Structural control, Inclusion polymerization

The importance of the size of the host tunnel in controlling the stereoregularity of the product obtained in inclusion polymerization reactions has been demonstrated by Miyata and coworkers, who investigated the polymerization of 2,3-dimethylbutadiene in the host structures formed by several DCA derivatives (see Figure 1) with a variety of tunnel dimensions. The results are summarized in Table 1. The relative sizes of the tunnels in the inclusion compounds... 

Large amounts of mn -l,4-poly(2,3-dimethyl-l,3-butadiene) can be prepared by inclusion polymerization [330-332]. Urea or thiourea are used as templates. Trans-, A polymer (99 °C) is also obtained with 71-allylnickel chlorides in combination with tetrachlor-l,4-benzoquinone. Anionic polymerization by butyllithium allows good control of the products micro structure over a wide range [97]. 

C1 Cj to C4, and C4 to C4), but the channel directs the addition selectively only to C2 to C4. Polymerization of these dienes takes place in DC A and ACA channels, but with less selectivity. In all of these cases, it is the tight fit of the monomer within the cross section of the channels that enforces the geometry of inclusion and subsequently the structure of the polymer. The fact that the selectivities noted above correlate well with the channel cross-sectional area (lower selectivity obtained in the broader channels of DCA and ACA compared to urea and thiourea) is consistent with the hypothesis that alignment is controlled primarily by repulsive forces. 

Optical activity in biopolymers has been known and studied well before this phenomenon was observed in synthetic polymers. Homopolymerization of vinyl monomers does not result in structures with asymmetric centers (The role of the end groups is generally negligible). Polymers can be formed and will exhibit optical activity, however, that will contain centers of asymmetry in the backbones [73]. This can be a result of optical activity in the monomers. This activity becomes incorporated into the polymer backbone in the process of chain growth. It can also be a result of polymerization that involves asymmetric induction [74, 75]. These processes in polymer formation are explained in subsequent chapters. An example of inclusion of an optically active monomer into the polymer chain is the polymerization of optically active propylene oxide. (See Chap. 5 for additional discussion). The process of chain growth is such that the monomer addition is sterically controlled by the asymmetric portion of the monomer. Several factors appear important in order to produce measurable optical activity in copolymers [76]. These are (1) Selection of comonomer must be such that the induced asymmetric center in the polymer backbone remains a center of asymmetry. (2) The four substituents on the originally inducing center on the center portion must differ considerably in size. (3) The location... 

There is no doubt that these principles are applicable too to other systems which undergo topochemically controlled polymerizations, such as the diacetylenes [33] and, to a lesser extent perhaps, to mixed crystals (to give copolymers) and to inclusion complexes, in urea for example. Trans, fraws-pentadiene in urea has been reported to give a stereoregular polymer [42]. Further, it is known that crystals of the urea channel complexes have chiral structures. Thus we expect that this polymerization in a single crystal would give rise to an asymmetric polypentadiene, and therefore would provide a further example of absolute synthesis. 

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