Macroinitiated polymerization

To prove the controlled nature of the polymerization initiated from the surface by the PGMA/BAA macroinitiator, polymerization of styrene was conducted using the already grown PS brush of 19nm as an initiator. It was... 

Several macrointermediates to obtain this kind of copolymer were used via free radical, ionic, and/or free radical-ionic coupling polymerization. In this manner, macroinitiators, macromonomers, and macromono-meric initiators will be discussed in this chapter. 

Macroinitiators are macromolecules having peroxygen and/or azo groups that can thermally initiate a vinyl polymerization to obtain block copolymers in one step. They can be classified as macroperoxyinitiators (MPl), macroazoinitiators (MAI), and macroazo-peroxyiniti-ators. 

Vinyl polymerization initiated by macroinitiator yield AB, ABA, or (AB) types block copolymers. Macroinitiators such as macrobis peroxides, polyazoesters, and... 

Block copolymers have been synthesized on an industrial scale mainly by anionic or cationic polymerization, although monomers for block components are limited to ones capable of the process. Intensive academic and technological interest in radical block copolymerization using macroinitiators is growing. This process can be implemented in plants with easier handling of materials, milder conditions of operation, and a variety of materials to give various kinds of block copolymers to develop a wide application area [1-3]. 

Monomer A is polymerized initiated with a pair of radicals formed by thermolysis of an active site of macroinitiator. Since growing chain A propagates from the residual segment of the initiator, polymer A thus formed retains unreacted active sites in the chain end. 

Utilization of another function of the macroinitiator was tried in emulsion polymerization [30]. An MAI composed of PEG (molecular weight of a segment is 1000) linked with AGP units was confirmed to be usable as a surface active initiator (Inisurf) for preparing PSt-b-PEG [30]. A higher molecular weight block copolymer was obtained in comparison with the case of solution copolymerization. 

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. 

Aliphatic disulfides are not thought to be effective as initiators in this context. However, Endo et a . K have described the use of the cyclic 1,2-disulfides 11 and 12 as initiators in a controlled radical polymerization. Polymerization of S at 120 °C gave a linear increase in molecular weight with conversion and the PS formed was used as a macroinitiator to form PS-6/oet-PMMA. The precise mechanism of the process has not been elucidated. 

The first use of sterically hindered hexasubstituted ethanes [e.g. 33] as initiators of polymerization was reported by Bledzki et al.77,78 The use of related initiators based on silylatcd pinacols [e.g. 34, 35] has been reported by Crivcllo et a/.,7l>82 jjan( os et ai i anOther initiators of this class include 36 fi X/ and 37.The rates of decomposition of hexasubstituted ethanes and the derived macroinitiators are known to vary according to the degree of steric... 

The proposed polymerization mechanism is shown in Scheme 9.12. Thermal decomposition of the hexasubstituted ethane derivative yields hindered tertiary radicals that can initiate polymerization or combine with propagating species (primary radical termination) to form an oligomeric macroinitiator. The addition of the diphenylalkyl radicals to monomer is slow (e.g. k[ for 34 is reported as KT M"1 s l at 80 °C84) and the polymerization is characterized by an inhibition period during which the initiator is consumed and an oligomeric macroinitiator is formed. The bond to the Cl I formed by addition to monomer is comparatively thermally stable. 

Otsu and Tazaki90 have reported on the use of triphenylmethylazobenzene (39) as an initiator. In this case, phenyl radical initiates polymerization and the triphenylmethyl radical reacts mainly by primary radical termination to form a macroinitiator. The early report91 that triphenylmethyl radical does not initiate MMA polymerization may only indicate a very low rate of polymerization. The addition of triphenylmethyl radical to MMA has been demonstrated in radical... 

It is of interest to speculate on the precise structure of the macroinitiator species in these polymerizations. The work of Engel et a .94 suggests the likelihood of a quinonoid intermediate (e.g. 45, Scheme 9.13), at least for the polymerizations involving triphenylmethyl radical (44). 

There will be a gradual loss of stable radical with these systems as the di- or triarylmethyl radicals produced from the macroinitiator can add monomer, albeit slowly.99 100 This side reaction provides a mechanism for mopping up the excess stable radical formed as a consequence of termination between propagating radicals and may be essential to maintaining polymerization rates. 

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

Ring-opening metathesis polymerization (ROMP) of 1,4-cyelooctadiene was used to prepare poly(l,4-B) terminated with halo end groups.647 This was then used as a macroinitiator of ATRP with heterogeneous Cu bpy catalysts to form PS- >/ti /r-poly(l,4-B)-WoeA -PS and PMMA-Moc.T-poly(l,4-B)-Wof A-PMMA. 

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. 

The first step of NCA polymerization is usually accomplished by the use of nucleophilic initiators. These initiators can be alkoxides, alcohols, amines, transition metals, and even water [53,54]. In order to synthesize a copolymer diblock, the polymerization of the second block and its connection to the previously formed block are performed in a single process. This is achieved by initiating the polymerization of the second NCA monomer using the first homopolypeptide as a macroinitiator. Precipitation and purification processes follow to isolate the... 

The polymerization of 2-(diethylamino)ethyl methacrylate, DEAEMA, was studied under different conditions. It was shown that the best system providing narrow molecular weight distribution polymers involved the use of p-toluenesulfonyl chloride/CuCl/HMTETA as the initiator/catalyst/ligand at 60 °C in methanol [72]. Taking advantage of these results, well-defined PDEAEMA-fr-PfBuMA block copolymers were obtained. The synthesis was successful when either fBuMA or DEAEMA was polymerized first. Poor results with bimodal distributions were obtained when CuBr was used as the catalyst. This behavior was attributed to the poor blocking efficiency of PDEAEMA-Br and the incomplete functionalization of the macroinitiator. 

The direct synthesis of poly(3-sulfopropyl methacrylate)-fr-PMMA, PSP-MA-fr-PMMA (Scheme 27) without the use of protecting chemistry, by sequential monomer addition and ATRP techniques was achieved [77]. A water/DMF 40/60 mixture was used to ensure the homogeneous polymerization of both monomers. CuCl/bipy was the catalytic system used, leading to quantitative conversion and narrow molecular weight distribution. In another approach the PSPMA macroinitiator was isolated by stopping the polymerization at a conversion of 83%. Then using a 40/60 water/DMF mixture MMA was polymerized to give the desired block copolymer. In this case no residual SPMA monomer was present before the polymerization of MMA. The micellar properties of these amphiphilic copolymers were examined. 

The transformation of the chain end active center from one type to another is usually achieved through the successful and efficient end-functionalization reaction of the polymer chain. This end-functionalized polymer can be considered as a macroinitiator capable of initiating the polymerization of another monomer by a different synthetic method. Using a semitelechelic macroinitiator an AB block copolymer is obtained, while with a telechelic macroinitiator an ABA triblock copolymer is provided. The key step of this methodology relies on the success of the transformation reaction. The functionalization process must be 100% efficient, since the presence of unfunctionalized chains leads to a mixture of the desired block copolymer and the unfunctionalized homopolymer. In such a case, control over the molecular characteristics cannot be obtained and an additional purification step is needed. 

Anionically prepared hydroxy-terminated PBd was reacted with AlEt3 to form the corresponding aluminum alkoxide macroinitiator, capable of initiating the polymerization of L-lactide [117]. Using ratios [PBd-OH]/[AlEt3] between 1 and 6, reaction temperatures between 70 and 120 °C and maintaining the conversion of the lactide polymerization below 90%, products with narrow molecular weight distribution were obtained. 

The triblock terpolymer polypropylene oxide)-h-poly[2-(dimethylami-no)ethyl methacrylate]-b-poly[oligo(ethylene glycol) methacrylate], PPO-fc-PDMAEMA-fc-POEGMA, was prepared using the PPO macroinitiator followed by the addition of CuCl, HMTETA, and DMAEMA for the polymerization of the second block and finally OEGMA for the synthesis of the final product (Scheme 54) [128]. 

Employing similar procedures, PPO-fc-POEGMA block copolymers and POEGMA-fc-PPO-fc-POEGMA triblock copolymers were prepared from the corresponding PPO macroinitiators [129]. The polymerizations were performed in a isopropanol/water (70/30) mixture at 20 °C using CuCl and bpy. The methacrylate monomer was almost quantitatively polymerized, and the polydispersities were lower than 1.25 in most cases. Less than 5% PPO homopolymer contamination was detected by SEC analysis. 

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