Trifunctional initiator

Living polymerization of isobutylene (IB) by di- and trifunctional initiators to make the nearly uniform rubber mid-block... 

Double hydrophilic star-block (PEO-fo-PAA)3 copolymers were prepared by a combination of anionic and ATRP of EO and fBuA [150]. Three-arm PEO stars, with terminal - OH groups were prepared by anionic polymerization, using l,l,l-tris(hydroxymethyl)ethane, activated with DPMK as a trifunctional initiator. The hydroxyl functions were subsequently transformed to three bromo-ester groups, which were utilized to initiate the polymerization of f-butyl acrylate by ATRP in the presence of CuBr/PMDETA. Subsequent hydrolysis of the f-butyl groups yielded the desired products (Scheme 74). 

Miktoarm stars of the A(BC)2 type, where A is PS, B is poly(f-bulyl acrylate) (PtBA), and C is PMMA [161] have been synthesized, by using the trifunctional initiator 2-phenyl-2-[(2,2,6,6-tetramethyl)-l-piperidinyloxy] ethyl 2,2-bis[methyl(2-bromopropionato)] propionate (NMP, ATRP) (Scheme 86). In the first step, a PS macroinitiator with dual < -bromo functionality was obtained by NMP of styrene in bulk at 125 °C. This precursor was subsequently used as the macroinitiator for the ATRP of ferf-bulyl acry-... 

The anionic polymerization of isocyanates using NaCN as an initiator was first reported in I960.998 The living coordinative polymerization of n-hexylisocyanate has been described using the titanium(IV) complexes (345) (348).999-1001 A trifunctional initiator has also been used to prepare star polyisocyanates.1002... 

Di- or trifunctional initiators have also been developed to design the polymer structures including ABA-type block copolymers, and star polymers and star block copolymers. 

Compounds containing two or more carbon-carbon double bonds also act as coupling agents and also as multifunctional initiators [Hadjichristidis et al., 2001 Quirk et al., 2000]. Such compounds can also be used to synthesize multifunctional initiators that subsequently produce star polymers. Consider l,3,5-tris(l-phenylethenyl)benzene (XL). Reaction with r-butyllithium produces a trifunctional initiator XLI, which initiates polymerization of a monomer such as styrene to form a 3-arm star polystyrene [Quirk and Tsai, 1998]. The 3-arm... 

A synthetic approach to poly(ester-urethanes) was recently published by Xue et al. [48]. Glycerol was employed as a trifunctional initiator in the enzymatic ROP of CL. The three-arm PCL triol was then reacted with methylene-diphenyl diisocyanate (MDI) and hexanediol to yield a three-arm PCL-based poly(ester-urethane) with shape-memory properties. 

Polyfunctional OH Components (Crosslinkers). For crosslinking, trifunctional alcohols are used mainly. Some are of the same type as the difunctional prepolymers—e.g., polyethers derived from trifunctional initiators (glycerol, trimethylolpropane, 1,2,6-hexanetriol) and propylene or butylene oxide—and are preferred. Low molecular weight alcohols are used also. Examples are the ones listed above and glycerol monoricinole-ate, glycerol triricinoleate and amino alcohols like triethanolamine. 

In most cases, linear prepolymers are used for the production of elastomers. Gas evolution via dry polyols or diamines is therefore not encouraged. When a foam is required, however, a slight adjustment is made in the construction of the prepolymer. We referred to it earlier when we discussed polyfunctional polyols. By including a trifunctional alcohol in the prepolymer recipe or by using a polyol from a trifunctional initiator (e.g., trimethylol propane), a three-dimensional character is introduced into the prepolymer. 

Three arm star polymers of IBVE were synthesized by living cationic polymerization using trifunctional initiators 8 and 9 with the same trifluoroacetate initiating functions but different cores [19, 20]. The experimental conditions were selected to obtain living polymerization. A series of acetic acid derivatives including trifluoroacetic acid and the IBVE-acid adduct were found to be efficient... 

Three arm amphiphilic star block copolymers of IBVE and 2-hydroxyethyl vinyl ether (HOVE) were prepared using the trifunctional initiator 8 with sequential cationic polymerization of two hydrophobic monomers, IBVE and AcOVE. Subsequent hydrolysis of the acetates led to the hydrophilic poly(HOVE) segments [38]. Two types of stars were prepared depending on which monomer was polymerized first three arm star poly(IBVE-h-HOVE), with the hydrophobic part inside and three arm star poly(HOVE-h-IBVE), with the hydrophobic part outside. When IBVE was polymerized first, the experimental conditions were the same as described in Sect. 2.2.1. After reaching quantitative monomer conversion, AcOVE was added and temperature was raised from 0 to 40 °C to accelerate the reaction since this monomer is less reactive than IBVE. When starting with AcOVE as a first block, both polymerizations were carried out at 40 °C. SEC analysis showed that MWDs were narrow for the two steps whatever the se-... 

The key to this initiation is the trifunctional vinyl ether (16) from which the corresponding trifunctional initiator (17) is prepared by the addition of 3 equivalents of trifluoroacetic acid. Note that the three initiating sites in 17 are well separated spatially from each other by the rigid and planar triphenylmethyl core to avoid intramolecular side reactions be-... 

Scheme 9 Synthesis of triarmed poly(vinyl ethers) with a trifunctional initiator...

Haloester-type trifunctional initiators are obtained from triols by a method similar to those for bifunctional haloesters. 3-arm star polymers of MMA are obtained with dichloroacetates MI-28 and MI-29, for which Ru-1 and Al(acac)3 are employed.228 The polymers have controlled molecular weights and narrow MWDs. Similarly, MI-30 and MI-31 with copper catalysts gave 3-arm star polymers of styrene, acrylates, and methacrylates suitable copper catalysts vary with each monomer.199,326 358 368 The obtained star polymers can be further transformed into star block copolymers comprised of hydrophilic/hydropho-bic368 or organic/inorganic326 segments by block copolymerizations of other monomers. 

A novel hydrocarbon-soluble trifunctional initiator was proposed by Quirk et al.25 It was prepared by the reaction of 3 mol of sec-butyllithium (s-BuLi) with l,3,5-tris(l-phenylethenyl)benzene (tri-DPE), as presented in Scheme 5. This initiator was found to be efficient for the polymerization of styrene only when THF was also added in the reaction mixture ([THF] [s-BuLi] = 20). The polymerization reaction was monitored by UV—vis spectroscopy. The limitations of the method include the extreme care that should be exersized over the stoichiometry of the reaction between s-BuLi and tri-DPE and the fact that a minimum arm molecular weight around 6 x 103 is required for a successful synthesis. For arm molecular weights lower than this limit, incomplete initiation was observed. If these requirements are fulfilled, well-defined three-arm polystyrene stars can be prepared. 

The same initiator was also used to produce a three-arm PBd star.26 Complete monomer consumption was observed, but SEC analysis showed a bimodal distribution. This behavior was attributed to the strong association effects of the trifunctional initiator in a nonpolar solvent. The problem was overcome when s-BuOLi was added in the reaction mixture in a ratio [s-BuLi] [s-BuOLi] = 2. s-BuOLi was shown to be capable of disrupting the initiator association without affecting appreciably the microstructure of the PBd chains. Therefore, a well-defined star polymer with low molecular weight distribution was obtained. 

Other ABC triblock copolymers were prepared using mono-, di-, and trifunctional initiators to produce ABC, ABCBA, and [ABC]3 linear and 3-arm star block copolymers. For example, a difunctional pSt macroinitiator (Mn=1100, Mw/Mn=1.17) was chain extended with tBA (Mn=l 1,200, Mw/Mn=1.16) which was subsequently chain extended with MA (Mn=27,250, Mw/Mn=l. 14), as shown in Fig. 21. 

If trifunctional initiators such as glycerol or trimethylolpropane are used as starters for the alkylene oxides polymerisation, star-like polyether triols are formed [1-13, 15-17, 54, 60, 69, 75] ... 

Three arm amphiphilic star-block copolymers of isobutyl vinyl ether (IBVE) and 2-hydroxyethyl vinyl ethers were also prepared by Higashimura et al. [23]. The synthetic strategy involved the sequential living cationic polymerization of IBVE and 2-acetoxyethyl vinyl ether, initiated by the trifunctional initiator system composed of tris(trifluoroacetate) and ethylaluminum dichloride, with excess of 1,4-dioxane as a carbocation-stabilizing Lewis base (Scheme 8). 

In addition, dihlock copolymers, triblock copolymers, star homopolymers, and block copolymers can be obtained via anionic polymerization methods using difunctional and trifunctional initiators. Suitable initiators include sulfonyldiphenol bisphenol A, and phloroglucinol. 

As a typical example, Frey etal. [16, 27] described the anionic polymerization of glycidol, which was considered also as a latent AB2 (= ABB ) monomer (Scheme 24.3). The polymerization proved to be very versatile and led to hb polymers with a rather narrow molar mass distribution (Mu,/M = 1.1-1.4) due to a chain growth-hke character of the reaction when only partial deprotonation to the initiating alkoxide (initiating site, triol in Scheme 24.3) was performed. This led to a more or less simultaneous growth of all chain ends, and allowed control of both the molar mass and polydispersity. By use of the trifunctional initiator (core molecule) and slow monomer addition, cychzation was suppressed such that the molar mass and polydispersity could be controlled. 

The use of click chemistry has also influenced the construction of more sophisticated star polymers, such as those with block copolymer arms. Maty-jaszewski has eloquently demonstrated the preparation of three-arm star block copolymers by again combining ATRP with CuAAC click couphng [109]. In these studies the ATRP of styrene, starting from a trifunctional initiator, yielded the three-arm star homopolymer bearing bromide end groups that subsequently were transformed by substitution with sodium azide. CuAAC reaction with PEO-alkyne... 

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