Core multifunctional initiators

The synthesis of eight arm star PIB was recently described by Jacob et al. [37], where eight PIB arms emanate from a calixarene core (multifunctional initiators 20 (tert-hydroxy derivative) and 21 (terf-methoxy derivative)). The synthetic strategy is shown in Scheme 3. 

Scheme 56. Preparation of calixarene-core multifunctional initiators and the structure of calix [4] arene modified with acetyl chloride groups [346]...

The core first method starts from multifunctional initiators and simultaneously grows all the polymer arms from the central core. The method is not useful in the preparation of model star polymers by anionic polymerization. This is due to the difficulties in preparing pure multifunctional organometallic compounds and because of their limited solubility. Nevertheless, considerable effort has been expended in the preparation of controlled divinyl- and diisopropenylbenzene living cores for anionic initiation. The core first method has recently been used successfully in both cationic and living radical polymerization reactions. Also, multiple initiation sites can be easily created along linear and branched polymers, where site isolation avoids many problems. 

Interestingly, the Fe2+ ion in the core can be easily removed by base, the complex dissociates and the individual polymer dimers can be analyzed. Block copolymers of 2-ethyl-2-oxazoline with other substituted oxazolines have also been made [121]. Ru2+(4,4 dichloromethyl-2,2 bipyridine)3 has also been used as the multifunctional initiator for the ATRP of styrene at 110°C [122], It is interesting to note that the Cu+ ions necessary for the polymerization reaction are solubilized via complexation with other bipyridine species. 

Linear-dendritic star copolymers [5], most frequently obtained via processes in which dendrimers function as multifunctional initiator cores for the poly-... 

The core-first approach is based on the initiation of polymerization by a multifunctional initiator. The number of arms is then defined by the number of functional units present on the core. In order to have a good control of the molecular structure of star-shaped polyesters, the initiation must be quantitative and fast. It is also mandatory to avoid possible side-reactions between the initiating species on the core. 

Although the core-first method is the simplest, success depends on initiator preparation and quantitative initiation under living conditions. This method is of limited use in anionic polymerization because of the generally poor solubility of multifunctional initiators in hydrocarbon solvents [12]. Solubilities of multifunctional initiators are less of an issue in cationic polymerizations, and tri- and tetrafunctional initiators have been used to prepare well-defined three- and four-arm star polymers by this method [7] Except for two reports on the synthesis of hexa-arm polystyrene [27] and hexa-arm polyoxazoHne [26], there is a dearth of information in regard to well-defined multifunctional initiators for the preparation of higher functionality stars. 

Star polymers may be synthesized in several ways. The arm-first method joins preformed arms together using a linking agent, and the core-first method utilizes a multifunctional initiator to grow the... 

A commercially available hyper-branched polyester derived from bis-MPA was used as the multifunctional initiating core for the ROP of e-CL and this led to the synthesis of hybrid dendritic linear star polymers. The reactivities of the chain-end hydroxymethyl groups in the dendrimer were significantly greater than in the isomeric hyper-branched case. 

A. Multifunctional Initiator Method For example, living polymerization may be initiated with a multifunctional initiating system to grow multiple polymer chains from a central initiator core to give multiarmed or star polymers [Scheme 8(A)]. The key to this approach is of course the development of multifunctional initiators, which has already been... 

In the multifunctional initiation (A) and the multifunctional termination (B), living polymers grow outward from the initiator core and inward into the terminator core, respectively, but both processes lead to similar polymers. If they operate properly, these methods give multiarmed polymers that carry arms in a precisely controlled or predetermined number per molecule (= the functionality number of the initiator or terminator). 

By the use of the polymer-linking method with 20a, a variety of starshaped poly(vinyl ethers) have been synthesized (Scheme 12) [208-212]. A focus of these syntheses is to introduce polar functional groups, such as hydroxyl and carboxyl, into the multiarmed architectures. These functionalized star polymers include star block (23a,23b) [209,210], heteroarm (24) [211], and core-functionalized (25) [212] star polymers. Scheme 12 also shows the route for the amphiphilic star block polymers (23b) where each arm consists of an AB-block copolymer of 1BVE and HOVE [209] or a vinyl ether with a pendant carboxyl group [210], Thus, this is an expanded version of triarmed and tetraarmed amphiphilic block copolymers obtained by the multifunctional initiation (Section VI.B.2) and the multifunctional termination (Section VI.B.3). Note that, as in the previously discussed cases, the hydrophilic arm segments may be placed either the inner or the outer layers of the arms. 

Two general strategies are possible for the synthesis of star-shaped copolymers The arm-first method is based on the reaction of living chains with plurifunctional electrophiles carrying at least three reacting groups alternatively, polymerization can be initiated by a multifunctional initiator according to the core-first method. 

There are two basic synthetic routes for star polymers (Scheme 12)-the core first method (polymerization from multifunctional initiators or microgels) and the arm first method (where growing polymer chain ends are reacted with a multifunctional terminating agent or a divinyl compound). Whereas the use of multifunctional initiators leads to stars with a well-known (but often low) number of arms, the use of microgels or divinyl compounds leads to a rather broad arm number distribution, where the average arm number can be quite high. 

The formation of PAA star polymers using the core first method has been demonstrated in the ATRP process by use of multifunctional initiators [111, 112]. In this method, the number of arms in the star polymer can be determined by the number of initiating sites on the initiator. Star-shaped PtBuA was prepared by the arm first method via ATRP, using divinylbenzene, 1,4-butanediol diacrylate, and EGDMA as coupling reagents [113]. 

Block copolymers between alkyl acrylates such as B-4,358 B-5,202,203 and B-6,203 on the other hand, have been synthesized by the macroinitiator methods mostly with copper catalysts. Star block copolymers with a soft poly(MA) core and a hard poly(isobomyl acrylate) shell were synthesized by using multifunctional initiators.358 Poly(tBA) segments in B-5 and B-6 can be converted into hydrophilic poly(acrylic acid).203 Block copolymers between />methylstyrene and styrene (B-7) were also prepared by the rhenium-catalyzed living radical polymerization in conjunction with an alkyl iodide initiator.169... 

This functionalization method can be applied for the synthesis of star polymers only when a multifunctional initiator is used. The living branches emanate from the core and therefore can be subjected to several terminating reactions with suitable electrophilic reagents. 

Based on the findings of the arm-first strategy, it is possible to obtain stars with well-defined arm length. The main problem of this strategy is the arm number distribution. Moreover, purification may cause many difficulties in the synthesis. In contrast, the core-first strategy requires multifunctional initiators and further polymerization initiated from the core. This is shown in Scheme 3. The maximum arm numbers of the stars are determined by the number of functionalities in the core. In the ideal case, the initiating efficiency of the core is close to unity, which will produce well-defined stars with precise numbers of arms. However, due to the steric hindrance and the limit of the polymerization techniques, it can be difficult to obtain full initiating efficiency. 

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],... 

The livingness of the CROP of 2-oxazolines allows the incorporation of chain-end functionalities using functional electrophilic initiators as well as functional nucleophilic terminating agents. Examples of reported functional initiators include allyl- [177], propargyl [178], and phtalimido [179] (as precursor for amines) functionalized tosylates. In addition, the use of multifunctional initiators has been utilized for the preparation of star-shaped poly(2-oxazoline)s with various core structures [180-183]. 

In the first case, the arms are grown from a single core with a given number of potentially active sites or a well-defined multifunctional initiator. In contrast to anionic multifunctional initiators, weU-defined soluble multifunctional cationic initiators are readily available. These multifunctional initiators with 3-8 initiating sites have been successfully applied for the synthesis of 3-8 arm star homo- and block copolymers of vinyl ethers, styrene and styrene derivatives, and IB. For example, six-arm star polystyrenes were prepared using initiator with six phenylethylchloride-type functions emanating from a central hexa-substituted benzene ring [250]. By subsequent end functionalization, a variety of end-functionaUzed A or (AB) (see above) star-shaped structures can also be obtained. 

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]. 

Scheme 27.4 Core-first synthesis of star-like polymers from multifunctional initiators.

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. 

ROP and RAFT polymerization techniques were combined to synthesize multiarm star-block copolymers having PeCL inner blocks and PDMAEMA outer blocks. A hyperbranched polyester core was used as a multifunctional initiator. It was calculated that the functionality of the star-blocks was equal to 19. Temperature and pH-responsive micelles were obtained in aqueous solutions. Equilibrium between unimolecular and mulrimolecular micelles was observed at pH 6.58 by dynamic LS and TEM measurements. In low-pH solutions, the PDMAEMA chains were fully protonated and therefore highly stretched, leading to maximum Rh values. When the pH was increased, the micellar Rh decreased as a result of the deprotonation of the dimethylamine groups. PDMAEMA is also a temperature-sensitive polymer, as it exhibits lower critical solution temperature (LCST) behavior. It precipitates from neutral or basic solutions between 32 and 58 °C. At pH 6.58, the Rh values were found to decrease with increasing temperature, due to the gradual collapse of the PDMAEMA outer blocks. 

Multifunctional initiators, produced by dendrimer techniques, have been used to synthesize dendrimer-like star-block copolymers by ATRP method (Hedrick et al., 1998). Starting from a hexafunctional initiator, e-caprolactone was polymerized and each hydroxyl group was then chemically transformed into two 2-bromo-isobutyrate moieties, which were used to initiate the ATRP of either MMA or a mixture of MMA and hydroxy ethyl methacrylate (HEMA) to produce, in the latter case, dendrimer-hke star-block copolymer composed of a poly(e-caprolactone) core... 

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