in g/mole) and of its distribution is essential for the practical application of a polymerization process, since its utility is greatly reduced unless the reaction can be carried out to yield polymer of a sufficiently high and specified molar mass. 

Control of the molar mass and MMD using LRP has important implications on the mechanical properties of the final polymer, making these techniques highly attractive for industrial applications. 

The number average molar mass, Mn, and molar mass distribution (MMD) are controlled by interplay of kinetic parameters, which will be described in more detail in Chapter 5. In principle there are two ways of terminating a growing chain bimolecular termination and transfer. These two modes of termination have led to the two main categories of living radical polymerization (a) reversible termination and (b) reversible chain transfer. 

Vicente et al. [30] used the heat of reaction and the open-loop observers developed in Section 7.2.5.3 to determine the concentration of monomer and CTA and hence to infer the instantaneous number-average molar masses during emulsion homo- and copolymerization reactions. In addition, the authors used the inferred values for online control of the molar mass distributions of copolymers with predefined distributions. They demonstrated that polymer latexes with unimodal MMD with the minimum achievable polydispersity index in free-radical polymerization (PI = 2) and bimodal distributions could be easily produced in linear polymer systems [15, 30]. 

The present chapter reviews recent developments (work published in 1997 and later) in the synthesis of model block copolymers with a primary focus on ionic polymerisations. During this period controlled radical polymerisation techniques have attracted considerable interest and are emerging as a new method providing the synthesis of model polymers and copolymers. It is not the purpose of this chapter to cover this development since ionic methods still allow for better control of the polymers synthesised. Radical polymerization methods are the subject of Chapter 3. The question of how important differences in the widths of the molar mass distributions are has prompted the inclusion of a section on the MMD of model block copolymers. 

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