Graft polymerization dienes

The expressions are an outcome of the terminal model theory with several steady-state assumptions related to free-radical fiux (14,23). Based on copolymerization studies and reactivity ratios, chloroprene monomer is much more reactive than most vinyl and diene monomers (Table 1). 2,3-Dichloro-l,3-butadiene is the only commercially important monomer that is competitive with chloroprene in the free-radical copolymerization rate. 2,3-Dichlorobutadiene or ACR is used commercially to give crystallization resistance to the finished raw polymer or polymer vulcanizates. a-Cyanoprene (1-cyano-l,3-butadiene) and /3-cyanoprene (2-cyano-1,3-butadiene) are also effective in copolymerization with chloroprene but are difficult to manage safely on a commercial scale. Acrylonitrile and methacrylic acid comonomers have been used in limited commercial quantities. Chloroprene-isoprene and chloroprene-styrene copolymers were marketed in low volumes during the 1950s and 1960s. Methyl methacrylate has been utilized in graft polymerization particularly for vinyl adhesive applications. A myriad of other comonomers have been studied in chloroprene copolymerizations but those copolymers have not been used with much commercial success. 

Asymmetric Diels-Alder reactions have also been achieved in the presence of poly(ethylene glycol)-supported chiral imidazohdin-4-one [113] and copper-loaded silica-grafted bis(oxazolines) [114]. Polymer-bound, camphor-based polysiloxane-fixed metal 1,3-diketonates (chirasil-metals) (37) have proven to catalyze the hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene. Best catalysts were obtained when oxovanadium(lV) and europium(III) where employed as coordinating metals. Despite excellent chemical yields the resulting pyran-4-ones were reported to be formed with only moderate stereoselectivity (Scheme 4.22). The polymeric catalysts are soluble in hexane and could be precipitated by addition of methanol. Interestingly, the polymeric oxovanadium(III)-catalysts invoke opposite enantioselectivities compared with their monomeric counterparts [115]. 

Although this method yields a mixture of homopolymer and graft copolymer, and probably also ungrafted backbone polymer, some of the systems have commercial utility. These are high-impact polystyrene (HIPS) [styrene polymerized in the presence of poly(l,3-buta-diene)], ABS and MBS [styrene-acrylonitrile and methyl methacrylate-styrene, respectively, copolymerized in the presence of either poly(l,3-butadiene) or SBR] (Sec. 6-8a). 

RESINS (Acrylonitrile-Butadiene-Styrene). Commonly referred to as ABS resins, these materials are thermoplastic resins which are produced by grafting styrene and acrylonitrile onto a diene-rubber backbone. The usually preferred substrate is polybutadiene because of its low glass-transition temperature (approximately —80°C). Where ABS resin is prepared by suspension or mass polymerization methods, stereospedfic diene rubber made by solution polymerization is the preferred diene. Otherwise, the diene used is a high-gel or cross-linked latex made by a hot emulsion process. 

Today, a large part of the more than one billion lbs/year of impact polystyrene and 500 million lbs/year of ABS produced domestically is made by graft copolymerization. Impact polystyrene may be synthesized by dissolving a diene rubber in styrene monomer, in the presence or absence of another solvent, prepolymerizing the solution, and completing the polymerization in bulk, solution, or suspension. R. B. dejong describes a process wherein he prepolymerizes in emulsion with styrene as the continuous phase and the water as the dispersed phase and completes polymerization in aqueous suspension. 

Heterogeneous diene polymerization catalysts based on modified and unmodified silica-supported lanthanide complexes are known as efficient gas-phase polymerization catalysts for a variety of support materials and activation procedures (see Sect. 9). Metal siloxide complexes M(()SiR3 )x are routinely employed as molecular model systems of such silica-immobilized/ grafted metal centers [196-199]. Structurally authenticated alkylated rare-earth metal siloxide derivatives are scarce, which is surprising given that structural data on a considerable number of alkylated lanthanide alkoxide and aryloxide complexes with a variety of substitution patterns is meanwhile available. 

Solution grafting has been the predominant approach for the immobilization of rare-earth metal precatalyst components [288]. The identification of the catalytically active surface species, commonly formed upon interaction with organoaluminum compounds, is difficult and assisted by molecular model complexes. Several types of support materials including magnesium chloride [289], silica [290], and organic (co-)polymers [291,292], were examined both in the gas-phase and the slurry polymerization of 1,3-dienes. 

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