Polymer synthesis, titanium-based

However, most of the available metal-based eatalysts for ROP and a-olefin polymerisation suffer from inherent toxicity, low abundance, high price and are listed as endangered elements, which is in contradiction with their application in green and sustainable procedures for polymer synthesis. In line with sustainable catalyst development, titanium is nontoxic (no known biological role), readily accessible and an abundant element (crustal abundance of 4136 ppm with the lowest supply risk searcity factor is about 2.5/8.5) making this element one of the most attraetive metals for use in sustainable polymerisation catalysis. 

Polymerization Catalysed by Acids and Bases. Carbonium ions and carbanions respectively are carriers of the chain transfer in cationic and anionic polymerizations respectively. Ionic polymerization mechanism was exploited for the synthesis of polymeric stabilizers in comparison with the free-radical polymerization only exceptionally. The cationic process was used for the synthesis of copolymers of 2,6-di-tert-butyl-4-vinylphenol with cyclopentadiene and/or for terpolymers with cyclopentadiene and isobutylene [109]. System SnCWEtsAlCla was used as an initiator. Poly(lO-vinylphenothiazin) was prepared by means of catalysis with titanium chlorides [110]. Polymers of 4-[a-(2-hydroxy-3,5-dimethylphenyl)ethyl]-vinylbenzene [111] and 3-allyl-2-hydroxyacetophenone [112] were also prepared under conditions of cationic polymerization. 

The discovery of living cationic polymerization has provided methods and technology for the synthesis of useful block copolymers, especially those based on elastomeric polyisobutylene (Kennedy and Puskas, 2004). It is noteworthy that isobutylene can only be polymerized by a cationic mechanism. One of the most useful thermoplastic elastomers prepared by cationic polymerization is the polystyrene-f -polyisobutylene-(>-polystyrene (SIBS) triblock copolymer. This polymer imbibed with anti-inflammatory dmgs was one of the first polymers used to coat metal stents as a treatment for blocked arteries (Sipos et al., 2005). The SIBS polymers possess an oxidatively stable, elastomeric polyisobutylene center block and exhibit the critical enabling properties for this application including processing, vascular compatibility, and biostability (Faust, 2012). As illustrated below, SIBS polymers can be prepared by sequential monomer addition using a difunctional initiator with titanium tetrachloride in a mixed solvent (methylene chloride/methylcyclohexane) at low temperature (-70 to -90°C) in the presence of a proton trap (2,6-dt-f-butylpyridine). To prevent formation of coupled products formed by intermolecular alkylation, the polymerization is terminated prior to complete consumption of styrene. These SIBS polymers exhibit tensile properties essentially the same as those of... 

Excellent progresses have been made over past decades in the development of new efficient catalysts for polyesters synthesis, and in the understanding of the properties of these materials. Even if an impressive quantity of information are available, several challenges are still open. One of the most important purpose in PET research is, without no doubt, the substitution of the toxic Sb(III) based catalyst with more friendly compounds. One of these possibilities, as described in section 1, is related to titanium (IV) based compounds. In effect Ti(0- Bu)4 is an active catalyst in PET synthesis but it is not often used because of the yellowing of the obtained polymers. However, many studies are evaluating the use of different additives and/or several Ti(fV) based complexes in PET synthesis in order to solve these problems and to understand the reasons of the discoloration. 

Transesterification between hydroxyl-esters, carboxy-esters, or two ester groups is one of the most important polyesterification technique used for preparing various aliphatic, aliphatic-aromatic or aromatic polyesters (also known as arylates). Titanium alkoxides are very efficient catalysts for transesterification polymerizations, however in some cases it can cause undesirable discoloration to the resulting polymer [15]. A representative example is the synthesis of a polyester based on dimethyl terephthalate using titanium isopropoxide [16] (Fig. 2.3). 

Finelli, L., et al. 2004. Comparison between titanium tetrabutoxide and a new commercial titanium dioxide based catalyst used for the synthesis of poly(ethylene terephthalate). Journal of Applied Polymer Science 92(3) 1887-1892. 

Ho, S. C. H. Wu, M. M. Xiong, Y. Novel cyclopolymerization polymers from nonconjugated dienes and 1-alkenes. PCT International Patent Application WO 95/06669 (Mobil Oil Corp.), March 9,1995. Hustad, P. D. Coates, G W. Insertion/isomerization polymerization of 1,5-hexadiene synthesis of functional propylene copolymers and block copolymers. J. Am. Chem. Soc. 2062,124, 11578-11579. Hustad, P. D. Tian, J. Coates, G. W. Mechanism of propylene insertion using bis(phenoxyimine)-based titanium catalysts an unusual secondary insertion of propylene in a group IV catalyst system. J. Am. Chem. Soc. 2002,124,3614-3621. 

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