Polymerisation Precipitation... Suspension

The first study of this kind was carried out by Chinese researchers in 2003 [144]. They prepared MIP beads for the SPE of tyrosine by simple suspension in water as well as by two-step swelling and suspension polymerisation. They found no substantial difference in the rebinding capacity of the beads prepared by the two methods. A more thorough analysis of various synthetic approaches to MIP beads was conducted a year later by Perez-Moral and Mayes [145]. They took a standard monomer mixture with propranolol as the template molecule and polymerised it by bulk polymerisation, suspension polymerisation, precipitation polymerisation, two-step-swelling polymerisation and emulsion core-shell polymerisation (see also Sect. 2.2.3). Care was taken to keep the polymerisation... 

For both suspension and mass polymerisations at less than 2% conversion, PVC precipitates from its monomer as stable primary particles, slightly below 1-p.m dia (4,10—12). These primary particles are stabilised by a negative chloride charge (4,13). Above 2% conversion, these primary particles agglomerate. Sectioning the PVC grains of either suspension or mass resins readily shows the skins primary particles at 1-p.m dia, and agglomerates of primary particles at 3—10-pm dia (4,7,8,14). 

Hydroxy propyl cellulose, like methyl cellulose, is soluble in cold water but not in hot, precipitating above 38°C. It was introduced by Hercules in 1968 (Klucel) for such uses as adhesive thickeners, binders, cosmetics and as protective colloids for suspension polymerisation. The Dow company market the related hydroxypropylmethyl cellulose (Methocel) and also produce in small quantities a hydroxyethylmethyl cellulose. 

Dispersion polymerisation may be considered a heterogeneous process which may include emulsion, suspension, precipitation and dispersion polymerisation. In dispersion and precipitation polymerisation, the initiator must be soluble in the continuous phase, whereas in emulsion and suspension polymerisation the initiator is chosen to be soluble in the disperse phase of the monomer. A comparison of the rates of polymerisation of MMA at 80 C for the three systems was given by Barrett and Thomas [11], as illustrated in Figure 17.10. The rate of dispersion polymerisation is much faster than either precipitation or solution polymerisation. TTie enhancement of the rate in precipitation polymerisation over... 

A suspension of droplets of vinyl chloride monomer (VCM) in water, of size 30-150 iJim, is formed by agitation with a stirrer. It is stabilised by a colloidal layer of partially hydrolysed polyvinyl acetate or other water-soluble polymer. When the suspension is polymerised at a temperature in the range 50-70 °C, PVC molecules form in the VCM droplets. As PVC is insoluble in VCM, it precipitates in the form of primary particles, initially... 

An examination of an upstream process may be worthwhile in that a small but valuable change in particle or liquid properties might be possible at httle cost. Processes such as precipitation, polymerisation or crystallisation are amongst those that would merit reconsideration if the output suspension were not readily sq>arated. 

Previously published work on the precipitation of polymer gel particles in the monomer phase during bulk and suspension polymerisation is reviewed by Ugelstad et al (9). More recent contributions relating to parameters which influence tVie colloidal stability of the precipitated polymer are considered here. 

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