Donor and Acceptor Monomers

While there is clear evidence for complex formation between certain electron donor and electron acceptor monomers, the evidence for participation of such complexes in copolymerization is often less compelling. One of the most studied systems is S-.V1 Al I copolymerization/8 75 However, the models have been applied to many copolymerizations of donor-acceptor pairs. Acceptor monomers have substituents such as carboxy, anhydride, ester, amide, imide or nitrile on the double bond. Donor monomers have substituents such as alkyl, vinyl, aryl, ether, sulfide and silane. A partial list of donor and acceptor monomers is provided in Table 7.6.65.- 

Common features of polymerizations involving such monomer pairs are  

However, these observations are not proof of the role of a donor-acceptor complex in the copolymcrization mechanism. Even with the availability of sequence information it is often not possible to discriminate between the complex model, the penultimate model (Section 7.3.1.2) and other, higher order, models.28 A further problem in analyzing the kinetics of these copolyincrizations is that many donor-acceptor systems also give spontaneous initiation (Section 3.3.6.3). 

Equilibrium constants for complex formation (A ) have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

Since intermediates usually cannot be observed directly, the exact nature of the donor-acceptor complex and the mechanisms for their interaction with radicals are speculative. At least three ways may be envisaged whereby complex formation may affect the course of polymerization  

---

In considering the origin of the results in Table III and the extraordinary rate exhibited by the HM/CHVE system, it is tempting to consider the role that ground-state charge transfer interaction between the donor and acceptor monomers in the... 

Each vertex of a buckyball cluster is attached by three H-bonds, and hence must have net donor (2D1A) or acceptor (1D2A) character that seems to preclude significant cooperativity. However, by suitably pairing each donor and acceptor monomer, one may produce connected dimers that are each of effective 3D3A pseudo-closed-CT character. Such cooperative dimer units may then be joined in proton-ordered fashion to form closed polyhedra that retain a high degree of cooperative stabilization. 

The monomer complex participation (MCP) mechanism suggests that alternation results from homopolymerization of a 1 1 complex formed between donor and acceptor monomers [Cowie, 1989 Furukawa, 1986] ... 

The Bond-Forming Initiation theory is based on the fact that cycloaddition and initiation of copolymerization compete in the reactions of donor and acceptor monomers. Experimental conditions played a great role in that dilute solutions favored cycloadditions, while high concentration favored polymerization. The main concept is that the intermediates in the cycloaddition reactions are also the initiators of the polymerization. 

Zwitterions are the propagating species in a number of spontaneous polymerizations that occur upon mixing electron donor and acceptor monomers. Several reviews of spontaneous polymerizations via zwitterion intermediates are available [317-323]. In this novel type of polymerization reaction there is no need to add an initiator or catalyst. Alternating copolymers have been obtained in some cases when at least one of the monomers contains a heterocycle. In these polymerizations a nucleophilic monomer (Mn) and an electrophilic monomer (Me) react to form a zwitterion [Eq. (89)]. This zwitterion is referred to as a genetic zwitterion. ... 

Scheme 2 Various donor and acceptor monomers can be combined to obtain block copolymers with amorphous or crystalline segments. In the left box, the polymerizable monomers are shown. On the right, the architectures of the resulting two main classes of D-A block copolymers are depicted amorphous-crystalline and crystalline-crystalline block copolymers...

Scheme 7 Monofunctional donor and acceptor monomers used (EAE Ethyl allyl ether AHE Allyl hexyl ether AIG Allyl isopropylidene glycerol ACG Allyl glycerol carbonate AR Allyl-D-ribofuranoside AIR Allyl-2,3-0-isopropylidene-D-ribofuranoside ATMR Allyl-2,3,5-tri-0-methyl-D-ribofuranoside EVE Ethyl vinyl ether BVE Butyl vinyl ether HBVE 4-hydroxybutyl-vinyl ether HVE Hexyl vinyl ether VIG Vinyl isopropylidene glycerol VCG Vinyl glycerol carbonate VIR Vinyl-2,3-0-isopropylidene-D-ribofuranoside VTMR Vlnyl-2,3,5-tri-0-methyl-D-ribofuranoside DEF Diethyl fumarate DEM Diethyl maleate).

Table 1 Comparison of polymerizability of allylether vs. vinylether monomers with DBF (equimolar amounts of donor and acceptor monomers PI 5 wt-% of Darocur 1173 ...

Finally, the higher reactivity with high final conversion levels observed for the ribose-based derivatives in this series can be explained by possible association via H-bonding between donor and acceptor monomers and also by the better accessibility of reactive unsaturations as a consequence of a favorable conformation within the complex (Table 1 VIR and AR, AIR). ... 

It is possible to adjust the relative proportions of donor and acceptor monomers to ensure a high level of fraetional conversion for the less reacting allylic function. Decreasing the proportion of donor monomer AHE or AIG in the blends with DEF was shown to increase noticeably the... 

---