COPOLYMERIZATION in .NET

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situation is much more like that which occurs in ionic copolymerizations. Ring-opening copolymerizations are complicated in several ways: 1. For copolymerizations proceeding by the activated monomer mechanism (e.g., cyclic ethers, lactams, N-carboxy-a-amino acid anhydrides), the actual monomers are the activated monomers. The concentrations of the two activated monomers (e.g., the lactam anions in anionic lactam copolymerization) may be different from the comonomer feed. Calculations of monomer reactivity ratios using the feed composition will then be incorrect. 2. Most ring-opening copolymerizations involve propagation depropagation equilibria, which require that experimental data be handled in the appropriate manner (Sec. 6-5b), but this is usually not done. The situation is more complicated in many systems because of additional equilibria between polymer and cyclic oligomer and by intermolecular chain transfer to polymer which result in reshuf ing of monomer units. It may be possible to avoid or minimize the reshuf ing equilibria by using mild initiators, low conversions, and highly reactive (more strained) monomers. However, the situation in industrial practice usually involves conditions that result in near-complete reshuf ing of monomer units. The effect of reshuf ing is evident in the anionic copolymerization of octamethylcyclotetrasiloxane (M1 ) and 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane (M2 ) (feed ratio 9 : 1) by potassium silanolate at 130 C [Ziemelis and Saam, 1989]. The initial copolymer is very rich in M2 , and this monomer is exhausted early. Further reaction results not only in conversion of M1 but also redistribution of the M2 units. Silicon 29 NMR shows the M2 units to be in blocks at short reaction times but statistically distributed at long reaction times. Similar results have been observed in the cationic copolymerization of methyl glyoxylate and 1,3-dioxolane [Basko et al., 2000]. 3. Counterion effects similar to those in ionic chain copolymerizations of alkenes (Secs. 6-4a-2, 6-4b-2) are present. Thus, copolymerizations of cyclopentene and norbornene with rhenium- and ruthenium-based initiators yield copolymers very rich in norbornene, while a more reactive (less discriminating) tungsten-based initiator yields a copolymer with comparable amounts of the two comonomers [Ivin, 1987]. Monomer reactivity ratios are also sensitive to solvent and temperature. Polymer conformational effects on reactivity have been observed in NCA copolymerizations where the particular polymer chain conformation, which is usually solvent-dependent, results in different interactions with each monomer [Imanishi, 1984]. 4. Some ROPs proceed with the simultaneous operation of two different mechanisms, for example, NCA copolymerizations initiated by some secondary amines proceed with both the amine and activated monomer mechanisms. The monomer reactivity ratios for any comonomer pair are unlikely to be the same for the two different propagations. Any experimentally determined r values are each composites of two different r values. 5. The initiator used is important for copolymerizations between monomers containing different polymerizing functional groups. Basic differences in the propagating centers (oxonium ion, amide anion, carbocation, etc.) for different types of monomer preclude some copolymerizations. Even when two different monomer types undergo polymerization with similar propagating centers, there may not be complete compatibility in the two crossover reactions. For example, oxonium ions initiate cyclic amine polymerization, but ammonium ions do not initiate cyclic ether polymerization [Kubisa, 1996]. 7-12a Monomers with Same Functional Group
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Copolymerizations between pairs of cyclic esters, acetals, sul des, siloxanes, alkenes, lactams, lactones, N-carboxy-a-amino acid anhydrides, imines, and other cyclic monomers
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RING-OPENING POLYMERIZATION
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have been studied [Frisch and Reegan, 1969; Ivin, 1987; Ivin and Saegusa, 1984; Kendrick et al., 1989; Sebenda, 1989; Tomalia and Killat, 1985]. Copolymerizations have also been achieved between closely re lated types of monomers such as cyclic ethers with cyclic acetals or lactones with lactides and cyclic carbonates [Basko et al., 2000; Keul et al., 1988; Kricheldorf et al., 1985]. The interpretation of experimental monomer reactivity ratios is subject to the cautions described above. When copolymerization proceeds with minimal complications, the r values can often be analyzed by considering the effects of ring size on the general tendency toward ring opening (see considerations in Sec. 2-5b) and the reactivity toward attack by the particular propagating species. For those polymerizations involving a cyclic propagating center (e.g., oxonium ions in cyclic ether polymerization), there is also the need to consider the effect of ring size on formation of the propagating center. Some monomers with no tendency toward homopolymerization are found to have some (not high) activity in copolymerization. This behavior is found in cationic copolymerizations of tetrahydropyran, 1,3-dioxane, and 1,4-dioxane with 3,3-bis(chloromethyl)oxetane [Dreyfuss and Dreyfuss, 1969]. These monomers are formally similar in their unusual copolymerization behavior to the radical copolymerization behavior of sterically hindered monomers such as maleic anhydride, stilbene, and diethyl fumarate (Sec. 6-3b-3), but not for the same reason. The copolymerizability of these otherwise unreactive monomers is probably a consequence of the unstable nature of their propagating centers. Consider the copolymerization in which M2 is the cyclic monomer with no tendency to homopolymerize. In homopolymerization, the propagation depropagation equilibrium for M2 is completely toward
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