ATRP initiators

In ATRP, alkyl halides (RX) are commonly used as the initiator. Fast initiation is important to obtain well-de ned polymers with low polydispersities (PDs). To obtain polymers with low PDs, 

Two parameters are important for a successful ATRP initiating system. First, initiation should be fast in comparison with propagation and, second, the probability of side reactions should be minimized. Several general considerations may be nsed for the initiator choice  

Tertiary alkyl halides are better initiators than secondary hahdes, which, in turn, are better than primary alkyl halides. Sulfonyl chlorides also provide faster initiation than propagaion. 

Using the same initiator, the sueeess of initiation can depend strongly on the choice of catalyst. For example, CQ4 is a good initiator for styrene and MMA with CuBr(bpy)3 as the catalyst, but the same is not true using the CuBr(dNbpy)2 eatalytic system (Matyjaszewski and Xia, 2001). 

The method or order of reagent addition (e.g., adding catalyst to the initiator or initiator to the catalyst) can be crucial. 

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In combination ATRP, the catalyst is again present in its more stable oxidized form. A slow decomposing conventional initiator e.g. AIBN) is used together with a normal ATRP initiator. Initiator concentrations and rate of radical generation arc chosen such that most chains arc initiated by the ATRP initiator so dispersities can be very narrow.290 The conventional initiator is responsible for generating the activator in situ and prevents build up of deactivator due to the persistent radical effect. Reverse or combination ATRP are the preferred modes of initiation for ATRP in emulsion or miniemulsion (Section 9.4.3.2).290 291... 

Poly(dimethyl siloxanc) with vinyl or hydrosilanc (Si-H) chain ends have been converted to ATRP initiator ends e.g. Scheme 9.62) by hydrosilylalion, Bis-functional dimethyl siloxane polymers prepared in this way were used in polymerizations of S, MA, tsobornyl acrylate and BA to form ABA triblock copolymers. 

Low molecular weight or polymeric ATRP initiators have been converted to dithiobenzoate RAFT agents by reaction with phcnylcthyl dithiobenzoate RAFT agent441,655 or by reaction with bis(thiobenzoyl) disulfides under ATRP conditions.483 It is likely that ATRP initiators can be transformed to other forms of RAFT agent by similar methods. 

Triblock copolymers can be prepared from diblock copolymers by a third monomer addition. They can also be prepared using a bis-funetional NMP or ATRP initiator or a bis-RAFT agent (for examples, see Table 9.13). Symmetrical trithiocarbonates (Table 9.15) should also be considered as bis-RAFT agents in... 

I he method of polymerization needs to be chosen for compatibility with functionality in the cores and the monomers to be used. Star block copolymers have also been reported. Mulli(bromo-compounds) may be used directly as ATRP initiators or they can be converted to RAFT agents. One of the most common... 

Several techniques have been applied in attaching the appropriate functionality to the polymer surface. For example, copolymeri/ation of a monomer containing functionality (alkoxyamine e.g. 358 or 359,744 ATRP initiator, e.g. 352,734 RAFT... 

The very small number of growing polymer chains, when compared to the monomer concentration results in a very low overall concentration of free control agent and leads to inefficient capping of chain ends. One solution to this problem is the addition of a free or unbound control agent to the polymerization medium. This can take the form of a low molecular weight alkoxyamine, ATRP initiator, RAFT agent or, alternatively, free deactivator such as nitroxide or Cu(II). This species is often called a sacrificial agent. This solution also leads to the formation of free polymer that must ultimately be removed from the brush. 

Grafting from silica particles, silicon wafers, and related surfaces usually involves attaching a chlorosilanc or alkoxysilane derivative. Thus alkoxyamincs (e.g, 361,744,749 3627 0) and a wide variety of ATRP initiators (e.g. 363751) have been attached directly to surfaces and used to initiate grafting from" processes. 

Polystyrene-Woc -polysulfone-/ /oc -polystyrene and poly(butyl acrylate)-Woc -polysulfone-/ /oc -poly(butyl acrylate) triblock copolymers were prepared using a macroinitiator.214 The hydroxyl-terminated polysulfone was allowed to react with 2-bromopropionyl bromide, an atomic transfer radical polymerization (ATRP) initiator, in the presence of pyridine. The modified macroinitiator could initiate die styrene polymerization under controlled conditions. 

ATRP and grafting from methods led to the synthesis of poly(styrene-g-tert-butyl acrylate)-fr-poly(ethylene-co-butylene)-fr-poly(styrene-g-ferf-butyl acrylate) block-graft copolymer [203]. ATRP initiating sites were produced along the PS blocks by chloromethylation as shown in Scheme 112. These sites then served to polymerize the ferf-butyl acrylate. The poly(ferf-butyl acrylate) grafts were hydrolyzed to result in the corresponding poly(acrylic... 

Fig. 5.8 Examples of polymers grafted from nanocarbons, (a) An ATRP initiator covalently attached to RGO via nitrene and carbodiimide chemistry was used for the growth of poly(2-(ethyl (phenyl)amino)ethyl-methacrylate). (b) A RAFT chain transfer agent is covalently attached to GO prior to polymerization of vinylcarbozole.

In a very similar way, hydroxy functionalized ATRP initiators such as 2,2,2-tribromoethanol can be used for the simultaneous polymerization of eCL and MMA (Scheme 25) [83]. Purposely, the ROP of eCL is promoted by Al(OfPr)3 added in catalytic amount so that the rapid alcohol-alkoxide exchange reaction (see Sect. 2.4) activates all the hydroxyl functions. In order to avoid initiation by the isopropoxy groups of Al(0/Pr)3. The in-situ formed zPrOH is removed by distillation of the zPrOH/toluene azeotrope. On the other hand, the ATRP of MMA is catalyzed by NiBr2(PPh3)3. The two aforementioned one-step methods provide block copolymers with controlled composition and molecular weights, but with a slightly broad MWD (PDI=1.5-2). 

In order to produce block copolymer brushes by ATRP directly from the surface, the ATRP initiator, (ll-(2-bromo-2-methyl)propionyloxy)imdecyltri-... 

We have repeated similar degrafting experiments for brush formation via ATRP. While there have been reports on degrafting using conventional radical polymerization [10,58], this discussion will be limited to brush formation by ATRP. In unpublished work [59], we immobilized an ATRP initiator, (1 l-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane) on StOber silica and conducted a styrene polymerization. Degrafting of the PS brushes was conducted by etching of the silica cores with HE From TGA analysis of the immobilized initiator and the corresponding PS brush system, we determined that there are 4.8 initiator molecules/nm and / = 0.06. The initiator density corresponds well to the values of 2.4-5.0 reported by Patten and co-workers [56,57] for the immobilization of (2-(4-chloromethylphenyl)ethyl)dimethylethoxysilane on a similar support. 

Depending on the monomer, one needs to adjust the components of the system as well as reaction conditions so that radical concentrations are sufficiently low to effectively suppress normal termination. The less reactive monomers, such as ethylene, vinyl chloride, and vinyl acetate, have not been polymerized by ATRP. Acidic monomers such as acrylic acid are not polymerized because they interfere with the initiator by protonation of the ligands. The car-boxylate salts of acidic monomers are polymerized without difficulty. New ATRP initiators and catalysts together with modification of reaction conditions may broaden the range of polymerizable monomers in the future. 

Sha et al. applied the commercially available dual initiator ATRP-4 for the chemoenzymatic synthesis of block copolymers. In a first series of publications, the group reported the successful synthesis of a block copolymer comprising PCL and polystyrene (PS) blocks [31, 32]. This concept was then further applied for the chemoenzymatic synthesis of amphiphilic block copolymers by macroinitiation of glycidyl methacrylate (GMA) from the ATRP functional PCL [33]. This procedure yielded well-defined block copolymers, which formed micelles in aqueous solution. Sha et al. were also the first to apply the dual enzyme/ATRP initiator concept to an enzymatic polycondensation of 10-hydroxydecanoic acid [34]. This concept was then extended to the ATRP of GMA and the formation of vesicles from the corresponding block copolymer [35]. 

A series of interesting block copolymer architectures has also been prepared by Zhang et al. In a first paper, the synthesis of H-shaped triblock copolymers was demonstrated from enzymatically obtained PCL diol after end-functionalization with a difunctional ATRP initiator [40]. This allowed the growth of two PS chains from each end of the telechelic PCL. When methanol instead of glycol was used as the initiator in the initial enzymatic CL polymerization, a PCL with one hydroxyl endgroup was obtained. Functionalization of this endgroup with the difunctional ATRP initiator and subsequent ATRP of styrene or GMA resulted in Y-shaped polymers (Scheme 3) [41, 42]. 

In a recent report, new nanocomposites of Au NPs and poly(4-vinylpyridine) were obtained through surface-initiated atom-transfer radical polymerization (SI-ATRP). The citrate-stabilized gold nanoparticles were first modified by the disulfide initiator [BrC(CH3)2COO(CH2)iiS]2 for ATRP initiation, and the subsequent polymerization of 4-vinylpyridine occurred on the surface of the gold particles. The assembled Au PVP nanocomposites are pH-responsive because of the pyridyl groups, which are facially protonated and positively charged. The micrographs show Au N Ps of around 15 nm size [92] (Scheme 3.14). 

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