Polymerization chiral monomers

Transition metal coupling polymerization has also been used to synthesize optically active polymers with stable main-chain chirality such as polymers 33, 34, 35, and 36 by using optically active monomers.29-31 These polymers are useful for chiral separation and asymmetric catalysis. For example, polymers 33 and 34 have been used as polymeric chiral catalysts for asymmetric catalysis. Due... 

In one approach, polymethacrylate-type monoliths have been fabricated by copolymerization of the chiral monomer 0-9-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine 1 or 0-9-(tert-butylcarbamoyl)-ll-[2-(methacryloyloxy) ethylthio]-10,ll-dihydroquinine 2 (see Figure 1.34a), the comonomer 2-hydroxyethylmethacrylate (HEMA), the crosslinker ethylenedimethacrylate (EDMA) in presence of the binary porogenic solvent mixture cyclohexanol and 1-dodecanol, directly in a single step within fused-silica capillaries. Initiation of the polymerization by either thermal treatment or UV irradiation yielded microglobular polymer morphologies, such as those well known from their corresponding nonchiral... 

Synthetic polymers This type of chiral selectors (polyacrylamides, polymethacrylamide, etc.) was first developed by the group of Blaschke by polymerization of chiral monomers with cross-linking agents to form a three-dimensional polymer network. 

The polymerization of racemic mixtures of chiral monomers gives different results depending on the structure of the monomer and the type of polymerization process. For monomers containing only one asymmetric carbon atom we can distinguish three processes ... 

A method for obtaining optically active polyiminomethylenes from achiral monomers was recently devised by Nolte, Drenth and co-workers (420). It consists in the copolymerization of an achiral monomer (e.g., phenyl isocyanide) with an optically active isocyanide endowed with a low tendency to polymerize. The chiral monomer is incorporated in one of the two helices and, due to its low reactivity, stops or slows down its growth. The other helix is unaffected by this phenomenon and continues to grow, permitting the almost complete conversion of the achiral monomer into an optically active polymer. 

Polymerization in which a tactic polymer is formed. However, polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific. For example, the polymerization of a chiral monomer, e.g., R)-propylene oxide ((i )-methyloxirane), with retention of configuration is not considered to be a stereospecific reaction however, selective polymerization, with retention, of one of the enantiomers present in a mixture of R)- and (S)-propylene oxide molecules is so classified. 

Note 2 A polymerization in which, starting from the racemate of a chiral monomer, two types of polymer molecules, each containing monomeric units derived from one of the enantiomers, form in equal amounts is termed racemate-forming enantiomer-differentiating polymerization . The resulting polymer is optically inactive (see Note 4 of Definition 2). 

Chirality induction can be achieved in homo- and copolymerization of vinyl monomers based on chiral monomer structure [1,3,8,9]. The first example of this type of polymerization was the copolymerization of (S)-a-methylbenzyl methacrylate with maleic anhydride the polymerization product showed [a]D +23° after removal of the chiral side group [73]. For another example, the copolymerization of an optically active styrene derivative (39) with N-phenylmaleimide (17, R = -Ph) followed by removal of the optically active side group and deboronation gave an optically active N-phenylmaleimide-styrene copolymer [74]. 

Fig. 6.23. Effect of thermal (a) and UV initiation (b), type of comonomer, and percentage of 1-dodecanol in the polymerization mixture on the mode pore diameter of quinidine-functionalized chiral monoliths. (Reprinted with permission from [56]. Copyright 2000 American Chemical Society). Reaction conditions polymerization mixture, chiral monomer 25 8 wt%, glycidyl methacrylate ( ) or 2-hydroxyethyl methacrylate ( ) 16 wt%, ethylene dimethacrylate 16 wt%, porogenic solvent 60 wt% (consisting of 1-dodecanol and cyclohexanol), polymerization time 20 h at 60°C (a) and 16 h at room temperature (b).

In research with Ziegler catalysts, Cossee (11) and Arlmann and Cossee (12) hypothesized that the insertion of propylene monomer takes place in a cis conformation into a titanium-carbon bond. Natta et al. (8) postulated that in the stereospecific polymerization, chiral centers on the surface are needed to produce isotactic polymers. These and other issues regarding the nature of the active sites have helped to increase the interest in investigations of homogeneous metallocene catalysis. 

Chiral polymers have been applied in many areas of research, including chiral separation of organic molecules, asymmetric induction in organic synthesis, and wave guiding in non-linear optics [ 146,147]. Two distinct classes of polymers represent these optically active materials those with induced chirality based on the catalyst and polymerization mechanism and those produced from chiral monomers. Achiral monomers like propylene have been polymerized stereoselectively using chiral initiators or catalysts yielding isotactic, helical polymers [148-150]. On the other hand, polymerization of chiral monomers such as diepoxides, dimethacrylates, diisocyanides, and vinyl ethers yields chiral polymers by incorporation of chirality into the main chain of the polymer or as a pedant side group [151-155]. A number of chiral metathesis catalysts have been made, and they have proven useful in asymmetric ROM as well as in stereospecific polymerization of norbornene and norbornadiene [ 156-159]. This section of the review will focus on the ADMET polymerization of chiral monomers as a method of chiral polymer synthesis. 

LI.2 Synthetic polymeric type CSPs. With the aim of mimicking nature and naturally occurring biopolymeric SOs like polysaccharides or proteins, researchers have developed several approaches for the preparation of new types of synthetic macromolec-ular SOs. These new polymeric SOs may be divided into (a) SOs synthesized from achiral monomers including helical polyacrylates and molecular imprint type CSPs and (b) SOs synthesized from chiral monomers including polyacrylamides and network polymers based on tartaric acid diamides. 

Network-polymeric CSPs based on cross-linked tartaric acid diamides. Recently, a new class of network-polymeric-based CSPs has been proposed by Allenmark et al. [I94j. CSPs of this class are based on A,A -diallyl-(/ .R)-tartaric acid diamide (DATD) as chiral monomers. The chiral monomers are polymerized and cross-linked with multifunctional hydrosilanes, yielding a network polymer, which... 

Kobayashi et al. developed chiral Lewis acids derived from A -benzyldiphenylproli-nol and boron tribromide and used these successfully as catalysts in enantioselective Diels-Alder reactions [89]. The corresponding polymeric catalyst 71 was prepared and used for the Diels-Alder reaction of cyclopentadiene with methacrolein [90]. Different polymeric catalysts 72, 73, 74 were prepared from supported chiral amino alcohols and diols fimctionalized with boron, aluminum and titanium [88,90]. In these polymers copolymerization of styrene with a chiral auxiliary containing two polymerizable groups is a new approach to the preparation of crosslinked chiral polymeric ligands. This chiral monomer unit acts as chiral ligand and as a crosslink. 

Catalyzed enantioselective Mukaiyama-aldol reactions have been developed extensively [101] and chiral polymer-supported Lewis acids are the catalysts of choice. Polymer-supported chiral A(-sulfonyloxazaborohdinones 86 and 87, prepared by copolymerization of styrene, divinylbenzene, and chiral monomers derived from L-valine and L-glutamic acid, respectively, have been used for aldol reactions [102]. The rates of reaction using the polymeric catalysts were slow and enantioselectivity was lower than was obtained by use of the low-molecular-weight counterpart (88). The best ee obtained by use of the polymeric catalyst was 90 % ee with 28 % isolated yield in the asymmetric aldol reaction of benzaldehyde with 89 (Eq. 27). 

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