Azo-initiators

GeneraHy, the commercially available azo initiators are of the symmetrical azonittile type ... 

Relatively high concentrations of organic peroxide or azo initiators are needed to obtain complete polymerization. After the reaction peak exotherm, polymerization slows down. Initiator concentrations must be high enough to complete conversion. Polymerization is inhibited by oxygen and copper, lead, and sulfur compounds (11). 

Polymethacrylates. Poly(methyl methacrylate) [9011-14-7] is a thermoplastic. Itis the acryUc resin most used in building products, frequendy as a blend or copolymer with other materials to improve its properties. The monomer is polymerized either by bulk or suspension processes. Eor glazing material, its greatest use, only the bulk process is used. Sheets are prepared either by casting between glass plates or by extmsion of pellets through a sHt die. This second method is less expensive and more commonly used. Peroxide or azo initiators are used for the polymerization (see Methacrylic polymers). 

Cittadini [60] and Corso [61] found that azo-initiated polymer liberates HCI more rapidly than peroxide-initiated material. Stromberg et al. [62] confirmed these findings and in addition found that PVC prepared by gamma irradiation was still more stable. Talamini and coworkers... 

Interfacial polycondensation between a diacid chloride and hexamethylenediamine in the presence of small amounts of ACPC also yield polymeric azoamid, which is a macroazo initiator.[27] In this manner, azodicarbox-ylate-functional polystyrene [28], macroazonitriles from 4,4 -azobis(4-cyano-n-pentanoyl) with diisocyanate of polyalkylene oxide [29], polymeric azo initiators with pendent azo groups [3] and polybutadiene macroazoinitiator [30] are macroazoinitiators that prepare block and graft copolymers. 

Azo Initiators as Transformation Agents for Biock Copolymer Synthesis... 

The thermal (or photochemical) decomposition of the azo group gives rise to a radically initiated polymerization. The reactive site F, the transformation site, however, can, depending on its chemical nature, initiate a condensation or addition type reaction. It can also start radical or ionic polymerizations. F may also terminate a polymerization or even enable the azo initiator to act as a monomer in chain polymerizations. 

The azo initiator initially present therefore has to be classified as a transfer agent—it is able to combine monomers polymerized by different polymerization modes with each other. Three different modes of block copolymer synthesis via azo transfer agents can be distinguished ... 

Figure 1 F = Transformation site MAI = macro-azo-initiator, having at least one azo group in the main chain.

The bifunctional azo initiator can initiate a polymerization of a monomer A forming a polymeric compound containing at least one azo group in its main chain [Figure 1, Scheme (1)]. Naturally, the monomer A has to have suitable sites for reacting with the transformation site F. The azo-containing polymers obtained in Scheme... 

Although most examples reported in the literature used azo initiators with two transformation sites F per molecule, in a few cases block copolymer synthesis was accomplished with transfer agents containing only one reactive site. The resulting macro-azo-initiators did contain exclusively terminal azo groups. 

II. MACRO-AZO-INITIATORS (MAIs)—KEY ELEMENTS IN BLOCK COPOLYMER SYNTHESIS... 

The number and location of azo functions in the polymeric azo initiator is substantial regarding its application for block copolymer synthesis. The cases illustrated in Table 1 will be discussed here. 

Bifunctional azo initiators with groups enabling them to participate in condensation or addition reactions can be classified as condensation radical and addition radical... 

The carboxyl terminated ACPA, 4,4 -azobis-(4-cya-nopentanoic acid), turned out to be a suitable reagent in condensation reactions. This compound can be prepared by Strecker s synthesis from levulinic acid following the method of Haines and Waters [12]. Regarding the formation of polymeric azo initiators, Matsakuwa et al. [13] reported on the condensation of ACPA with various diols and diamines in the presence of a condensation agent, I-methyl-2-chlorpyridinium iodide, and a cata-... 

Macro-azo-initiators containing crown ether units were successfully synthesized by Yagci et al. [37,38] condensing ACPC with the c s or trans forms of 4,4 -diaminodibenzo-18-crown-6 (Scheme 8). The polymeric... 

Furthermore, macro-azo-initiators have also been synthesized by direct condensation of AIBN with diols (Pinner reaction, [42,43]) or formaldehyde [44,45]. The... 

By means of a ring-opening polymerization of the condensation type Vlasov et al. [50] synthesized polypeptide based MAIs with azo groups in the polymeric backbone. The method is based on the reaction of a hydracide derivative of AIBN and a N-carboxy anhydride. Containing one central azo group in the polymer main chain, the polymeric azo initiator was used for initiating block copolymerizations of styrene and various methacrylamides. 

As was explained, block copolymer formation by azo initiators always involves at least one radically polymeri-... 

Moreover, free radical block copolymerization has been performed by means of low-molecular initiators containing two azo groups of different thermal reactivity. The first thermal treatment at a relatively low temperature in the presence of a monomer A results in a polymeric azo initiator. The more stable azo functions being situated at the end of A blocks can be subjected to a second thermal treatment at a higher temperature in the presence of monomer B. 

A bifunctional azo initiator containing terminal CCI3 groups was obtained by an addition type reaction using ACPA and trichloroacetyl diisocyanate [114]. In this... 

In the polymerization of St initiated with type II MAI composed of polyvinylpyrrolidone (PVP), block efficiency was kept to 80% when feed concentration was above 3 mol/L, but it drastically decreased below 3 mol/ L (Fig. 2) [36,37]. AIBN, the typical low-molecular weight azo initiator, shows a drastic decrease in its initiation efficiency below a critical feed monomer concentration, i.e., 0.5 mol/L. In the case of MAI, it seems that a similar decrease in initiation efficiency occurs at much higher critical monomer concentration due to immobility of macroinitiating radicals. 

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