Heterogeneous Polymerisation

Next to homogeneous reaction conditions, multi-phase or heterogeneous polymerisation conditions frequently occur. Suspension and emulsion polymerisation are examples, but also condensation polymerisation with phase separation of water during cure. The low-temperature production of inorganic polymer glasses (IPGs) is a special case of suspension polymerisation involving clay particles in a reactive silicate solution. 

Starting from the cure reaction mechanism, a proper cure rate law, describing the evolution of the system from initial to final state, can be proposed. In the case of a mechanistic approach, in which the reaction model consists of a set of chemical reaction steps, a set of (stiff) coupled differential equations has to be solved to describe the evolution of the important reacting species of the system. In this case, effects of the composition of the fresh reaction mixture (such as a stoichiometric unbalance of resin and hardener, the concentration of accelerator, initiator or inhibitor) and the influence of additives (such as moisture and fibres in composites) can be studied. Because this set of equations may be rather complex and/or even partly unknown, various simplifications have to be made. 

A simplified mechanistic model for the epoxy-anhydride cure of Reaction scheme 3 is given in Reaction scheme 5. 

In this reaction mechanism, the initiation step is slow, whereas the acylation step is much faster than the esterification step [36,37]. Based on this reaction scheme, the concentration dependence of initiator, anhydride and epoxy are described. 

For the free radical copolymerisation of unsaturated polyester resins containing an inhibitor, the following simplified mechanism could be used. 

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We may wish to control whether most of the cations are unpaired with an anion, or paired, or unpaired but complexed, or paired with an anion which is held in the surface of a crystal, as in a heterogeneous polymerisation. 

Even though the subject of heterogeneous polymerisation can rightly be considered mature, there remain many unresolved issues and the need for fundamental research is substantial, both in more completely understanding existing processes and in the development of new routes to polymer colloids. 

The field of controlled radical polymerisation (CRP) has seen rapid growth during the past decade, growth that in more recent years has begun to embrace work on CRP in dispersed media. The two-phase nature of heterogeneous polymerisations, however, imposes severe constraints on what can be achieved and restricts the scope for extending the established methods of CRP to heterogeneous systems. 

Emulsion polymerisations are heterogeneous polymerisation processes, which unlike bulk or solution polymerisations, are not uniform in composition throughout the reaction medium, but rather contain polymerising colloidal particles dispersed within an otherwise inert aqueous environment. There are three... 

Suspension polymerisation (49,298) is a heterogeneous polymerisation process that is related to, but is not the same as, emulsion polymerisation. Suspension polymerisation differs from conventional emulsion polymerisation in that the droplets are much larger, and an oil-soluble initiator is used to start polymerisation within the droplets. To prepare a suspension, monomers with relatively low water-solubility are dispersed along with a stabiliser in water by stirring. The stabiliser... 

A feature of condensation polymerisation in dispersion is the fast rate of reaction relative to bulk condensation processeso The heterogeneous polymerisation can often be carried out 50-70 C below the normal bulk polymerisation temperatureo... 

A further discussion of the living radical polymerisation techniques and its application in heterogeneous polymerisation techniques can be found in Chapter 5. 

Although LRP techniques are well understood in bulk or solution, in heterogeneous polymerisations the already complex kinetics are further complicated by partitioning of the activating species in the various environments and by the rate of transportation of these... 

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