Complexes of monomers

It is also possible that complexation of monomer or propagating species could influence the regiospecificity of addition. However, since the effect is likely to be an enhancement of the usual tendency for head-to-tail addition, perhaps it is not surprising that such effects have not been reported. 

Concerning the initiation, we follow Russell [75a] in assuming that it is a second order reaction, the rate of which is independent of monomer concentration. Most probably it involves a complex of monomer with the stannic chloride monohydrate [75a]. In view of the well-known tendency of stannic chloride to form 6-co-ordinate rather than 5-coordinate structures, this idea does not seem too far-fetched. Since the monomer is in great excess over stannic chloride hydrate, the concentration of the complex, Pj-SnCl HjO, will be sensibly equal to that of the monohydrate, i.e. of either the stannic chloride or the water, whichever is the scarcer. Therefore, the rate of initiation Vi5 will be given by... 

The distinction between systems in which a complex of initiator and co-initiator reacts with the olefin (scheme (1)) and those in which the coinitiator reacts with a complex of monomer and metal halide (scheme (2)) was made very early in the history of this subject [9]. 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

Chapman and Breslow synthesized zinc(II) complexes of monomer and dimers derived from 1,4,7-triazacyclododecane with phenyl 48 and 4,4 -biphenyl linkers 49 (55). They were examined as catalysts for the hydrolysis of 4-nitrophenyl phosphate (NP2 ) and bis(4-nitrophenyl) phosphate (BNP ) in 20% (v/v) DMSO at 55°C. On the basis of the comparison of the pseudo-first-order rate constants, the dinuclear zinc(II) complexes 48 with 1,3-phenyl and 1,4-phenyl linkers are ca. 5 times more efficient than monomer or 49 in the hydrolysis of NP2, leading to the conclusion that the two zinc(II) ions are simultaneously involved in the hydrolysis, as in the enzyme alkaline phosphatase. For the hydrolysis of BNP, a longer dimer 49 is ca. six times more effective than 1,3-phenyl-linked dimer 48 and monomers. 

Although any coordination polymerisation involves a monomer coordination in each step, literature data that might concern complex formation between the monomer heteroatom and the metal atom at the active site are rather scant. This is due to the lability of such complexes of monomers with active sites if they were stable enough, they would not undergo any further rearrangement, which might lead to polymer chain growth. Thus, models have been studied that have consisted of a metal complex, of no effectiveness as a catalyst, and a monomer, or of a catalyst and a non-polymerisable heterocycle. 

Better evidence for steric hindrance to complexing of monomer is provided by the copolymerization of ethylene with internal acyclic and... 

Simple 2,2-dibutyl-l,3,2-dioxastannolanes form solid complexes of monomer units with certain nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1 1 stoichiometry and pentacoordinate tin atoms.62 Such complexes are less stable for more-substituted stannylene acetals, such as those derived from carbohydrates.62 Unfortunately, the precise structures of these complexes have not yet been defined. Addition of nucleophiles to solutions of stannylene acetals in nonpolar solvents has been found to markedly increase the rates of reaction with electrophiles,63 and transient complexes of this type are likely intermediates. Similar rate enhancements were observed in reactions of tributylstannyl ethers.57 Tetrabu-tylammonium iodide was the nucleophile used first,57 but a wide variety of nucleophiles has been used subsequently tetraalkylammonium halides, jV-methylimidazole,18 and cesium fluoride64,65 have been used the most. Such nucleophilic solvents as N,N-dimethylformamide and ethers probably also act as added nucleophiles. As well as increasing the rates of reaction, in certain cases the added nucleophiles reverse the regioselectivity from that observed in nonpolar solvents.18,19... 

Complexes of monomers with acceptors can initiate polymerizations of the respective monomers even without photoexcitation. The properties of DA complexes and the conditions of the thermal activation are determined by the type of donor (monomer), acceptor, and solvent [300,303]. Ion radicals from the monomer, dipolar intermediates (zwitterions), as well as the relatively... 

The entropy change at polymerization has the tendency to increase with growing complexity of monomer structure [19], The values of the heats of polymerization and of the entropy changes of some common monomers are summarized in Table 1. 

Absorption or complexing of monomer has been suggested to occur even in homopolymerization. It is, in principle, unlikely, for anionic chain-ends add monomer rapidly, and no convincing evidence has ever been presented for its occurrence. 

Zinc(II) complexes of monomers and dimers derived from 1,4,7-triazacyclodo-decane 24 were recently examined by Breslow and co-workers [23] as catalysts... 

Simple 2,2-dibutyl-l,3,2-dioxastannolanes form solid complexes of monomer units with nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1 1 stoichiometry and pentacoordinate tinatoms. Such complexes are less stable for more substituted stannylene acetals, e.g., those derived from carbohydrates. Addition of nucleophiles to solutions of stannylene acetals in non-polar solvents has been found to markedly increase the rates of reaction with electrophiles and transient 1 1 complexes of this type are... 

KH Okhapkin, I.M., Nasimova, I.R., Makhaeva, E.E., and Khokhlov, A.R., Effect of complexation of monomer units on pH- and temperature-sensitive properties of poly(A-vinylcaprolactam-co-methacrylic e id). Macromolecules, 36, 8130, 2003. 

Moreover, efficiency of phase inversion imprinting can be improved with pre-forming complex of monomer-template (Table 2) in copolymerization [70]. The THO-acrylic acid or methacrylic acid precomplex monomer was copolymerized with acrylonitrile in DMSO. The resultant viscous solution contents were used for phase inversion in water after template copolymerization. Template copolymers can improve binding capacity of THO. From H-NMR analysis, this is due to tailor-made modification of a copolymer backbone for the template molecule. Also, comparison was made between copolymers of acrylic acid and methacrylic acid in THO selectivity of the imprinted polymers. Presence of the methacryl methyl group is more efficient in the tailor-made structure of the THO template. 

The polymerization of methyl methacrylate in acidic solution by iron metal was reported earlier [213]. Palit and co-workers [214] studied the mechanism of methyl methacrylate in the presence of ferric chloride. They proposed that the hydroxyl radical formed by the chemical decomposition of the system containing ferric salt is the active species for initiating polymerization. Narita et al. reported [215] the polymerization of acrylamide initiated by ferric nitrate and suggested that a complex of monomer and metallic salt generates an active monomer radical capable of initiating vinyl polymerization. 

---