Precipitated macroradicals

Relatively stable macroradicals are precipitated when they are insoluble in their monomers. Thus, poly (vinyl chloride) has been obtained by a process in which the solid polymer was removed continuously as it precipitated from the monomer (23). These precipitated macroradicals have been described as popcorn (21) or trapped free radicals (22). Macroradicals obtained by the polymerization of acrylonitrile which have been widely studied (4) have been used to prepare block copolymers (35). 

Some of the difference in average molecular was attributable to the continued propagation of the precipitated macroradicals. In addition, some coupling of the macroradicals probably occurred when they were dissolved in tetrahydrofuran to obtain the gel permeation chromato-... 

Stage 2 Aggregation of precipitated macroradicals and macromolecules produces microdomains (10-20 nm) at less than 0.01% conversion. Stage 3 Aggregation of microdomains produces domains (100-300 nm). 

Stage 2 Microdomains (0.10-0.02 jxm, <0.01% conversion) are produced by the aggregation of precipitated macroradicals and macromolecules. 

When the emulsion polymerization is conducted in the absence of an emulsifier, this process is termed emulsifier free or soapless emulsion polymerization [68-73]. In this case, the particle formation occurs by the precipitation of growing macroradicals within the continuous... 

When a monomer such as acrylonitrile is polymerized in a poor solvent, macroradicals precipitate as they are formed. Since these are living polymers, polymerization continues as more acrylonitrile diffuses into the precipitated particles. This heterogeneous solution polymerization has been called precipitation polymerization. 

The analysis of the reaction serum (the continuous phase without polymer particles) at the end of polymerization led to the conclusion that the molecular weight of the soluble oligomers of styrene and PEO macromonomer varied from 200 to 1100 g mol-1. This indicates that the critical degree of polymerization for precipitation of oligomers in this medium is more than ten styrene units and only one macromonomer unit per copolymer chain. Several reasons for the low molecular weight of the soluble copolymers were proposed, such as the thermodynamic repulsion (or compatibility) between the PEO chain of the macromonomer and the polystyrene macroradical, the occurrence of enhanced termination caused by high radical concentration, and, to a lower extent, a transfer reaction to ethanol [75]. 

In regions poor in initiator, initiation occurs slowly. The propagation reaction is enhanced by high monomer concentration and leads to macromolecular chains. However, their length is limited by the presence of alcohol molecules which inhibit the chain growth either by dilution or by precipitation of macroradicals. Low-molecular-... 

The free-radical kinetics described in Chapter 6 hold for homogeneous systems. They will prevail in well-stirred bulk or solution polymerizations or in suspension polymerizations if the polymer is soluble in its monomer. Polystyrene suspension polymerization is an important commercial example of this reaction type. Suspension polymerizations of vinyl ehloride and of acrylonitrile are described by somewhat different kinetic schemes because the polymers precipitate in these cases. Emulsion polymerizations aie controlled by still different reaetion parameters because the growing macroradicals are isolated in small volume elements and because the free radieals which initiate the polymerization process are generated in the aqueous phase. The emulsion process is now used to make large tonnages of styrene-butadiene rubber (SBR), latex paints and adhesives, PVC paste polymers, and other produets. 

While it is assumed that termination by coupling takes place when maleic anhydride and styrene are copolymerized in a good solvent such as acetone, insoluble macroradicals precipitate when these monomers are copolymerized in a poor solvent such as benzene (7). Insoluble macroradicals obtained by bulk polymerization of acrylonitrile (1, 11) and the solution copolymerization of maleic anhydride and styrene in benzene (7) have been used as seeds for the preparation of block copolymers. 

Stage 1 Macroradicals or macromolecules with a chain length of 10-20 units starts to precipitate from the monomer phase at less than 0.001% conversion. 

The above examples are all heterogeneous solution polymerizations. Heterogeneous solution polymerization occurs in most cases when the Hildebrand solubility parameter values of the solvent and polymer differ by at least 1.8h(122). The presence of macroradicals in these precipitated polymers has been demonstrated by electron spin resonance (esr). 

When acrylic acid or vinylpyrrolidone was heated with a precipitated acrylonitrile polymer believed to contain macroradicals, block like polymers were formedLikewise poly(acrylonitrile-b-methyl methacrylate) was reportedly produced when acrylonitrile was heated with precipitated methyl methacrylate polymer. Acrylonitrile-styrene copolymers however, were formed only when styrene and acrylonitrile precipitated polymer were heated in the presence of DMF. Presumably the DMF swells the polyacrylonitrile permitting diffusion of styrene monomer into the free radical containing coils. Block copol3miers of methyl methacrylate and various vinyl monomers have also been reported prepared by heating monomers and methyl methacrylate macroradicals in l-propanol(126). Interestingly poly-... 

After 96 hrs., either methyl methacrylate (MMA), acrylonitrile (AN), styrene (S), or vinylpyrrolidone (VP)(1 ml) was added to the precipitated SAN macroradicals, and allowed to react 96 additional hours. Other S-AN-ZnCla copolymerizations in the respective molar ratios of 5 10 1 (0.02 0.04 0.004 moles) were carried out in tert.-butyl alcohol (200 ml) with tBPP (0.15 g). The polymerizations were carried out at 50°C and 70°C in amber bottles for 4 hrs. 

The block copolymers obtained from the reaction of SAN macroradicals were purified by solvent fractionation. In general the gross polymer was dissolved in a suitable solvent and then fractionally precipitated by adding a poor solvent. 

Various polymers including poly(acrylonitrile), poly(vinyl chloride) and poly (acrylic acid) are insoluble in their monomers. In these cases, growing chains precipitate at a critical chain length and termination does not occur via mutual coupling of macroradicals. Then, V > Vt and Vpoi is proportional to D with 0.5 < p < 1.0. 

The total activation energy for radiation polymerization varies within the limits of 0-41.8 kJ/mol. A small value of this energy causes that the radiation polymerization reaction rate is temperature independent within a few tens of Kelvins. A characteristic feature of the radiation polymerization is the presence of the retrospective effect (post-effect). It consists on the fact that after the cessation of irradiation the polymerization continues over many hours. This phenomenon concerns primarily the polymerization in solid phase or in solution precipitation. The reason for this phenomenon is the reduced mobility of macroradicals in solid phase and difficulties ending the chains by recombination. 

In order to increase the lifetime of radicals, Richards proposed to polymerize a monomer which is a non-solvent for its polymer. In these conditions, a high content of macroradicals are trapped in the precipitated matrix and may be swollen by an appropriate solvent containing a reducing agent, such as sodium naphthalene, which allows radical to anion transformation. 

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