Star-like polymers

As a result of this process, star-like polymers have been produced with a wide range of molecular weights [91]. 

Further repeated intermolecular addition of the methacrylate ion onto the pendant methacrylate double bonds yields star-like polymers. Like the divinyl-benzene process in other anionic and in carbocationic living systems designed star polymers are difficult to obtain [78-80]. A recent detailed characterization of the PMMA stars has found that the number of arms typically varies between 10 and 100 and that each sample has a fairly wide distribution in the number of arms [78],... 

Living ROMP of a macromonomer with MW = 2600 yields star-like polymers with 10 to 100 arms on average possessing a somewhat broad MW distribution (i.e., 1.2—1.4). 

Many micellar catalytic applications using low molecular weight amphiphiles have already been discussed in reviews and books and will not be the subject of this chapter [1]. We will rather focus on the use of different polymeric amphiphiles, that form micelles or micellar analogous structures and will summarize recent advances and new trends of using such systems for the catalytic synthesis of low molecular weight compounds and polymers, particularly in aqueous solution. The polymeric amphiphiles discussed herein are block copolymers, star-like polymers with a hyperbranched core, and polysoaps (Fig. 6.3). 

Fig. 6.3 Different types of micelles and micelle analogous structures a) amphiphilic block copolymers, b) star-like polymers with a hyperbranched core, c) polysoaps.

A macromonomer is a macromolecule with a reactive end group that can be homopolymerized or copolymerized with a small monomer by cationic, anionic, free-radical, or coordination polymerization (macromonomers for step-growth polymerization will not be considered here). The resulting species may be a star-like polymer (homopolymerization of the macromonomer), a comblike polymer (copolymerization with the same monomer), or a graft polymer (copolymerization with a different monomer) in which the branches are the macromonomer chains. 

This occurs if small amounts of a multifunctional monomer, R-A, are reacted with A-B bifunctional monomers (Figure 5-26). In the initial stages of the polymerization, linear chains of A-B units are the main product. But, as the reaction approaches completion, these linear species disappear in favor of multi-chain star-like polymer molecules. 

Another approach to incorporate a block structure was to use the multifunctional precursor shown in Fig. 35 and grow blocks from the core of the hyperbranched structure. Such star-like polymers with 80 pnBA blocks were obtained [ 130]. In a similar way, hyperbranched polymers from VBC were used to initiate the ATRP of nBA [130] and St [264]. Dendrigraft polystyrene was found to display a lower intrinsic viscosity and higher thermal stability than linear polystyrenes [264]. 

Generally PSAs are well known for their very viscoelastic behavior, which is necessary for them to function properly. It was therefore important to characterize first the effect of the presence of diblocks on the linear viscoelastic behavior. Since a comprehensive study on the effect of the triblock/diblock ratio on the linear viscoelastic properties of block copolymer blends has recently been reported [46], we characterized the linear viscoelastic properties of our PSA only at room temperature and down to frequencies of about 0.01 Hz. Within this frequency range all adhesives have a very similar behavior in terms of elasticity, as can be seen in Fig. 22.10. The differences appear at low frequency, a regime where the free iso-prene end of the diblock chain is able to relax. This relaxation process is analogous to the relaxation of an arm of a star-like polymer [47], and causes G to drop to a lower plateau modulus, the level of which is only controlled by the density of triblock chains actually bridging two styrene domains [46]. 

Recent advances in polymer chemistry, in particular, in controlled radical polymerization, have enabled the synthesis of complex macromolecular architectures with controlled topology, which comprise chemically different (functional) blocks of controlled length in well-defined positions. Block co- and terpolymers, molecular and colloidal polymer brushes, and star-like polymers present just a few typical examples. Furthermore, miktoarm stars, core-shell stars and molecular brushes, etc. exemplify structures where chemical and topological complexity are combined in one macromolecule. 

Interest in star polymers can be easily understood they are composed of several polymer chains attached at one end to a single branching point that corresponds to the core [8]. As such, they represent useful models for the experimental evaluation of theories about the solution properties and rheological behavior of branched polymers. Star-like polymers can also serve as nanoscale building blocks for the construction of well-defined polymer networks and conetworks [9]. 

Figure 27.2 Various types of star-like polymer.

Five synthetic strategies for the construction of star-like polymers have been identified, mainly through variations in the core construction. In particular, con-trolled/living radical polymerization (C/LRP) techniques, that originally were developed during the mid-1990s, have provided quite simple routes to polymers with a star-like architecture [12-14, 21]. 

Scheme 27.1 The nodulus synthetic approach to star-like polymers. The asterisk represents an active center (carban-ion, carbocation, or carbon-centered radical) or the dormant form of a growing chain.

Scheme 27.2 The macromonomer approach" to synthesizing star-like polymers.

The (co)polymerization of macromonomers based on PS and/or PEO by ring-opening metathesis polymerization (ROMP) [34] offers the advantage of a quantitative process, to yield star-like polymers that are free from any macromonomeric precursors. 

In this case, chlorosilane reagents (XVIII, XIX), halomethyl benzene reagents (XX), and 1,1-diphenylethylene (DPE) derivatives (XXI) carrying aUsyUialide functions are typical coupling agents used to deactivate anionically derived polymers, mostly PS, PB, and PI. A variety of star-like polymers of precise functionality, including multiarm stars, star-block copolymers, asymmetric and miktoarm stars, and other branched architectures, are accessible in this way [1, 12-14, 35]. This... 

Scheme 27.3 Arm-first synthesis of star-like polymers from multifunctional linking reagents.

Scheme 27.5 Core-first approach to star-like polymers from reactive nano/microgels...

Michel Schappacher is Senior Research Engineer at the French National Center for Scientific Research (CNRS). He was educated at the Univereity Louis Pasteur (ULP), Strasboui France, where he received a "Doctorat d Universite in 1981 in synthetic organic chemistry under the supervision of Prof. R. Weiss. In 1988 he joined Dr. A. Deffieux s team at the Laboratoire de Chimie des Polymdes Oi aniques, University of Bordeaux France. He worked on modular synthetic approaches using living polymerization reactions for the precise design of new polymer architectures including maaocyclic polymere, star-like polymers, copolymer brushes, hyperbranched polymers, and their AFM visualization and characteriza-... 

Figure 1 (a) Linear PE chain, (b) Polymer ring, (-CH2-)48- (c) PE chain with one branching point, (d) Comb-like polymer, (e) Star-like polymer, (f) Ladder polymer, (g) Randomly branched polymer. 

Equation [24], when applied to an/-arm star-like polymer (see Figure 1(e) where/= 6) with N//units per arm, gives... 

While end-functional polymers are clearly important industrial products for materials synthesis, they are also interesting from an academic point of view. Many complex macromolecular architectures can be realized from end-functional polymeric starting materials. Two mono-telechelic polymers can, for example, be joined to form a diblock copolymer, or several such polymers can be joined to form star-like polymers. Mono-telechelic polymers can also be attached to a linear, multifunctional polymer to yield graft copolymers. Depending on the functional end group, such polymers can attach to macroscopic or nanoparticle surfaces or form conjugates with bio-oligomers [5] or biomacromolecules [6]. 

Many compounds with multiple active halogen atoms have been used to initiate bi- or multi-directional growth to form ABA block copolymers and star-like polymers and copolymers [158]. Active halogens can be incorporated at the chain ends of polymers... 

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