Microstructure reactivity ratios from

The cationic copolymerization of oxetanes provides a good example that shows agreement between two treatments, namely determination of the reactivity ratios from kinetic treatment and microstructure studies 11 12). 

High-resolution nuclear magnetic resonance spectroscopy, especially 13C NMR, is a powerful tool for analysis of copolymer microstructure [Bailey and Henrichs, 1978 Bovey, 1972 Cheng, 1995, 1997a Randall, 1977, 1989 Randall and Ruff, 1988], The predicted sequence length distributions have been verihed in a number of comonomer systems. Copolymer microstructure also gives an alternate method for evaluation of monomer reactivity ratios [Randall, 1977]. The method follows that described in Sec. 8-16 for stereochemical microstructure. For example, for the terminal model, the mathematical equations from Sec. 8-16a-2 apply except that Pmm, Pmr, Pm and Prr are replaced by p, pi2, p2j, and p22. 

The crystallinity, brittleness, and melting point of poly(L-LA) can be decreased by incorporation of comonomer units such as l,5-dioxepan-2-one (DXO). The large difference in reactivity ratio between the DXO and the lactides leads to a microstructure with a more block-like nature than is expected from a... 

The microstructure of acrylamide-sodium acrylate copolymers was determined by NMR (36). The monomer sequence distribution was found to conform to Bernouillian statistics and the reactivity ratios of both monomers were close to unity. These results which differ from those obtained for copolymers prepared in solution or emulsion (37) confirmed a polymerization process by nucleation and interparticular collisions. 

The above conclusion on the role of reactivity ratios on microstructure assumes the absence of rapid transesterification reactions between chains. Since such processes might also tend to randomize the microstructure, it seemed important to isolate the role of interchain transesterification. A unique experiment was designed in which a 13c labeled carbonyl in acetoxy benzoic acid monomer (B ) was reacted with the dimer of HBA-HNA. At 99% enrichment, the only resonances in the carbonyl region of the spectrum will arise from the enriched benzoic acid carbonyl. In the absence of any interchain transesterification the microstructure of the polymer would consist only of B -B dyads (see scheme 1). 

The basicity of esters is similar to that of related ethers, therefore, in copolymerization random structures could be expected. THF (pKb = 5.0) and BCMO (pKb = 5.65) were copolymerized with e-caprolactone (eCL pKb = 5.31) and P-propiolactone (PPL, pKb = 10.06), eCL and cyclic ethers enter the copolymer with nearly the same rates. The reactivity ratios determined by the Mayo-Lewis method are rt(BCMO) = 0.24, r2(eCL) = 0.44 and rt(THF) = 0.7, r2(eCL) = 0.3 4C). r, should be higher for THF(Mj) because the reversibility of THF polymerization has not been taken into account (cf. Sect. 15.1.3.1). rt and r2 values suggest the formation of nearly random copolymers. The microstructure of BCMO-eCL copolymers was studied by NMR spectroscopy, and the new signals B and A due to heterodyad appeared [cf. Eq. (15-41)]. Simultaneously the ratio of signals C/D decreased from 2 to a lower value ... 

Random copolymers of styrene/isoprene and styrene/acrylonitrile have been prepared by stable free radical polymerization. By varying the comonomer mole fractions over the range 0.1-0.9 in low conversion SFRP reactions it has been demonstrated that the incorporation of the two monomers in the copolymer is analogous to that found in conventional free radical copolymerizations. The composition and microstructure of random copolymers prepared by SFRP are not significantly different from those of copolymers synthesized conventionally. These two observations support the conclusion that the presence of nitroxide in the SFR process does not influence the monomer reactivity ratios or the stereoselectivity of the propagating radical chain. Rather, the SFR propagation mechanism is essentially the same as that of the conventional free radical copolymerization process. 

A study of the microstructure of copolymers made use of a simple solution procedure for the copolymerization of methyl acrylate (MA) with A-vinylcarba-zole (NVK). The ratio of the feed composition ranged from (0.1429 moles of MA to 1.00 of NVK) to (7 moles of 1 of NVK)—a 49-fold range in composition. The experiments were carried out to high conversions [41]. Preparation 2-8 is based on this work. The reactivity ratios calculated from this series were approximately... 

Copolymers of acrylamide (AM) with sodium-3-acrylamidO 3 methylbutanoate have been prepared with a range of compositions yet similar molecular weights (Table 1) in order to assess the effect of microstructure on solution properties. Particular attention has been focused on statistical monomer sequence lengths along the polymer backbone as calculated from experimental reactivity ratios (rj xr2 = 0.56) for the monomer pair . 

The reactivity ratios rj/r2 of 0.01 1.1 were strongly in favor of the 2-methylcyclopentadiene isomer. Furthermore, examination of the product microstructure by IR and NMR spectroscopy indicated that the most important contributing repeat unit was (17), resulting from 2-methylcyclopentadiene by 1,4 addition. The repeat units (18) and (19) resulted from 1- and 2-methylcyclopentadiene, respectively, by 3,4 addition, and structure (20) was virtually absent. 

The microstructural features of stereoregular copolymers prepared directly from monomers can be related to conditional monomer placement probabilities, which are, in turn, related to monomer reactivity ratios and monomer feed ratios. Thus, the conditional probability that an A unit follows a B unit in a copolymer chain, P(a/b), can be calculated from BA and BB dyad distributions or from the monomer reactivity ratio for B,r, and the ratio of, monomers A and B in the feed (A /B ), according to the following equations. 

Polymerization was carried out in benzene in the presence of bis-(7r-allylnickel halides). The latter were prepared from nickel carbonyl and allyl halide (allyl bromide, crotyl chloride, bromide, or iodide etc.). The results of the polymerization runs are reported in Table I. The data indicate that all of the bis(7r-allylnickel halides) initiate by themselves the stereospecific butadiene polymerization yielding a polymer with 97-98% 1,4-units. The cis-l,4/trans-l,4 ratio depends on the halide in the dimeric r-allylnickel halide but not on the nature of allylic ligand. The case of bis(7r-crotylnickel halides) shows the effect of halide on microstructure, for whereas (C4H7NiCl)2 initiates cis- 1,4-polybutadiene formation, trans-1,4 polymers are produced by (C4H7NiI)2. The reactivity increase in the series Cl < Br < I. 

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