Physical Forms of Polymerisation
After discussing the chemical forms of polymerisation such as step and chain polymerisations, this section will discuss the physical forms of polymerisation. In general, there are 6 physical forms of polymerisation which monomers can be polymerised. In some cases, the polymerisations have a better size control which makes it attractive for industry.
The simplest case of polymerisation is bulk polymerisation which only consists of monomer and initiator (or catalyst in some cases). However, chain polymers are extremely exothermic which means difficult to control the heat evolved in during the bulk polymerisation of vinyl monomers. This highly exothermic can be rationalised with thermodynamic equation (ΔG= ΔH - TΔS) where during the polymerisation the entropy is negative so to make the polymerisation proceed spontaneously enthalpy must be negative, i.e. exothermic process.
Besides that, highly viscous polymerising mixutre, even at low conversion, can cause a stirring problem which can cause local heat spots to occur. These local heat spots can lead to degradation or discolouration of polymers and MWD becomes very broad.
Therefore, bulk polymerisation is usually used for less exothermic step polymerisation with exceptions such as chain polymerisation of poly(methyl methacrylate) or PMMA for plastic baths, contact lenses, and dental fillings and poly(ethylene oxide) (PEO) on an industrial scale.
To address the problem of overheating, a solvent can be used in polymerisation process which leads to second type of physical forms of polymerisation which is solution polymerisation. In this process, solvent acts as a diluent and dissipates the heat of polymerisation. With this process, a low viscosity of polymerising mixture can be achieved so it is easier to stir the mixture. However, this process can get chain-transfer of radicals to solvent which would lower Mn. and requires additional step to remove the solvent in order to isolate the polymer. Solution polymerisation can be used for most of living anionic polymerisation such as Kraton process for thermoplastic elastomers and free radical polymerisation of vinyl acetate and acrylic esters in various organic solvents.
The other type of polymerisation is precipitation polymerisation where the monomer and initiator is water-soluble but the polymer is insoluble. With the insoluble polymer is produced in this process, there is no viscosity problem and also a better control over thermal dissipation. However, this process has poor control over molecular weight and also it is difficult to achieve high MW. Precipation polymerisation can be used in some cases such as polymerisation of pyrrole in water to form conducting polymer polypyrrole and polymerisation of styrene in either n-hexane or methanol.
Precipitation polymerisation can also be done in the presence suitable polymer stabiliser such as poly(N-vinyl pyrrolidone) (PNVP) or poly(vinyl alcohol) (PVA) and this form of polymerisation is called dispersion polymerisation. Therefore, this polymerisation has the advantages of good control over heat dissipation and viscosity and it can be carried in aqueous or non-aqueous media.
In this polymerisation, stabilising polymer adsorbs onto precipitating polymer nuclei and prevents further growth via steric stabilisation. Hence, microscopic particles of polymer are suspended in solvent that do not aggregate further. These particles are called latexes and it has typical range between 0.1 to 10 μm, in some cases it could have size range in order of nanometre. This process is widely used to make solvent-based (non-aqueous) latexes for gloss paint with other applications ranging from coating for textiles to floor polish.
In constrast with dispersion polymerisation, suspension polymerisation consists of water-immiscible monomer, water-insoluble initiator, water-soluble polymeric stabiliser and water as solvent. This process is suitable for monomers such as vinyl chloride, vinyl alcohol and methyl methacrylate with common stabilisers such as PVA or cellulosic derivatives. In this process, the polymerisation occurs within the droplets containing monomer and efficient stirring is important in this process to achieve polydisperse polymer 'beads'. Besides that, this process has some industrial utility as water is used as solvent which is cheap and has good heat capacity and also low viscosity since beads is insoluble polymer.
The final type of polymerisation is emulsion polymerisation where it consists of water-immiscible monmer, water-soluble initiator, surfactant and water as solvent. This method is preferred for co-polymerising many vinyl monomers.
In this polymerisation, monomers are in big droplets which are stabilised by surfactants to make it soluble in water. Besides that, surfactan forms micelles in this polymerisation which act as the locus of polymerisation in this process. Some of monomers diffuse into the solvent and form monomer-swollen micelles. During the heating, initiator decomposes to form radical molecules which then diffuse into the monomer-swollen micelles and then monomer feeds growing particles. In this process, 99% of polymerisation happens in the micelles and no more than one polymer radical in one micelle. Besides that, at any given time during polymerisation, the polymerising micelles contain either one (growing) or zero (dormant) polymer radical.
In emulsion polymerisation, the kinetics changes dramatically due to compartmentalisation of polymerisation. Normally, the MW of polymers is inversely proportional to the rate of polymerisation but in emulsion polymerisation high MW polymer can be achieved at high reaction rates due to Smith-Ewarts kinetics.
Emulsion polymerisation is widely used and also very common technique in big chemical companies due to several advantages:
Hence, the overall process of emulsion can be seen from two point of views. Chemically, the monomer forms growing polymer chain with a radical one end and anionic initiator fragment at the other end. Besides that, physically monomer-swollen micelles and styrene-in-water emulsion forms a surfactant-stabilised polymer latex particles with the surface charge due to surfactant and/or initiator fragment.
The simplest case of polymerisation is bulk polymerisation which only consists of monomer and initiator (or catalyst in some cases). However, chain polymers are extremely exothermic which means difficult to control the heat evolved in during the bulk polymerisation of vinyl monomers. This highly exothermic can be rationalised with thermodynamic equation (ΔG= ΔH - TΔS) where during the polymerisation the entropy is negative so to make the polymerisation proceed spontaneously enthalpy must be negative, i.e. exothermic process.
Besides that, highly viscous polymerising mixutre, even at low conversion, can cause a stirring problem which can cause local heat spots to occur. These local heat spots can lead to degradation or discolouration of polymers and MWD becomes very broad.
Therefore, bulk polymerisation is usually used for less exothermic step polymerisation with exceptions such as chain polymerisation of poly(methyl methacrylate) or PMMA for plastic baths, contact lenses, and dental fillings and poly(ethylene oxide) (PEO) on an industrial scale.
To address the problem of overheating, a solvent can be used in polymerisation process which leads to second type of physical forms of polymerisation which is solution polymerisation. In this process, solvent acts as a diluent and dissipates the heat of polymerisation. With this process, a low viscosity of polymerising mixture can be achieved so it is easier to stir the mixture. However, this process can get chain-transfer of radicals to solvent which would lower Mn. and requires additional step to remove the solvent in order to isolate the polymer. Solution polymerisation can be used for most of living anionic polymerisation such as Kraton process for thermoplastic elastomers and free radical polymerisation of vinyl acetate and acrylic esters in various organic solvents.
The other type of polymerisation is precipitation polymerisation where the monomer and initiator is water-soluble but the polymer is insoluble. With the insoluble polymer is produced in this process, there is no viscosity problem and also a better control over thermal dissipation. However, this process has poor control over molecular weight and also it is difficult to achieve high MW. Precipation polymerisation can be used in some cases such as polymerisation of pyrrole in water to form conducting polymer polypyrrole and polymerisation of styrene in either n-hexane or methanol.
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| Polymerisation of pyrrole |
Precipitation polymerisation can also be done in the presence suitable polymer stabiliser such as poly(N-vinyl pyrrolidone) (PNVP) or poly(vinyl alcohol) (PVA) and this form of polymerisation is called dispersion polymerisation. Therefore, this polymerisation has the advantages of good control over heat dissipation and viscosity and it can be carried in aqueous or non-aqueous media.
In this polymerisation, stabilising polymer adsorbs onto precipitating polymer nuclei and prevents further growth via steric stabilisation. Hence, microscopic particles of polymer are suspended in solvent that do not aggregate further. These particles are called latexes and it has typical range between 0.1 to 10 μm, in some cases it could have size range in order of nanometre. This process is widely used to make solvent-based (non-aqueous) latexes for gloss paint with other applications ranging from coating for textiles to floor polish.
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| Polypyrrole-PNVP latex |
In constrast with dispersion polymerisation, suspension polymerisation consists of water-immiscible monomer, water-insoluble initiator, water-soluble polymeric stabiliser and water as solvent. This process is suitable for monomers such as vinyl chloride, vinyl alcohol and methyl methacrylate with common stabilisers such as PVA or cellulosic derivatives. In this process, the polymerisation occurs within the droplets containing monomer and efficient stirring is important in this process to achieve polydisperse polymer 'beads'. Besides that, this process has some industrial utility as water is used as solvent which is cheap and has good heat capacity and also low viscosity since beads is insoluble polymer.
The final type of polymerisation is emulsion polymerisation where it consists of water-immiscible monmer, water-soluble initiator, surfactant and water as solvent. This method is preferred for co-polymerising many vinyl monomers.
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| Schematic diagram of emulsion polymerisation |
In emulsion polymerisation, the kinetics changes dramatically due to compartmentalisation of polymerisation. Normally, the MW of polymers is inversely proportional to the rate of polymerisation but in emulsion polymerisation high MW polymer can be achieved at high reaction rates due to Smith-Ewarts kinetics.
Emulsion polymerisation is widely used and also very common technique in big chemical companies due to several advantages:
- good thermal and viscosity control,
- very high monomer conversion so minimal organic waste,
- water is cheap, non-toxic, non-flammable, and high heat capacity,
- it has low VOC which means more environmentally friendly.
Hence, the overall process of emulsion can be seen from two point of views. Chemically, the monomer forms growing polymer chain with a radical one end and anionic initiator fragment at the other end. Besides that, physically monomer-swollen micelles and styrene-in-water emulsion forms a surfactant-stabilised polymer latex particles with the surface charge due to surfactant and/or initiator fragment.
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