Free radicals, polymerization of organic

Primary, and Secondary Reactions in Photoinitiated Free-Radical Polymerization of Organic Coatings... 

Polymer colloids can be formed by the free radical polymerization of organic olefinic monomers in liquids which are solvents for the monomers and non-solvents for the resulting polymerso Ordinarily a stabilizer is added which is capable of adsorption at thje newly forming interface during the period of particle formatlono... 

Photoinitiation. Since photolysis of polysdanes generates sdyl radicals, which can add to carbon—carbon double bonds, these polymers have been used for the free-radical polymerization of unsaturated organic monomers (135,136). Though about one-tenth as efficient as other organic photoinitiators, polysdanes are nevertheless quite insensitive to oxygen effects, which somewhat compensates for their lower efficiency. 

By contrast, much of the work performed using ruthenium-based catalysts has employed well-defined complexes. These have mostly been studied in the ATRP of MMA, and include complexes (158)-(165).400-405 Recent studies with (158) have shown the importance of amine additives which afford faster, more controlled polymerization.406 A fast polymerization has also been reported with a dimethylaminoindenyl analog of (161).407 The Grubbs-type metathesis initiator (165) polymerizes MMA without the need for an organic initiator, and may therefore be used to prepare block copolymers of MMA and 1,5-cyclooctadiene.405 Hydrogenation of this product yields PE-b-PMMA. N-heterocyclic carbene analogs of (164) have also been used to catalyze the free radical polymerization of both MMA and styrene.408... 

Dioxiranes constitute a new class of organic peroxides that possess great potential as oxidants with a variety of applications in synthetic organic chemistry.5 7 A new convenient route for the synthesis of silanol polymers has been developed by the selective oxidation of =Si—H bonds with dimethyldioxirane. A series of styrene-based silanol polymers and copolymers were synthesized (Scheme l).8 9 The precursor polymers and styrene copolymers containing =Si—H bond were first synthesized by free radical polymerization of the corresponding monomers or copolymerization of the... 

Polyvinyl chloride. Organic peroxides are used to catalyze the free radical polymerization of vinyl chloride monomer in water. The organic peroxide is selected to generate free radicals thermally at the temperature of polymerization. 

To begin, let s consider the anionic polymerization of styrene. For an initiator, we will choose an organometallic compound an organic compound bonded to a metal atom) such as butyllithium, C4H9 Li+. Although the details differ, you should recognize the overall similarity of the mechanism for this anionic polymerization to that for the free radical polymerization of ethylene, above (initiation, propagation, and termination). 

Organic peroxides are used to initiate free-radical polymerization of ethylene, butadiene, styrene, vinyl chloride, vinyl acetate, and methyl methacrylate. They are also used to cure unsaturated polyesters, occasionally to cross-link thermoplastics such as polyethylene and polyacrylates, and increasingly for grafting and compatibiliza-tion of polymer blends. A variety of organic peroxides offer useful reactivity over a temperature range from 0 to 130°C or more, for different polymers and different processes. 

Commercially, suspension polymerizations have been limited to the free radical polymerization of water-insoluble liquid monomers to prepare a number of granular polymers, including polystyrene, poly(vinyl acetate), poly(methyl methacrylate), polytetrafluoroethylene, extrusion and injection-molding grades of poly(vinyl chloride), poly(styrene-co-acrylonitrile) (SAN), and extrusion-grade poly(vinylidene chloride-covinyl chloride). It is possible, however, to perform inverse suspension polymerizations, where water-soluble monomer (e.g., acrylamide) is dispersed in a continuous hydrophobic organic solvent. 

Origins of free radical polymerization of ethylene to produce LDPE were discussed in Chapter 1, stemming from the seminal work of chemists at ICI in the early 1930s. Agents that foster free radical polymerization of ethylene are called "initiators" and sometimes "catalysts." (The latter is not technically correct, since the agents are consumed in the process.) Organic peroxides are the most commonly used initiators for free radical polymerization of ethylene. 

In the polymerization reactor, organic peroxides dissociate homolytically to generate free radicals. Polymerization of ethylene proceeds by a chain reaction. Initiation is achieved by addition of a free radical to ethylene. Propagation proceeds by repeated additions of monomer. 

Figure 2.4 Key organic peroxides used to initiate free radical polymerization of ethylene.

Chapter 1 is used to review the history of polyethylene, to survey quintessential features and nomenclatures for this versatile polymer and to introduce transition metal catalysts (the most important catalysts for industrial polyethylene). Free radical polymerization of ethylene and organic peroxide initiators are discussed in Chapter 2. Also in Chapter 2, hazards of organic peroxides and high pressure processes are briefly addressed. Transition metal catalysts are essential to production of nearly three quarters of all polyethylene manufactured and are described in Chapters 3, 5 and 6. Metal alkyl cocatalysts used with transition metal catalysts and their potentially hazardous reactivity with air and water are reviewed in Chapter 4. Chapter 7 gives an overview of processes used in manufacture of polyethylene and contrasts the wide range of operating conditions characteristic of each process. Chapter 8 surveys downstream aspects of polyethylene (additives, rheology, environmental issues, etc.). However, topics in Chapter 8 are complex and extensive subjects unto themselves and detailed discussions are beyond the scope of an introductory text. 

Copovidone is manufactured by free-radical polymerization of vinylpyrrolidone and vinyl acetate in a ratio of 6 4. The synthesis is conducted in an organic solvent owing to the insolubility of vinyl acetate in water. 

The concept of controlled hydrolytic stability based on substituted phospha-zenes can be extended to organic polymers. Free radical polymerization of acrylic acid in aqueous solution in presence of 39 yields a degradable hydrogel with imidazolyl groups as controlling sites with respect to the hydrolytic stability. ... 

---