Commercial polymer radical chain polymerization

Anionic polymerization offers fast polymerization rates on account of the long life-time of polystyryl carbanions. Early studies have focused on this attribute, most of which were conducted at short reactor residence times (< 1 h), at relatively low temperatures (10—50°C), and in low chain-transfer solvents (typically benzene) to ensure that premature termination did not take place. Also, relatively low degrees of polymerization (DP) were typically studied. Continuous commercial free-radical solution polymerization processes to make PS, on the other hand, operate at relatively high temperatures (>100° C), at long residence times (>1.5 h), utilize a chain-transfer solvent (ethylbenzene), and produce polymer in the range of 1000—1500 DP. 

RADICAL CHAIN POLYMERIZATION 3-14 SPECIFIC COMMERCIAL POLYMERS... 

Free radical chain-growth addition polymerization of vinyl monomers is an important route to commercial polymers. The chain-growth mechanism involves three steps initiation of a chain, propagation of the growing chain, and termination of the reactive intermediates. 

From an industrial stand-point, a major virtue of radical polymerizations is that they can often be carried out under relatively undemanding conditions. In marked contrast to ionic or coordination polymerizations, they exhibit a tolerance of trace impurities, A consequence of this is that high molecular weight polymers can often be produced without removal of the stabilizers present in commercial monomers, in the presence of trace amounts of oxygen, or in solvents that have not been rigorously dried or purified, Indeed, radical polymerizations are remarkable amongst chain polymerization processes in that they can be conveniently-conducted in aqueous media. 

Another class of chain scission positive resists is the poly(olefin-sulfones). These materials are alternating copolymers of an olefin and sulfur dioxide, prepared by free radical solution polymerization. The relatively weak C-S bond, 60 kcal/mole compared with 80 kcal/mole for a carbon-carbon bond, is readily cleaved upon irradiation (Gs values for these polymers are typically 10), and several sensitive resists have been developed based on this chemistry (53). One material that has been made commercially available is poly (butene-1-sulfone) (54). 

Polypropylene (PP) is a stereospecific polymer prepared by polymerization using organo-metallic catalyst system. Commercial PPs have up to 95% isotactic content, which means that pendant methyl groups are almost all on the same side of the chain. When PP is exposed to ionizing radiation, free radicals are formed that cause chemical changes. Because PP is highly crystalline, these radicals are relatively immobile and consequently may not be available for reaction for long periods of time.96... 

Chain polymerizations are less often performed in die bulk, because of problems with the control of the reaction. [An interesting exception is poly(methyl methacrylate), a polymer that is soluble in its own monomer (not all polymers are), and which is synthesized commercially by chain (free radical) polymerization very slowly in bulk (Figure 3-44). The resulting polymer has outstanding optical properties (clarity) because there are very few impurities.] In bulk polymerizations there is a tendency for the reaction mass to form a gel (i.e., have an extraordinarily high viscosity) and hot spots can develop. At the extreme, the reaction rate can accelerate to runaway proportions (for reasons we will discuss when we consider kinetics) with potentially disastrous (explosive) consequences. Viscosity and heat control can be achieved, if necessary, by carrying out the polymerizations to a relatively low conversion, with the unreacted monomer being separated and recycled. Another way to control the viscosity and heat transfer problems of chain polymerizations is to perform the polymerization in solution A major concern with this method is that chain transfer to sol-... 

Random copolymers are often formed by chain polymerizations when two or more monomers are polymerized together. Many commercial polymers belong to this group, e.g. styrene/acrylonitrile (SAN), polyvinyl chloride/ polyvinylidene dichloride (Saran film), polyvinylidene difluoride/polyhexa-fuoropropene (Viton) which are all produced using free radical initiators (section 1.8.1). Ethylene/propylene elastomers are random copolymers (section 1.15.1.4) and they are obtained with Ziegler catalysts. 

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