Polowi 'nski Template Polymerization

In order to prove the ladder structure of obtained products, hydrolysis was carried out. By hydrolysis, both ester and CN groups were converted to oligomeric polyacids with the number of units twice as high as the number of units in the template.

Another example is a synthesis of a ladder polymer suggested by Bamford71 and examined afterwards by Jantas.72 Poly(vinyl alcohol) esterified by methacrylic chloride was used as a template. Polymerization can be represented by the following reaction:

The polymerization can begin at any group present and may proceed in any direction. In other words, since the reactive groups are independent and initiation occurs randomly, polymerization (zipping) leads to isolated reactive groups which are both potential crosslinking sites and breaks in the ladder structure. Crosslinking, as an intermolecular reaction, can be minimized by conducting the reaction at an appropriate dilution. If, however, the initiation is not random, but starts at one end of the template, we might expect a very regular ladder structure. Such case was considered by Bamford.71 Active radicals in multimonomer - poly(vinyl methacrylate) were created from -CCl3 end-groups in the presence of manganese carbonyl Mn2(CO)10 by irradiation with light, λ = 435.8 nm. The radicals are expected to initiate selectively at one end of the template.

Another type of multimonomer has been synthesized and examined by Jantas.73 Selecting an appropriate dilution, concentration of initiator, and temperature, even if initiation is random, the polymerization leads to the ladder-type structure of the product as shown in the example of template polymerization of multimethacrylates according to following reaction:

More examples of such “ zipping up” reactions are reviewed in Table 4.2. Polymerization of multimonomers can be discussed in terms of more general prob-

lems - polymerization of multifunctional monomers. Generally speaking, two types of reaction can take place when monomer has more than one double bond: intermolecular or intramolecular reaction as shown in Figure 4.8. The reaction proceeds according to the case A as “pure template” process, while the case B leads to the crosslinked structure. Moreover, in the last case, a part of double bonds is unreacted. From the amount of double bonds and end-groups analysis,72-75 a percentage of template reaction (case A) can be estimated. This “percentage of template reaction” depends on the polymerization conditions and the structure of multimonomer. This value seems to be a good measure of “multimonomer ability” to template polymerization.

From the examples of template polymerization carried out under special condi- tions,72-75 we can see that intermolecular polymerization can be neglected. Indeed, if the concentration of monomer used is very low, and concentration of initiating radicals is

rather high, one can expect intramolecular polymerization leading to the product with ladder-type structure.

Table 4.2: Examples of multimonomers polymerization

4.9 RING-OPENING POLYMERIZATION

A very interesting method of synthesis of polypeptides from N-carboxy-α-amino acid anhydrides was proposed by Ballard and Bamford.76 Opening the ring of N-carboxy-α-amino acid anhydrides by primary or secondary amines can be illustrated by the following reaction:

The R NHgroup thus created can interact with the next molecule of the anhydride which leads to the addition of this molecule, decarboxylation, and restoration of the active center. Repetition of such a reaction gives the kinetic chain. Eventually, polypeptide

with repeating units of -(NR3CR1R2CO)- was obtained. If a polypeptide containing a terminal secondary amine group is used as an initiator, a template effect can be expected. Ballard and Bamford reported76 unusual features when polysacrosine dimethylamide was used to initiate polymerization of DL-phenylalanine NCA (N-carboxy-DL-phenylalanine anhydride), NCPA. Interaction between polysarcosine unit and molecule of phenylalanine NCA can be illustrated as follows:

These types of interaction were confirmed by IR analysis by examination of N-H stretching vibrations in monomer and C=O stretching vibrations of the polysarcosine groups. At any step of the reaction, the monomer molecule was adsorbed onto polysacrosine. If the polysacrosine template contains at one end inactive (CH3)2N- group, and at the other end CH3NHgroup (active as an initiator), block copolymer polysacrosine/polyphenylalanine can be obtained. It was observed by the authors76 that the rate of the reaction depends on the chain length of the template (chain effect) and also on the type of solvent used. The rate of polymerization was much higher in comparison with the polymerization in the presence of low molecular weight analogue. In nitrobenzene and chloroform, when polysacrosine with degree of polymerization 30 was used, the ratio of the rate of template polymerization to the rate of blank reaction was more then 10. It was also observed that the initial rate became independent of the concentration of monomer if concentration was sufficiently high for adsorption at all sites of the template.77

A very special type of template polymerization was presented by a group of Japanese scientists.78,79 The method used was based on the observation that during radical

polymerization of 2,2-diphenyl-4-methylene-1,3-dioxolane, elimination of benzophenone occurs according to reaction:

Statistical, not a block, copolymer was used. Methyl-substituted derivative was also used:

Polymerization of this compound leads to even better efficiency of “newborn polymer” with the structure:

Especially high efficiency (93%) was found when copolymer containing 60 mol% of styrene units was used. The efficiency was in this case much higher than obtained for polymerization of homopolymer-type prepolymer, both types - methylene substituted and unsubstituted. Improvement of solubility of the newborn polymer by the methyl substituent allowed to analyze the product by GPC method. It was found that molecular weight of the newborn polymer was lower then molecular weight of template polymer. It was the evidence that the newborn polymer is not connected with the template.

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