Star polymers core-first approach

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

The most important method for the synthesis of star polymers by GTP involves the use of a diftmctional monomer either as an arm-first or as a core-first approach. Usually, EGDM is used as the difunctional monomer. However, other multifunctional monomers, such as tetramethylene glycol dimethacrylate, trimethylol propane trimethacrylate, and 1,4-butylene dimethacrylate have been used. 

The core-first approach was also adopted for the synthesis of star polymers. According to this procedure, the catalyst was reacted with the difunctional monomer to give the multifunctional initiator, followed by the addition of norbomene." Unfortunately, a multimodal product was obtained revealing the presence of linear chains, dimers, and star stmctures. 

Miktoarm polymers are essentially heteroarm star polymers where two or more arms of the star are chemically unique. Therefore, the same general approaches for the synthesis of star polymers also apply to miktoarms, with some additional constraints. Many research groups have sequentially performed orthogonal polymerization techniques to access a variety ABC and ABCD miktoarm polymers, in a core-first approach. 

The core-first approach has the advantage that well-defined star polymers with a constant number of arms and identical arm lengths can be prepared, which is crucial for the study of structure-property relationship. In the case of NMP, this route requires the synthesis of multifunctional alkoxyamines. The first example was reported by Hawker using a tri-frmctionalized alkoxyamine based on TEMPO. Well-defined PS star polymers were obtained. A variety of star polymers were also prepared using multifunctional (3, 6, or 12 arms) TEMPO-based alkoxyamines. " The core of the star... 

GTP, on the other hand, offers good potential for the synthesis of star-and comb-branched polymers. The GTP chain ends are living even at temperatures as high as 70°C and, thus, are capable of participating efficiently in branching reactions. Both the arm-first and core-first approaches have been reported for the synthesis of multiarm star-branched acrylic polymers. However, very few details of experimental conditions and polymer characterization have been reported for star-branched pol)uners synthesized using the arm-first approach. 

Two examples of the core-first approach for the synthesis of star polymers by GTP have been reported. Trimethylolpropane triacrylate is converted to a silyl enol ether that is used to initiate the polymerization of ethyl acrylate (Scheme 6). A pol)aner with a = 2190 and MJM = 1.39 was obtained [9]. A cyclic tetramer of methyl hydrogen siloxane was converted to a core containing four initiating groups using a Pt-catalyzed hydrosilylation reaction. The tetrafiinctional initiator was used to initiate the polymerization of MMA to form a four-arm star PMMA (Scheme 7), with about 20 to 150 MMA repeat... 

As we conjectured in the introduction, the fundamental role of topology in this approach to entangled polymer dynamics would indicate that changes to the topology of the molecules themselves would radically affect the dynamic response of the melts. In fact rheological data on monodisperse star-branched polymers, in which a number of anionically-polymerised arms are coupled by a multifunctional core molecule, pre-dated the first application of tube theory in the presence of branching [22]. Just the addition of one branch point per molecule has a remarkable effect, as may be seen by comparing the dissipative moduli of comparable linear and star polymer melts in Fig. 5. 

All of the aforementioned reports showed star polymer formation originating from a core. The so-called arm-first approach has also been demonstrated. 

The first step for the core-first stars is the synthesis of multifunctional initiators. Since it is difficult to prepare initiators that tolerate the conditions of ionic polymerization, mostly the initiators are designed for controlled radical polymerization. Calixarenes [39, 58-61], sugars (glucose, saccharose, or cyclodextrins) [62-68], and silsesquioxane NPs [28, 69] have been employed as cores for various star polymers. For the growth of the arms, mostly controlled radical polymerizations were used. There are only very rare cases of stars made from nitroxide-mediated radical polymerization (NMRP) [70] or reversible addition-fragmentation chain transfer (RAFT) techniques [71,72], In the RAFT technique one has to differentiate between approaches where the chain transfer agent is attached by its R- or Z-function. ATRP is the most frequently used technique to build various star polymers [27, 28],... 

This method was first reported by Eschwey and Burchard [241] and developed by Rempp and coworkers [242-244]. Living polymer chains initiate the polymerization of divinylbenzene (DVB), leading to the formation of living star polymers composed of a polydivinylbenzene core from which emanate the arms, which have contributed to its formation. The core contains the same number of active sites as the arms of the star. These living anions can be used for the polymerization of a second monomer leadingto the formation of miktoarm stars of the AnBn type. The polymers prepared by this approach have PS as A arms and as B arms PtBuMA, PBuMA, PEO or PtBuA [242-246] (Scheme 82). 

A fourth strategy involves the polymerization of a monomer out of multifunctional initiators, following a divergent ( core-firsf ) approach (Scheme 27.4) [6, 12-14]. All main CLP methods have been applied to the synthesis of core-first star polymers, including chain growth by LAP [43] and hving cationic polymerizations [44], ROMP [45], nitroxide-mediated polymerization (NMP) [46], ATRP [47], or RAFT polymerization via the so-called R-approach [48]. The conden-sative chain-growth polymerization method, as developed by Yokozawa etal, has also been applied to the synthesis of well-defined, core-first, star-shaped polyamides [49]. 

The first examples of star polymers synthesized via LRP were reported by Matyjaszewski and co-workers (92,93). At a later stage they reported on the synthesis of star polymers via the so-called arm first procedure (94X In this approach polymer chains are synthesized with a reactive chain end that allows coupling to the core of the star (Fig. 6). 

A star polymer is composed of / linear arms each arm comprises N monomer units. As shown in Figure 3, the arms are grafted by one of the terminal segments onto a multiftinc-tional core. Two major strategies, that is, the core first and the arms first, are used in synthesis of star polymers "Hie arms first approach " enables one to prepare arms with well-defined length. However, the control over the number of arms successively attached to the core is poor, which leads to significant polydispersity of the samples. "Hie core first ... 

Another approach to core shell polymers, or multiarmed star polymers is the arm first approach, where a growing polymer formed by a CRP is copolymerized with a difunction monomer to form a crosslinked core with the attached first formed arms [131,376,377]. Other surfaces include organic resins and latexes [315]. 

As mentioned above, some difficulties arose in the synthesis of dendrimer-like star-branched polymers by the two aforementioned methodologies based on the core-first divergent and arm-first convergent approaches described in Sections 5.2.1 and 5.2.2. In fact, most of the polymers are limited to 4G stages and a few 10 g/mol orders in molecular weight except for two cases reported by Gnanou et al. (Tables 5.1 and 5.2). Moreover, the structures of the resulting polymers could not be well characterized and structural imperfections were indicated in several cases. 

This method is referred to as the core-first or arm-out or divergent approach. According to this procedure, multifunctional compounds capable of simultaneously initiating the polymerization of several arms are used. There are several requirements a multifunctional initiator has to fulfill in order to produce star polymers with controllable molecular weights, uniform arm lengths, and low molecular-weight distribution. All initiating sites must be equally... 

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