Macromolecular initiators

An alternative procedure consisted of trichloroacetylation of the end groups and redox initiation with sulfur dioxide and pyridine309. In these two methods the block efficiency was not high. An improved method was proposed by Furukawa in which a macromolecular initiator is prepared from polyetherglycol and azobiscyanovaleroyl dichloride310 311. ... 

Block or graft copolymem can be obtained by cationic polymerization of THF with macromolecular initiators. The recommended groups for the initiation are the dioxolenium cation, the acyl cation and the super acid ester, each of which can be introduced into the backbone polymer by reaction with silver salts of strong acids. Introduction of the dioxolenium group into polystyrene was carried out by the following route32 ... 

In a more conventional approach, poly(S-b-CL) and poly(MMA-b-CL) block copolymers have been prepared from the same components as described previously, but in a two-step process via macromolecular initiators [47]. In a first step. 

Successive addition of monomers to the end of macromolecular initiator is the usual technique for the synthesis of tailored blockcopolymers. Anionic polymerization of pivalolactone, a-pyrrolidone— and the NCA of T-methyl-D-glutamate -2 was started from the end group of a prepolymer consisting carboxylate group or acyl lactam group or amino group. Living polymer of C-capro-lactone was expected to be formed by the initiated polymerization from polymer carbanion under kinetic controlled condition. 

The technique of the sulfur coupling reaction can be used also to prepare multiblock polymers. The technique of deactivation of carbanionic polymer with oxygen or sulfur is able to yield numerous interesting organic compounds such as novel macromolecular initiators, new macromolecular additives, and telechelic polymers. Finally, the coupling reactions can be used to build block polymers. 

This technique is based on the use of a linear polymer with pendant functional groups that can be activated to initiate the polymerization of a second monomer. Based on this definition, the linear precursor polymer can be considered as a multifunctional macromolecular initiator. The importance of the grafting from technique by cationic polymerization of the second monomer increased considerably with the advent of living cationic polymerization. The advantage is the virtual absence of homopolymer formation via chain transfer to monomer. 

The method of synthesis20-25 consists in the preparation by anionic polymerization of a monodisperse first block, followed by chemical modification of its living end in order to introduce a terminal primary amine function into the molecule this to aminated polymer is then used as a macromolecular initiator for the polymerization of the N-carboxy anhydride (NCA) of the desired amino add. 

Besides the use of micromolecular multiinitiators, block copolymers can be obtained from macromolecular initiators. In a first step, a polymeric initiator is generally synthesized by reacting a mono- or difunctional polymer with a functional initiator. Various macromolecular initiators were prepared in this way including quite different sequences polystyrene [13, 18, 19, 25, 26], poly(dimethylsiloxane) [27], polymethylmethacrylate) [13,15,28], polyvinylacetate [28], polyvinylchloride [29, 30], polyesters [30], polycarbonate [31,32], polybutadiene [13, 25, 33], polyamide [34], polyethylene glycol) [35] or polyaromatic [36], An excellent review of the synthesis and uses of such macroinitiators was written by Nuyken and Voit [37]. Thus, only few typical examples are going to be mentioned below. 

It is seen that free radical micromolecular or macromolecular initiators have been successfully employed for the synthesis of di-, tri- or multiblock copolymers. However, once again, the structure of these block copolymers depends upon the termination step of the polymerization, and especially on the recombination or disproportionation of macroradicals produced. Besides, such a method also generates homopolymers. Separation and purification of these different structures are usually very difficult or even impossible. Moreover, the copolymers obtained usually exhibit a broad polydispersity, a defect inherent in the classical radical process. 

How adsorption of the side chains to a flat substrate effects the backbone conformation has been observed in further microscopic detail for brush molecules with a methacrylate backbone and poly-(n-butyl acrylate) side chains. These poly(/v-butyl acrylate) brushes were prepared by living radical grafting from a multifunctional macromolecular initiator.38 The synthetic approach allowed observation of the same batch of molecules without (macroinitiator) and with poly(n-butyl acrylate) side chains (brush). 

Synthesis of amphiphilic copolymers having a hydrophobic polystyrene tail with a charged helical poly(isocyanide) headgroup were prepared by polymerization of the peptide-derived isocyanide 50a with the macromolecular nickel initiator 63 (Scheme 42) [71]. The macromolecular initiator was prepared by reaction of (f-BuNC)4Ni(II)(Cl04)2 (46a) with polystyrene, which was end-capped with an amino group. The block copolymer 64 thus ob-... 

Figure 3. GPC analysis of poly-TBA-poly PEMA block-copolymer (A) and of the poly-TBA used as the macromolecular initiator(B).

Capping of the PS "Front Block with 1,1-DPE to afford a highly hindered, less basic macromolecular initiator for methacrylate polymerization. 

Grafting from is based on the use of a macromolecular initiator, that is, a polymer backbone containing dioxolenium salts as precursors of carbenium and oxocarbeniura ions (i.e., labile halides or acylhalides). The last two functional groups can be transformed into a carbenium ion by reaction with a silver salt containing a stable anion. When the monomer used is a strong nucleophile (such a cyclic imino ethers) the labile halide itself can initiate graft copolymerization. 

Another approach for the preparation of BCMO graft copolymers was based on the use of macromolecular initiators 38). Polystyrene or polyTHF containing dioxole-nium groups in the chain were prepared and used as initiators of BCMO polymerization as shown for polystyrene ... 

Macromolecular initiators have also been applied successfully. The left column of Table VI represents several examples of such initiators. Poly(tetrahydrofuran) and poly(norbomene) were used as macroinitiators for ATRP to m block copolymers. 

Table VL Macromolecular initiators with one or two functionalities (F) were reacted with a second monomer using ATRP with a blockcopolymer or a triblockcopolymer as result...

The superbase DBU was chosen as catalyst for the ROP of lactide because of its fast kinetics, high efficiency, and prevention of transesterification. However, it was decided to add bis(3,5-trifiuoromethyl)phenyl cyclohexylthiourea (thiourea) as co-catalyst to further increase the efficiency and overcome possible steric hindrance arising from the bulkiness of macromolecular initiators. The thiourea was synthesized according to Pratt et al. [41]. Briefly, 3,5-bis(trifluoromethyl)phenyl isothiocyanate (3.37 ml, 18.5 mmol) and anhydrous tetrahydrofuran (20 ml) were added to a flame-dried two-neck round bottom flask. Cyclohexylamine (2.11 ml, 18.5 mmol) was added dropwise via a syringe at room temperature to the stirring solution. After... 

Acetates of tin, lead, manganese, antimony, and zinc as well as esters of orthotitanates catalyze the reactions Optimum temperatures for these reactions are between 230-260 C at 0.03-1 mm Hg pressure. Block copolymers can also form by ring-opening polymerizations of lactones, when carboxy-terminated macromolecular initiators are used" ... 

At the same time, Wnek et al. [98] published a series of papers concerned with the synthesis of other copolymers. Two synthetic routes were developed. The first one used doped PA as a macromolecular initiator for the synthesis of graft copolymers, and the second one used the transformation reaction as a route for the preparation of block copolymer. 

By using the grafting from technique, PMMA-g-(y3-butyrolactone) (34) copolymers were synthesized. Anionically polymerized PMMA was treated with 18-crown-6 complex of potassium hydroxide resulting in a macromolecular initiator (2) (Fig. 6). 

An interesting application of anionic polymerization is the living dispersion polymerization (LDP) which was reported by Kim et al. [302]. LDP is one of the best methodologies to prepare pm sized polymer particles. The authors did LDP-copolymerization of styrene and divinylbenzene using poly(t-butylstyryl)lithium as macromolecular initiator/stabilizer. 

Lutsen, L., et al. (1998). Poly(methylphenylsilylene)-block-polystyrene copolymer prepared by the use of a chloromethylphenyl end-capped poly(methylphenylsilylene) as a macromolecular initiator in an atom transfer radical polymerization of styrene. Eur. Polym. J., 34(12) 1829-1837. 

Stalmach, U., de Boer, B., Post, A.D. et al. (2001) Synthesis of a conjugated macromolecular initiator for nitroxide-mediated free radical polymerization. Angewandte Chemie International Edition, 40,428. 

More recently, Wagner and coworkers used rhodium catalysts for the alkylcatecholborane-mediated hydroboration of PBD.In their first study, the authors used catecholborane-mediated hydroboration to give alkoxyamines via nitroxide oxidation with TEMPO (Fig. 20A). With RhCl(PPh3)3 as the catalyst, hydroboration of PBD occurred preferentially at the vinyl —CH=CH2 bonds on the side chains, whereas internal olefins were converted to a smaller extent. The total functionalization was approximately 50%. In their next study, the authors used the same chemistry to achieve 74% hydroboration/nitroxide oxidation of C=C bonds in PBD. They subsequently used alkoxyamine substituents as macromolecular initiators for the controlled nitroxide-mediated polymerization of styrene or butyl acrylate to generate comb-shaped polymers (Fig. 20B). Fligh conversions were achieved after 24 h, and PD Is were low in aU cases (M /M = 1.3-1.5). 

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