Dual initiators

Elimination of unreacted monomers can be accompHshed by two methods dual initiators to enhance conversion of monomers to product (73—75) and steam stripping (70,76). Several process improvements have been claimed for dewatering beads (77), to reduce ha2e (78—81), improve color (82—86), remove monomer (87,88), and maintain homogeneous copolymer compositions (71,72,89). 

An interesting development regarding the synthesis of block copolymers involves the use of bifunctional or dual initiators, which are compounds capable of performing two mechanistically distinct polymerizations. The advantage of this procedure is that there is no need for intermediate transformation or activation steps. If the different initiation sites are equally active for the... 

In the second dual photo/thermal initiation strategy, the approach described above is augmented by the inclusion of a thermal initiator. Upon illumination, active centers produced by fragmentation of the photoinitiator start the polymerization reaction. The heat evolved from the exothermic photopolymerization elevates the temperature of the system and results in the production of additional active sites by the thermal initiator. This dual initiating strategy provides both the cure on demand (temporal control) afforded by photopolymerization, and the completeness of cure provided by the thermal initiator. 

Becer CR, Paulus RM, Hoppener S et al. (2008) Synthesis of poly(2-ethyl-2-oxazoline)-h-poly(styrene) copolymers via a dual initiator route combining cationic ring opening polymerization and atom transfer radical polymerization. Macromolecules 41 5210-5215... 

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]. 

The second strategy for the chemoenzymatic synthesis of block copolymers from enzymatic macroinitiators employs an individual modification step of the enzymatic block with an initiator for the chemical polymerization (route B in Fig. 4). This strategy has the advantage that it does not depend on a high incorporation rate of the dual initiator. On the other hand, quantitative end-functionalization becomes more... 

Apart from ATRP, the concept of dual initiation was also applied to other (controlled) polymerization techniques. Nitroxide-mediated living free radical polymerization (LFRP) is one example reported by van As et al. and has the advantage that no further metal catalyst is required [43], Employing initiator NMP-1, a PCL macroinitiator was obtained and subsequent polymerization of styrene produced a block copolymer (Scheme 4). With this system, it was for the first time possible to successfully conduct a one-pot chemoenzymatic cascade polymerization from a mixture containing NMP-1, CL, and styrene. Since the activation temperature of NMP is around 100 °C, no radical polymerization will occur at the reaction temperature of the enzymatic ROP. The two reactions could thus be thermally separated by first carrying out the enzymatic polymerization at low temperature and then raising the temperature to around 100 °C to initiate the NMP. Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-MeCL for the synthesis of chiral block copolymers. 

Also, Kerep and Ritter reported a radical chain transfer agent as a dual initiator, FRP-1 [45]. The first step builds on the fact that hydroxyl groups are much better nucleophiles in enzymatic ROP than thiols. Due to the chemoselectivity of the enzyme, PCLs with predominantly thiol endgroups were obtained, which were subsequently used as macroinitiator for styrene. The authors report that the reaction yield can be further increased by microwave irradiation. Although thiols provide less control over the radical polymerization than RAFT agents, the subsequent radical polymerization successfully leads to the synthesis of PCL-Z -PS. 

Remark 4 The GCD algorithm can start with either the primal subproblem, as shown in Figure 6.10, or with the dual subproblem, depending on whether a good primal or dual initial point is available. 

The thickness of UV-cured samples is generally less than 100 /am. To cure sample thicknesses up to 1 cm or more, it is possible to use a dual initiation process. The heat of reaction developed during the UV cure of the material located close to the surface is used to decompose a peroxide that initiates the polymerization in the bulk. 

Evidence favoring an associative mechanism was obtained in dual initiator studies. Under an associative mechanism with two different initiators in the same reactor each set of chains would grow at slightly different rates. Thus the MWD of the resulting polymer should be higher than the one with one initiator. This is the case when dimethylphenylsilyl ketene acetal and TMS were used to polymerize MMA with TBA biacetate as catalyst [38] (Scheme 22). 

In an interesting experiment, Yagci et al. polymerized cyclohexene oxide (CHO) via a photosensitized cationic polymerization with an initiator that contained a TEMPO moiety capable of CRP [281]. Anthracene was reacted with N-ethoxy-2-methyl pyridinium hexafluorophosphate, which produced a radical cation that could then be trapped with TEMPO to create the dual initiating species capable of both cationic and nitroxide-mediated polymerizations (Scheme36). 

The initial contribution to the development of such dual processes has been made by Heise and Palmans, who reported the combination of enzymatic ROP and ATRP [13], The authors applied dual initiator 1 and showed that during the course of the enzymatic ROP of CL the initiator is consumed to about 95%... 

Table 12.1 Initiators and monomer used in dual initiator approach to block copolymers. CL caprolactone 4MCL 4-methyl caprolactone MMA methyl methacrylate CMA glycidyl methacrylate FOMA perfluorooctyl methacrylate 10-HA 10-hydroxydecanoic acid.

Moreover, Heise and Palmans investigated the possibility of conducting both polymerizations as a one-pot cascade reaction in the presence of the dual initiator, ATRP catalyst, CL and methacrylates [16]. The study revealed that certain ATRP catalysts have a strong inhibiting effect on the enzyme. A nickel catalyst, for example, completely inhibited the enzyme, while certain copper catalysts had no... 

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

The use of the dual initiator strategy has opened up a whole new area of block copolymer synthesis using enzymes and over the previous five years many reactions and systems have been extensively reported. In general, the previous reports have shown that the use of supercritical C02 has allowed the two reaction mechanisms of eROP and free radical polymerization to occur simultaneously to yield well-defined block and graft copolymers. 

It is generally accepted that the eROP of CL is initiated by a nucleophilic attack of a hydroxy compound on the enzyme activated monomer (8-10). Various hydroxy-fimctionalized (macro)initiators have been reported in the past for the enzymatic synthesis of functionalized polymers and block copolymers, respectively (8-11). As outlined above, only a high initiation efficiency of the dual initiator in the eROP leads to high block copolymer yield. This efficiency mainly depends on two factors side reactions caused by competitive water initiation and the initiator design. We achieved the reduction of the water initiation by developing a thorough drying protocol prior to the enzymatic reaction (12). In order to study the influence of the initiator structure, several dual... 

Figure 1. Initiator conversion in enzymatic polymerizations of CL, employing two different dual initiators. (Reproducedfrom reference 12. Copyright 2005 American Chemical Society.)...

Dual initiators, i.e., 2-bromo(33 -diethoxypropyl)-2-methylpropiQnate (BrDEP), with two different initiating functions, produce unusual diblocks such as poly(vinyl methyl ether)-b-poly(acrylic acid) as shown. 

Systems using several initiators in a single polymerization reaction have also been proposed. The basis for this concept is the fact that at low polymer conversion, an acrylamide emulsion is more stable under shear than the monomer-containing emulsion prior to polymerization. Then a dual initiator system is used, with one initiator used at low temperature to start the polymerization, ufrile the second, less reactive initiator is utilized, at S0°C to complete the reaction [16]. In another example, inverse emulsions were polymerized in the presence of a biphase initiator system containing both a thermal oil-soluble initiator and a water-soluble initiator (redox couple). This invention was found to increase reproducibility and to improve the shelf-life of polymer emulsions [17]. 

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