Chain transfer side reactions

Chain-transfer side reactions (see Scheme 1.1) can also cause substantial increases in D-values. Macrocychzation is particularly poor in this respect, leading to a complete equihbrium and an especially broad molar mass distribution (see e.g. Figure 1-4). On the other hand, a reversible polymerization devoid of macro-cyclization, but accompanied by the segmental exchange reactions, can fulfill the criteria of the living process [95-99]. However, in this case the D-value also increases with conversion, reaching at equilibrium a value which is predicted by Equation 1.24 and characteristic of the most probable molar mass distribution. Figure 1.7 illustrates the dependence of M /M determined for LA polymerization initiated with Sn(ll) alkoxide [98]. 

PDI=1.2-1.4) were prepared at the strict condition to control kinetically the copolymerization step. Actually, long reaction times are much more favorable to transfer side reactions with a concomitant broadening of the MWD and the formation of cyclic chains. 

The anionic polymerization of cyclic esters with higher-membered ring sizes (five and more) proceeds on the alkoxide active centers. Less-nucleophihc carboxyl-ates are unable to initiate the polymerization of these weakly or medium strained monomers, while the relatively high nudeophiUcity of the alkoxides gives rise to chain transfer to polymer with chain rupture side reactions (see Scheme 1.1). As discussed earlier in Section 1.2.3, only an intramolecular transfer wiU lead to a departure from Uvirigness. The kinetic scheme of polymerization accompanied by chain-transfer reactions is shown in Equation 1.42a-<. 

In addition, subsequent chain transfer reactions may occur on side chains and the larger the resulting polymer, the more likely will it be to be attacked. These features tend to cause a wide molecular weight distribution for these materials and it is sometimes difficult to check whether an effect is due inherently to a wide molecular weight distribution or simply due to long chain branching. 

In general, the activation energies for both cationic and anionic polymerization are small. For this reason, low-temperature conditions are normally used to reduce side reactions. Low temperatures also minimize chain transfer reactions. These reactions produce low-molecular weight polymers by disproportionation of the propagating polymer ... 

The chain transfer agents (11 X=CH2, A=CH2) are misnamed macrornonomcrs since in this context they do not behave as macromonomers. Copoiymerization when it occurs is a side reaction. The mechanism is shown in Scheme 6.19 for MAA Mrimer (63). The final product (65) is a also a macromonomer5 and formation of the adduct (64) and chain transfer is reversible (see also Section 6.2.7.2 and Section 9.5.2)/6 76 79 109... 

Other complexes also react with propagating radicals by catalytic chain transfer.110 These include certain chromium,151 152 molybdenum152 1" and iron154 complexes. To date the complexes described appear substantially less active than the cobaloximes and are more prone to side reactions. 

Transfer to monomer is of particular importance during the polymerization of allyl esters (113, X=()2CR), ethers (113, X=OR), amines (113, X=NR2) and related monomcrs.iw, 8, lb2 The allylic hydrogens of these monomers arc activated towards abstraction by both the double bond and the heteroatom substituent (Scheme 6.31). These groups lend stability to the radical formed (114) and are responsible for this radical adding monomer only slowly. This, in turn, increases the likelihood of side reactions (i.e. degradative chain transfer) and causes the allyl monomers to retard polymerization. 

The most important side reactions are disproportionation between the cobalt(ll) complex and the propagating species and/or -elimination of an alkcnc from the cobalt(III) intermediate. Both pathways appear unimportant in the case of acrylate ester polymerizations mediated by ConTMP but are of major importance with methacrylate esters and S. This chemistry, while precluding living polymerization, has led to the development of cobalt complexes for use in catalytic chain transfer (Section 6.2.5). 

The authors concluded that the side reactions normally observed in amine-initiated NCA polymerizations are simply a consequence of impurities. Since the main side reactions in these polymerizations do not involve reaction with adventitious impurities such as water, but instead reactions with monomer, solvent, or polymer (i.e., termination by reaction of the amine-end with an ester side chain, attack of DMF by the amine-end, or chain transfer to monomer) [11, 12], this conclusion does not seem to be well justified. It is likely that the role of impurities (e.g., water) in these polymerizations is very complex. A possible explanation for the polymerization control observed under high vacuum is that the impurities act to catalyze side reactions with monomer, polymer, or solvent. In this scenario, it is reasonable to speculate that polar species such as water can bind to monomers or the propagating chain-end and thus influence their reactivity. 

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