Polymerisation molar mass distribution

Fast high-pressure fractionation of tails in the molar-mass-distribution and/or the chemical-composition-distribution of a (co)polymer [47] during the polymerisation process will indicate eventual drifts in (co)polymer composition. Several important polymer properties depend strongly on such tails in the (co)polymer distribution. On--line fractionation data are strongly needed. 

The breakthrough in metallocene catalyst development occurred in the early 1980s when a metallocene catalyst, instead of an aluminium alkyl, was combined with methylaluminoxane (MAO) [8, 9, 10]. This catalyst system boosted the activity of metallocene-based catalyst and produced uniform polyethene with the narrow molar mass distribution typical for single-site catalysts. Efforts to polymerise propene failed, however the product was found to be fully atactic, indicating complete lack of stereospecificity of the catalyst [10]. 

The polymerisation of vinyl acetate in microemulsions stabilised with cetyltrimethylammonium bromide surfactant was investigated as a function of concentrations of monomer and initiator and temperature. Molar mass distributions, phase behaviour and latex characterisations were analysed. 44 refs. 

The polymerisation reactions are statistically driven processes. Therefore, unlike some natural polymers such as DNA, synthetic polymers always show, due to the reaction mechanisms involved in the production processes, a certain distribution of molar mass and not a distinct molecular weight. The molar mass of S5mthetic polymers can range from some thousand g/mol up to some million g/mol. As an example. Figure 1.4 shows the normalised molar mass distribution (MMD) curves of two different polyethylene samples. 

For a typical condensation polymerisation, the molar mass distribution function is generally in the range 3—20, but is sometimes even greater. On the other hand, in vinyl polymerisation the values typically wiU be in the range 1.05—3.0. The narrowest molar mass distributions are observed with anionic and certain cationic initiated polymerisations. Molar mass effects are observed with aU polymer systems but they are more important in the physical properties of amorphous polymers than in their crystalline analogues. 

For a technical polymer, the molar mass distribution (MMD) is a direct result of the statistics of the polymerisation process. In the process itself, it results from ... 

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. 

Step polymerisation Monomer concentration drops rapidly to zero early in the reaction. Polymer molar mass rises steadily during reaction. Long reaction times increase molar mass and are essential to obtain very high molar masses. At all stages of the reaction every possible molecular species from dimers to polymers of large degrees of polymerisation are present in a calculable distribution. 

Typical chain transfer reactions involve the abstraction of an atom from a neutral saturated molecule, which may be solvent or a chain transfer agent added to the polymerisation mixture specifically to control the final size and distribution of molar masses in the polymer product. The chain transfer reaction may be represented as in Reaction 2.7. 

The macromonomer route assures uniform distribution of dendrons along the polymer backbone. Acrylates and styrenes with pendant dendrons proved to be well suited for polymerisation [57]. However, steric hindrance between monomers bearing higher-generation dendrons and the spatial requirements of the end of the chain cause problems The monomer reacts with the chain end only in the case of slight, if any, steric hindrance. Thus the method only provides access to dendronised linear polymers with relatively low molar masses... 

Polymers consist of long chains of varying length that cannot be characterised by a constant molar mass. Indeed, cross-linked polymers can be thought of as a single molecule, so that the molar mass is the total mass of the object. However, it is helpful to have a measure of the degree of polymerisation or the distribution of chain lengths in a polymer, and... 

In section 1.3.1 it is pointed out that polymers, unlike other chemical compounds, do not have fixed molar masses. The molar masses are very high and there is a distribution of molar masses, or chain lengths, that depends on the polymerisation conditions. The distribution of molar masses can have a profound influence on the physical properties of the polymer, so it is important to have methods of specifying and determining the distribution. 

The simplest explanation is that there is a rubber-like network present and that this has a maximum extensibility due to the degree of entanglement, which is constant for a given grade of polymer and depends on its molar mass and method of polymerisation. This limiting extensibility is not to be confused with the limit of applicability of the affine rubber model for predicting orientation distributions discussed in section 11.2.1 because the limiting extension can involve non-affine deformation. 

PAM Macromolecules The macromolecules were made of poly (acrylamide), thereafter abbreviated PAM. They were synthesized according to a free radicalar polymerisation process. We have used two types of macromolecules. Their average molar mass are Mw(l) =1.5 10 g/mol and Mw(2) = 6 10 g/mol, and their mass distribution index are Mw/Mn(l) = 2 and Mw/Mn(2) = 4. 

Transesterification reactions could occur during anionic ROP of six-membered carbonates (Scheme 4.5). The intramolecular nucleophilic attack on carbonyl carbon atom (back-biting) leads to cyclic oligomers. The control of the polymerisation is rather poor, and bimodal distribution of molar masses is often observed (Matsuo et al., 1998b Pahovnik and Hadjichristidis, 2015). 

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. 

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