Copolymerization reversibility

The effect of reversibility in Scheme (15-1) is shown by the influence of [THF], and temperature on reactivity ratio values for the copolymerization of THF with PO. A quantitative description of a system with one monomer copolymerizing reversibly is given below. 

There have been reports in which j3-pinene was copolymerized by a radical copolymerization, reversible addition-fragmentation chain transfer (RAFT). As comonomers, methyl acrylate or n-butyl acrylate have been used (9,10). 

High molecular weight polymers or gums are made from cyclotrisdoxane monomer and base catalyst. In order to achieve a good peroxide-curable gum, vinyl groups are added at 0.1 to 0.6% by copolymerization with methylvinylcyclosiloxanes. Gum polymers have a degree of polymerization (DP) of about 5000 and are useful for manufacture of fluorosiUcone mbber. In order to achieve the gum state, the polymerization must be conducted in a kineticaHy controlled manner because of the rapid depolymerization rate of fluorosiUcone. The expected thermodynamic end point of such a process is the conversion of cyclotrisdoxane to polymer and then rapid reversion of the polymer to cyclotetrasdoxane [429-67 ]. Careful control of the monomer purity, reaction time, reaction temperature, and method for quenching the base catalyst are essential for rehable gum production. 

Plastic packagiag materials are thermoplastic, ie, reversibly fluid at high temperatures and soHd at ambient temperatures. These materials may be modified by copolymerization, additives ia the blead, aHoyiag, and surface treatment and coating. Properties of principal plastic packagiag materials are givea ia Table 1. 

The steady structure determined by the value of Kw (Fig. 1) for the entire class of carboxylic CP obtained by precipitation copolymerization is one of the most important factors determining the possibility of reversible bonding of proteins absorbed by carboxylic CP with a high sorption capacity [16,19]. Thus, for the MA-HHTT system (Fig. 2), a complete desorption of enzyme is carried out on crosslinked copolymers characterized by low Kw values. In crosslinked structures exhibiting looser structure (Kw P 1), owing to the mobility of chain fragments of CP especially in the process of desorption, the macromolecules of sorbed protein are irreversibly captured as a result of a marked polyfunctional interaction. 

With most common monomers, the rate of the reverse reaction (depropagation) is negligible at typical polymerization temperatures. However, monomers with alkyl groups in the a-position have lower ceiling temperatures than monosubstituted monomers (Table 4.10). For MMA at temperatures <100 °C, the value of is <0.01 (Figure 4.4). AMS has a ceiling temperature of <30 °C and is not readily polymerizable by radical methods. This monomer can, however, be copolymerized successfully (Section 7.3.1.4). 

The reaction between the PMMA and PS model radicals (4 and 5, generated from the unsymmetrical azo-compound 3) has been studied as a model for crosstermination in MMA-S copolymerization (Scheme 7.13).178,179 The value for tcross reaction was 0.56. In disproportionation, transfer of hydrogen from the PS model 5 to the PMMA radical 4 was ca 5.1 times more prevalent than transfer in the reverse direction (from 4 to 5). The value of kJklc(90°C) is between those of Atd/ tc(90oC) for the self-reaction of these radicals... 

Monomers not amenable to direct homopolymerization using a particular reagent can sometimes be copolymcrizcd. For example, NMP often fails with methacrylates (e.g. MMA, BMA), yet copolymerizalions of these monomers with S are possible even when the monomer mix is predominantly composed of the methacrylate monomer,15j This is attributed to the facility of cross propagation and the relatively low steady state concentration of propagating radicals with a terminal MMA (Section 7.4.3.1). MMA can also be copolymerized with S or acrylates at low temperature (60 C).111 Under these conditions, only deactivation of propagating radicals with a terminal MMA unit is reversible, deactivation of chains with a terminal S or acrylate unit is irreversible. Molecular weights should then be controlled by the reactivity ratios and the comonomer concentration rather than by the nitroxide/alkoxyamine concentration. 

Polymerization equilibria frequently observed in the polymerization of cyclic monomers may become important in copolymerization systems. The four propagation reactions assumed to be irreversible in the derivation of the Mayo-Lewis equation must be modified to include reversible processes. Lowry114,11S first derived a copolymer composition equation for the case in which some of the propagation reactions are reversible and it was applied to ring-opening copalymerization systems1 16, m. In the case of equilibrium copolymerization with complete reversibility, the following reactions must be considered. 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to <12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477... 

Related alkoxides, such as (272), and the amide (277) display a similar activity to the acetate,964 suggesting that both alkoxide and carbonate intermediates are formed during the reaction, ll NMR spectroscopy has been used to demonstrate that (334) reacts reversibly with CHO to generate an alkoxide intermediate which subsequently inserts C02. The amide complex initiates the copolymerization by first inserting C02 into the Zn—N bond and subsequent elimination of trimethylsilyl isocyanate.965... 

Anionic block copolymerizations of MM A with lactones proceeded smoothly to give copolymers with Mw/Mn = 1.11-1.23 when the monomers were added in this order. However, when the order of addition was reversed, no copolymerization took place [3c], i.e., no addition of MMA to the polylactone active end group occurred (Scheme 12). 

Yasuda et al. [122] extended the above work to the block copolymerization of ethylene with lactones. 5-Valerolactone and s-caprolactone were combined with the growing polyethylene end at ambient temperature and the expected AB-type copolymers (100 1 to 100 89) were obtained at high yield. Reversed addition of the monomers (first MMA or lactones and then ethylene) induced no block copolymerization at all, even in the presence of excess ethylene, and only homo-poly(MMA) and homo-poly(lactone) were produced. 

The molecular weight distribution of a polymer produced with a chain shuttling catalyst/CSA system is highly dependent on reaction conditions. The extent of reversibility with the catalyst/CSA pairs was therefore further explored through a series of polymerizations over a range of monomer conversions (i.e., yield). A representative example from this secondary screening process is described below for precatalyst 17. Several members from this well-studied bis(phenoxyimine)-based catalyst family [39] were identified as poor incorporators in the primary screen. A series of ethylene/octene copolymerizations using 17 was performed across a... 

These reactions, performed many times, show, in addition to the reversal of the absolute configuration of the product with the change in the configuration at C-8 and C-9 of the alkaloids, a small but reproducible difference in the e.e. of the product. It is evident that the diastereomeric nature of quinine vs. quinidine and cinchonidine vs. cinchonine expresses itself via small but important energy differences in the best fits of the transition states. Noteworthy in this respect is the fine work of Kobayashi (20), who observed larger differences (in the e.e. s of products) when the diastereomeric cinchona alkaloids were used as catalysts after having been copolymerized with acrylonitrile (presumably via the vinyl side chain of the alkaloids). 

Analysis of the Copolymerizabilities of Monomers The composition of the copolymers formed was determined by measuring the relative amounts of each monomer, NIPAAM and AAM, that remained in solution after a copolymerization. Copolymerizations were terminated by addition of 1 ml of reaction mix to 9 ml of 0.1% phosphoric acid at 50 C, followed by centrifugation of a 0.4 ml aliquot at 6,500 x g for 5 minutes in an Eppendorf microfuge. After 100 fold dilution of an aliquot of the supernate, 200 pi of this was injected onto an IBM reversed phase Cig HPLC column pre-equilibrated with 2% acetonitrile in 0.1% aqueous phosphoric acid and the eluent monitored at 214 nm. The monomers were eluted using a 0.1% aqueous phosphoric acid (solvent A) acetonitrile (solvent B) gradient as follows for 5 minutes the solvent was 98% solvent A and 2% solvent B, followed by a linear gradient to 80% A and 20% B over 10 minutes. After 5 more minutes at 80% A and 20% B, the solvent was returned to 98% A and 2% B. 

The above theory can be extended to deal with other more complex cases. For example, the two ends of a biopolymer need not behave identically, and, as noted earher, MTs are helical polymers of asymmetric protomer units. Thus, two sets of on- and off-constants might be necessary. In other cases, such as in the polymerization of tubulin in the presence of tubulin-colchicine complex (Sternlicht et one may need to consider copolymerization. The kinetics of microtubule depolymerization are the reverse of elongation, and are gener-... 

---