Homopropagation

It is known that the penultimate unit influences the conformation of both model radicals and propagating radicals.32 3 Since addition requires a particular geometric arrangement of the reactants, there are enthalpic barriers to overcome for addition to take place and also potentially significant effects on the entropy of activation. Comparisons of the rate constants and activation parameters for homopropagation with those for addition of simple model radicals to the same monomers also provide evidence for significant penultimate unit effects (Section 4.5.4). 

Table 8.1 Solvent Effect on Homopropagation Rate Constants for VAc at 30°C7 f...

The distinction between the rates of homo- and copolymerization apparently is misapprehended by some workers. For example, a recent review 141) discusses the results of McGrath 142) who reported butadiene to be more reactive in polymerization in hexane than isoprene, whether with respect to lithium polybutadiene or polyisoprene, although the homopropagation of lithium polyisoprene in hexane was found to be faster than of polybutadiene. The miscomprehension led to the erroneous statement14l) McGrath 142) results regarding the rate constants for butadiene and isoprene place in clear perspective the bizarre assertion 140) that butadiene will be twice as reactive as isoprene (in anionic co-polymerization). 

The coordination polymerisation of heterounsaturated monomers, such as aldehydes [101-103] and ketones [104], isocyanates [105] and ketenes [106,107], in homopolymerisation systems has not been widely described in the literature. However, the coordination copolymerisation of heterounsaturated monomers not susceptible to homopropagation, such as carbon dioxide [71,108-113], with heterounsaturated monomers such as cyclic ethers has been successfully carried out and is of increasing interest. 

In the above schemes, k and k22 are the respective monomer homopropagation rate constants, and k 2 and k2 are the monomer heteropropagation rate constants. The expression for the copolymer composition in terms of comonomer feed concentrations is as follows ... 

The ability of a /i-olefin to copolymerise with ethylene in the presence of Ziegler-Natta catalysts arises from minimisation of steric interactions at the catalytic active site by ethylene units the steric hindrance, which prevents homopropagation of the /1-olefin, is overcome when the /<-olefin monomer is... 

However, the most important goal that might be reached by the application of coordination catalysts for the polymerisation of heterounsaturated monomers is the possibility of the enchainment of heterounsaturated monomers, not susceptible to homopropagation, via their copolymerisation with heterocyclic monomers. This concerns primarily the coordination copolymerisation of carbon dioxide and oxacyclic monomers such as epoxides, leading to aliphatic polycarbonates [8 12]. Representative examples of the copolymerisations of heterocyclic monomers and hardly homopolymerisable heterocumulenes, in the presence of coordination catalysts, are listed in Table 9.4 [1]. 

A similar mechanism was proposed when 1,5-dithiocin 838g underwent polymerizations with methyl methacrylate (MMA) and styrene (STY). The activated double bound of 838g was found to have a profound affect on reactivity. In fact, co-polymerization of 838g with MMA at 70 °C the 5-terminated sulfanyl radicals preferred to undergo homopropagation, while cross-propagation is favored for MMA-terminated radicals. Both monomers possessed an electron-deficient acrylate double bond with similar possibilities for conjugative stabilization of the adduct radical by the ester functionality, which would explain the apparent equal reactivity of the MMA radical to either monomer. 

The first three types of copolymers can be prepared by polymerizing the two monomers simultaneously. In this case, the distribution of comonomers is determined by their relative concentrations and reactivity ratios. The reactivity ratios (ri and r2) are the ratios of the rate constants of homopropagation and cross-propagation [Eq. (21)]. 

Thus, how should block copolymers between styrene and a vinyl ether be prepared Starting with styrene or with a vinyl ether In the former system, the propagating styryl cation is intrinsically more reactive but present at much lower concentration. A rough estimate of the ratio of cation reactivities is = 103 but the ratio of carbocations concentrations is = I0 S. Thus, the ratio of apparent rate constants of addition is 10-2. Macromolecular species derived from styrene should add to a standard alkene one hundred times slower than those derived from vinyl ethers. Thus, one cross-over reaction St - VE will be accompanied by =100 homopropagation steps VE - VE. Therefore, in addition to a small amount of block copolymer, a mixture of two homopolymers will be formed. Blocking efficiency should be very low, accordingly. 

Obviously, although a new monomer is fed equimolar to the growing end at each step, it is imperative to prevent homopropagation (into a sequence such as A-B-B— instead of the intended A-B-C----)- Monitoring the product distribution along with the multistep additions in fact reveals that the further the synthesis proceeds, the more the amount of such undesired products. Further examination also indicated that sequence selectivity is higher for sequences where the reactivity of monomers progressively decreases [177]. 

In homopolymerization, the former monomer propagates irreversibly, the homopropagation of THF is highly reversible. It has been pointed out that, for heterocyclic monomers, it is the nature of penultimate unit that governs the reversibility of a given reaction step [304]. Thus, addition of THF to BCMO active center is irreversible, because the backward reaction would require the closure of 4-membered ring. On the other hand, addition of BCMO to THF active centers is reversible ... 

There is another question that has to be discussed the direction of an attack in the S), 2 reaction vs. the position of the anion in the onium salts. Indeed, the simple and attractive picture proposed by Szwarc in order to explain the differences between the reactivities of maaoanions and macroion pairs in the anionic homopropagation of styrene is based on the assumption that an ion pair has to dissociate partially when the transition state of propagation is reached ). Szwarc, after observing a similar reactivity of the polystyryl anion and polystyryl cesium ion pair, also assumed that no partial dissociation was needed for the large Cs cation. The... 

This order is opposite to the order of nucleophilicities, which one could expect to be in line with the order of reactivities in homopropagation. Apparently, bond scission in the macrocation involved is more advanced in the transition state than bond formation of the incoming monomer. This is a known feature of the borderline Sm2 mechanism, which is on the edge of becoming an St 1 substitution. 

Referential formation of the isotactic dyads (L-L and D-D units), i.e. the higher rates of homopropagation than of crosspropagation has been explained by Okada in terms of stereoselection due to the steric hindrance when, e.g. the D-monomer approaches the growing center composed of L-unit ... 

Thus, the data above indicate that the higher the monomer basicity the lower its homopropagation rate constant. 

Thus the observed order of reactivities in homopropagation is parallel to the order of reactivities in reaction of standard monomer with different active species and reverse to that observed for reaction of different monomers with standard active species. 

Surprisingly, the addition of TBA to a solution of living poly-PEMA also leads to the quantitative formation of a block copolymer as Is shown by GPC analysis (Figure 4). This proves that the rate constant of the Initiation for the TBA polymerization by the PEMA azlrldlnlum Ion Is of the same order of magnitude as the homopropagation constant for TBA or k2i 1 11 ... 

These are formed by a ring closure of the zwltterIons AAAL and AAAAL. This confirms the assumption that In this copolymerization the TBA monomer Is consumed In a considerable proportion by cationic homopropagation. 

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