Star Co polymers

A variety of mnltifnnctional CTAs have been used for the preparation of star (co)polymers via RAFT polymerization, the core (or hub) of the star being introduced via functionalization of either the R substiment (R-group approach or R approach) or the Z substituent (Z-group approach or Z approach). Typical examples of fimctional CTAs used for the production of star (co)polymers in R- and Z-group approaches are shown in Fig. 11.40. The difference between the two approaches is shown schematically in Fig. 11.41, while a mechanism for RAFT star polymerization proposed byBameretal. (2007) is presented in Fig. 11.42. 

Problem 11.24 Besides choosing the Z-approach, what are the key factors that determine the success of the RAFT star polymerization process to achieve large proportion of well-de ned star polymers of controlled stmctures . 

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Multiarm star (co)polymers can be defined as branched (co)polymers in which three or more either similar or different linear homopolymers or copolymers are linked together to a single core. The nomenclature that will be used follows the usual convention ... 

The synthesis of branched polymers by cationic polymerization of vinyl monomers has been reviewed recently [219] therefore, these will be briefly considered here. Star-shaped or multiarm star (co) polymers can be prepared by three general methods ... 

Difunctional monomers such as DVB or divinyl ether have been found to be efficient in the synthesis of star (co)polymers having a cross-linked core from which homopolymer or block copolymer arms radiate outwards. However, so far only core last method has been reported in cationic polymerization. This method is particularly suited to prepare stars with many arms. The average number of arms per molecule is a function of several experimental and structural parameters. 

Figure 11.40 Functional CTAs for the production of star (co)polymers (I) Flexakis (thiobenzoylthiomethyl) benzene (Stenzel-Rosenbaum et al., 2001), used in R-approach (II) tri(thiobenzoylthiomethyl) benzene (Dureault et al., 2004), used in Z-approach.

Figure 12.15 Schematic representations showing the differences between various types of star (co)polymers (a) star homopolymers (b) star block copolymers (c) and (d) miktoarm star copolymers. Solid lines and dotted lines represent polymer chains differing in composition and/or molecular weight.

TBiBPE (see Scheme P12.7.1) is a multifunctional ATRP initiator that can be used for the synthesis of star polymers by ATRP. Several other multifunctional initiators used in the preparation of star (co)polymers are shown in Fig. 12.17. These contain both bromide functionality to initiate ATRP and hydroxyl functionality to initiate ROP. 

Figure 12.17 Several multifunctional initiators used for making star (co)polymers. (VIII) 2-((2,2-bis(hydroxymethyl) butoxy) carbonyl)-2-methylpropane-l,3-diyl bis(2-bromo-2-methyl propanoate) (Yang etal.,2008). (IX) l,l,l-tris(4-(2-bromoisobutyryloxy) phenyl) ethane (Matyjaszewski etal., 1999). (X) 2-(hydroxymethyl)-2-methyl-3-oxo-3-(2-phenyl-2-(2,2,6,6-tetramethyl piperidin-l-yloxy)ethoxy) propyl pent-4-ynoate (Altintas et al., 2008). (XI) propargyl 2-hydroxylmethyl-2-(or-bromoisobutyryloxymethyl)propion-ate (Altintas et al., 2008).

Discuss feasible routes for the synthesis of narrow-disperse (a) four-armed star polymer, core-(PCLso)4 (where CL = caprolactone) and (b) four-armed star diblock copolymer, core-(PCLso-fc-PStso)4 (where PSt = polystyrene). Calculate the theoretical molecular weights of the star (co)polymers. [Ans. (a) 22,936 (b) 44,332.]... 

The purpose of this review is to show how anionic polymerization techniques have successfully contributed to the synthesis of a great variety of tailor-made polymer species Homopolymers of controlled molecular weight, co-functional polymers including macromonomers, cyclic macromolecules, star-shaped polymers and model networks, block copolymers and graft copolymers. 

Samarium enolates 60 can be easily prepared by reduction of ct-bromocarboxylic acid esters with SmT. These enolates mediated well-defined synthesis of star-shaped block co-polymers 61 (Scheme 21 ).32 32l Sml3 also mediated the formation of samarium enolates. Phenacyl thiocyanate 6233 and cr-haloketone 6434 are converted to samarium(lll) enolate intermediates 63 and 65, respectively, which undergo addition to benzaldehyde derivatives affording the corresponding oy i-unsaturatcd ketones as shown in Schemes 22 and 23. 

An important group of surface-active nonionic synthetic polymers (nonionic emulsifiers) are ethylene oxide (block) (co)polymers. They have been widely researched and some interesting results on their behavior in water have been obtained [33]. Amphiphilic PEO copolymers are currently of interest in such applications as polymer emulsifiers, rheology modifiers, drug carriers, polymer blend compatibilizers, and phase transfer catalysts. Examples are block copolymers of EO and styrene, graft or block copolymers with PEO branches anchored to a hydrophilic backbone, and star-shaped macromolecules with PEO arms attached to a hydrophobic core. One of the most interesting findings is that some block micelle systems in fact exists in two populations, i.e., a bimodal size distribution. 

ADMET is a step growth polymerization in which all double bonds present can react in secondary metathesis events. However, olefin metathesis can be performed in a very selective manner by correct choice of the olefinic partner, and thus, the ADMET of a,co-dienes containing two different olefins (one of which has low homodimerization tendency) can lead to a head-to-tail ADMET polymerization. In this regard, terminal double bonds have been classified as Type I olefins (fast homodimerization) and acrylates as Type II (unlikely homodimerization), and it has been shown that CM reactions between Types I and II olefins take place with high CM selectivity [142], This has been applied in the ADMET of a monomer derived from 10-undecenol containing an acrylate and a terminal double bond (undec-10-en-l-yl acrylate) [143]. Thus, the ADMET of undec-10-en-l-yl acrylate in the presence of 0.5 mol% of C5 at 40°C provided a polymer with 97% of CM selectivity. The high selectivity of this reaction was used for the synthesis of block copolymers and star-shaped polymers using mono- and multifunctional acrylates as selective chain stoppers. 

Living polymerization processes pave the way to the macromolecular engineering, because the reactivity that persists at the chain ends allows (i) a variety of reactive groups to be attached at that position, thus (semi-)telechelic polymers to be synthesized, (ii) the polymerization of a second type of monomer to be resumed with formation of block copolymers, (iii) star-shaped (co)polymers to be prepared by addition of the living chains onto a multifunctional compound. A combination of these strategies with the use of multifunctional initiators andtor macromonomers can increase further the range of polymer architectures and properties. 

Anionic polymerization has proven to be a very powerful tool for the synthesis of well-defined macromolecules with complex architectures. Although, until now, only a relatively limited number of such structures with two or thee different components (star block, miktoarm star, graft, a,to-branched, cyclic, hyperbranched, etc. (co)polymers) have been synthesized, the potential of anionic polymerization is unlimited. Fantasy, nature, and other disciplines (i.e., polymer physics, materials science, molecular biology) will direct polymer chemists to novel structures, which will help polymer science to achieve its ultimate goal to design and synthesize polymeric materials with predetermined properties. 

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