Cascade polymerization

Scheme 4 One-pot chemoenzymatic cascade polymerization combining enzymatic ROP and NMP for the synthesis of (chiral) block copolymer [43]...

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. 

Figure 12.9 Block copolymers by one-pot enzymatic ROP and nitroxide-mediated living free-radical cascade polymerization [24].

Figure 4. Conversion of CL (d) and t-BMA (o) in a consecutive one-pot cascade polymerization (arrow marks addition of ATRP catalyst) in comparison with the conversion of CL (m) and t-BMA ( ) in a homopolymerization. Lines are added to guide the eye for data set of the cascade polymerization. (Reproduced with permission from reference 17. Copyright 2006 John Wiley and Sons, Ltd.)...

Fig. 13 Shaping molecular mass and comonomer distribution using the cascaded polymerization process...

Fig. 12 Left. One-Pot enzymatic ring opening and iiving free radical cascade polymerization [44], Right. Block copolymers by combination of enzymatic ROP and carbene-catalyzed ROP [120]...

Our interest from the outset has been in the possibility of crosslinking which accompanies inclusion of multifunctional monomers in a polymerizing system. Note that this does not occur when the groups enclosed in boxes in Table 5.6 react however, any reaction beyond this for the terminal A groups will result in a cascade of branches being formed. Therefore a critical (subscript c) value for the branching coefficient occurs at... 

Dry-Film Resists Based on Radical Photopolymerization. Photoinitiated polymerization (PIP) is widely practiced ia bulk systems, but special measures must be taken to apply the chemistry ia Hthographic appHcations. The attractive aspect of PIP is that each initiator species produced by photolysis launches a cascade of chemical events, effectively forming multiple chemical bonds for each photon absorbed. The gain that results constitutes a form of "chemical amplification" analogous to that observed ia silver hahde photography, and illustrates a path for achieving very high photosensitivities. 

Continuous polymerization in a staged series of reactors is a variation of this process (82). In one example, a mixture of chloroprene, 2,3-dichloro-l,3-butadiene, dodecyl mercaptan, and phenothiazine (15 ppm) is fed to the first of a cascade of 7 reactors together with a water solution containing disproportionated potassium abietate, potassium hydroxide, and formamidine sulfinic acid catalyst. Residence time in each reactor is 25 min at 45°C for a total conversion of 66%. Potassium ion is used in place of sodium to minimize coagulum formation. In other examples, it was judged best to feed catalyst to each reactor in the cascade (83). 

The second aspect of biocompatibility is a leaching problem. Ion-selective electrode materials, especially components of solvent polymeric membranes, are subject to leaching upon prolonged contact with physiological media. Membrane components such as plasticizers, ion exchangers and ionophores may activate the clotting cascade or stimulate an immune response. Moreover, they can be potentially toxic when released to the blood stream in significant concentrations. 

Microtubules have a key role in mitosis and cell-proliferation. They are dynamic assemblies of heterodimers of a and f3 tubullin. In the cell-reproduction cascade tubulin polymerizes fast and subsequently depolymerizes. Tubulin dimers are unusual guanyl nucleotide binding (G) proteins, which bind GTP reversibly at a site in the (3-tubulin. GTP irreversibly hydrolyzes to GDP during polymerization. 

A number of new resist materials which provide very high sensitivities have been developed in recent years [1-3]. In general, these systems owe their high sensitivity to the achievement of chemical amplification, a process which ensures that each photoevent is used in a multiplicative fashion to generate a cascade of successive reactions. Examples of such systems include the electron-beam induced [4] ringopening polymerization of oxacyclobutanes, the acid-catalyzed thermolysis of polymer side-chains [5-6] or the acid-catalyzed thermolytic fragmentation of polymer main-chains [7], Other important examples of the chemical amplification process are found in resist systems based on the free-radical photocrosslinking of acrylated polyols [8]. 

The initial products of the reactions between sugars and proteins may enter a cascade of reactions yielding fluorescence, browning, and polymerization of proteins ("cross-linking"). The brown pigments, so-called melanoidins, are polymers whose composition has not yet been established completely. Melanoidins bind calcium and may thus interfere with de- and remineralization in caries. 

Chain growth polymerizations (also called addition polymerizations) are characterized by the occurrence of activated species (initiators)/active centers. They add one monomer molecule after the other in a way that at the terminus of each new species formed by a monomer addition step an activated center is created which again is able to add the next monomer molecule. Such species are formed from compounds which create radicals via homolytic bond scission, from metal complexes, or from ionic (or at least highly polarized) molecules in the initiating steps (2.1) and (2.2). From there the chain growth can start as a cascade reaction (propagation 2.3) upon manifold repetition of the monomer addition and reestablishment of the active center at the end of the respective new product ... 

Chemoenzymatic polymerizations have the potential to further increase macro-molecular complexity by overcoming these limitations. Their combination with other polymerization techniques can give access to such structures. Depending on the mutual compatibility, multistep reactions as well as cascade reactions have been reported for the synthesis of polymer architectures and will be reviewed in the first part of this article. A unique feature of enzymes is their selectivity, such as regio-, chemo-, and in particular enantioselectivity. This offers oppormnities to synthesize novel chiral polymers and polymer architectures when combined with chemical catalysis. This will be discussed in the second part of this article. Generally, we will focus on the developments of the last 5-8 years. Unless otherwise noted, the term enzyme or lipase in this chapter refers to Candida antarctica Lipase B (CALB) or Novozym 435 (CALB immobilized on macroporous resin). 

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