Star core-first method

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. 

The core first method has been applied to prepare four-arm star PMMA. In this case selective degradation of the core allowed unambiguous proof of the star structure. However, the MWD is a little too large to claim that only four-arm star polymers are present [81], Comb PMMAs with randomly placed branches have been prepared by anionic copolymerization of MMA and monodisperse PMMA macromonomers [82], A thorough dilute solution characterization revealed monodisperse samples with 2 to 13 branches. A certain polydispersity of the number of branches has to be expected. This was not detected because the branch length was very short relative to the length of the backbone [83]. Recently, PMMA stars (with 6 and 12 arms) have been prepared from dendritic... 

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. 

In summary, a core-first method was used to prepare allyl-end functionalized octa-arm PIB stars by end quenching the living PIB stars by the use of allyltri-methylsilane. Characterizations by NMR and GPC are consistent with the expected structure and show close to quantitative end-functionalization. 

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... 

Six arm star polystyrenes were prepared by the core-first method using initiator 13 with six phenylethylchloride-type functions emanating from a central hexa-substituted benzene ring [24]. 

Synthesis of star-shaped copolymers by the core-first method. 868... 

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. 

The core-first method, which uses an active multifimctional core to initiate growth of polymer chains, was apphcable to make hybrid POSS-core star-shaped polyoxazohnes that showed an increase in Tg, compared to that of polyoxazohne initiated by methyl p-toluenesulfonate (MeOTs) with poly(2-methyl-2-oxazohne) (POZO) [76]. Other hybrid star-shaped polyoxazohnes initiated by cube-OTs or cube-benzyl revealed the same phenomenon. This was attributed to the reduction of segmental mobifity of POZO in starshaped polyoxazolines, which was caused by the incorporation of hard, compact POSS moiety to the core of star polymer with the core-first technique. The conclusions were drawn that the thermal stabilities of star-shaped polymers increased as the POSS wt % was increased, and this was used as a measure of the effect of the inorganic POSS unit on polymer thermal properties. 

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. 

Scheme 12 Synthesis of star-shaped polyelectrolytes by the a arm first and b core first methods...

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]. 

A new core-first method, termed as a star from in situ generated core, was used as an alternative strategr to synthesize CCS polymers" ATRP was applied for homopolymerization of ethylene glycol diacrylate (EGDA), a commercially available... 

In the case of star polymers (Scheme 2.28), these include symmetric, asymmetric, and miktoarm stars, which are prepared by reacting active chain ends with a core using the arm-first method or via the core-first method. Miktoarm stars have been reported using anionic, cationic, and ATRP methodologies and typically have AB o "... 

The divergent or core-first method This consists of the initiation of a controlled polymerization from a multifunctional initiator, which serves finally as the star central core. This technique requires a controlled and quantitative initiation followed by a living-like polymerization process without transfer or termination. In most cases, stars obtained by this approach possess a limited number of branches typically between three to five or six. A precise determination of the number of star branches using conventional characterization techniques is not easy and generally average values can be obtained. 

To address this issue, Hadjichristidis developed the synthesis of 3- and 4-arm star copolypeptides by high-vacuum polymerization using the core-first method. They prepared 3-arm stars containing poly(Z-Lys) and poly(Bn-Glu) block copolymers that were simultaneously grown off of a 2(aminomethyl)-2-methyl-1,3-propanediamine initiator core (Scheme 2). This method produced well-defined star copolymers with narrow molecular weight distributions. Attempts to prepare 4-arm star copolymers... 

Star polyethers have been synthesized by a core-first method. AB2- and A2B-type mikto-arm star copolymers consisting of aromatic polyether arms as the A segment and polystyrene arms as the B segment were synthesized by using orthogonal trifunctional initiators (Scheme 32) [73]. 

Star polymers exhibit interesting properties, especially their lower bulk and solution viscosities compared to linear analogues of the same molar mass. In addition, these architectures contain a higher amount of chain-end functionalities, which may be of high importance regarding many applications. Star polymers are usually prepared from CLRP by two different methods (1) the core-first method, and (2) the arm-first method. ... 

This method has been extended by Rempp and co-workers [39,40] as a general core-first method to prepare star-branched polymers, as shown in Scheme 4. The plurifimctional metalorganic initiator (A) was prepared by potassium naphthalene-initiated polymerization of DVB in tetrahydrofuran (THF) at -40°C with [DVB]/[K ] ratios of 0.5-3. Microgel formation was reported outside of this stoichiometric range or when m-DVB was used instead of either / -DVB or the commercial mixture. Within the prescribed stoichiometric ratios, star polymers (B) with arm functionalities varying from 8 to 42 were reported. As expected for this type of process, the polydispersities were described as being quite broad and were attributed primarily to a random distribution of core sizes and functionalities. 

In the second method, the core-first method, a polyfunctional core is used to initiate the polymerization of the branches of the star. This method allows for easy access to chain end functionalization by simple deactivation of the active sites. The extension of the core-first method to the preparation of functional star-shaped polymers in nonpolar solvents will be discussed. 

Star polymers generated using the core-first method have a large distribution in functionalities. To minimize the distribution in functionalities of the stars, the in-out technique was developed. The in-out method is a combination of the two techniques mentioned above and first generates a small arm-first star with living active sites, then uses this core to initiate the polymerization of the star branches. The resulting stars can be functionalized, and the control over the distribution in functionalities is greatly improved. 

III. STAR-SHAPED POLYMERS VIA "CORE-FIRST" METHODS... 

The arm-first methods are efficient at synthesizing well-defined star-shaped macromolecules. Difficulty arises, however, in the functionalization of the outer chain ends, which is only possible through the use of functional initiators to generate the precursor chains [58]. Core-first methods were developed extensively in polar solvents to access star-shaped macromolecules exhibiting functional groups at the outer chain ends. Once such species are obtained, they can serve as valuable intermediates in the elaboration of a large scope of macro-molecular architectures. Attempts to prepare polyfunctional initiators have been described by Nagasawa and co-workers [62], who s)mthesized core-first star... 

Several factors influence the functionality and the solubility of the plurifunc-tional initiator, including the solvent and counter-ion. In polar solvents, the solubility of the polyfunctional initiator is greater, the ionic associations are minimized, and the rate of initiation is greater compared to nonpolar solvents. Divinylbenzene (DVB) was chosen as the bifunctional compound for the preparation of a plurifunctional initiator in nonpolar solvents [65]. Motivation for the development of the core-first method for the polymerization of oxi-ranes in polar solvents was driven by the need to rend available functional poly(ethylene oxide) star-shaped macromolecules. The initiator systems developed by Burchard are based on lithium as a counterion, which is why it could not be applied to the preparation of multifunctional PEOs under mild conditions. 

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