Copolymers with Bernoullian sequence distributions

As stated earlier, copolymers whose sequence distributions can be described by Bernoullian statistics (i.e. so-called random or statistical copolymers) can result either from Bernoullian or non-Bernoullian processes. Those formed by genuine Bernoullian mechanisms are relatively rare, reflecting the likeli- 

Several examples of NMR studies of copolymers that exhibit Bernoullian sequence distributions but arise from non-Bernoullian mechanisms have been reported. Komoroski and Schockcor [11], for example, have characterised a range of commercial vinyl chloride (VC)/vinylidene chloride (VDC) copolymers using carbon-13 NMR spectroscopy. Although these polymers were prepared to high conversion, the monomer feed was continuously adjusted to maintain a constant comonomer composition. Full triad sequence distributions were determined for each sample. These were then compared with distributions calculated using Bernoullian and first-order Markov statistics the better match was observed with the former. Independent studies on the variation of copolymer composition with feed composition have indicated that the VDC/VC system exhibits terminal model behaviour, with reactivity ratios = 3.2 and = 0.3 [12]. As the product of these reactivity ratios is close to unity, sequence distributions that are approximately Bernoullian are expected. 

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The ROMP of [2.2]paracyclophane-l,9-diene (128) yields poly(p-phenylenevinylene) (129) as an insoluble yellow fluorescent powder. Soluble copolymers can be made by the ROMP of 128 in the presence of an excess of cyclopentene387, cycloocta-1,5-diene388 or cyclooctene389. The UV/vis absorption spectra of the copolymers with cyclooctene show separate peaks for sequences of one, two and three p-phenylene-vinylene units at 290, 345 and about 390 nm respectively, with a Bernoullian distribution. The formation of the odd members of this series must involve dissection of the two halves of the original monomer units by secondary metathesis reactions. 

Thus, copolymers of the same composition can have qualitatively different sequence distributions depending on the solvent in which the chemical transformation is performed. In a solvent selectively poor for modifying agent, hydrophobically-modified copolymers were found to have the sequence distribution with LRCs, whereas in a nonselective (good) solvent, the reaction always leads to the formation of random (Bernoullian) copolymers [52]. In the former case, the chemical microstructure cannot be described by any Markov process, contrary to the majority of conventional synthetic copolymers [ 10]. 

Brown and Cudby [6] and Randall [7] have analyzed the C-spectra of a series of propylene-butene-1 copolymers prepared using an isospecific catalyst system. The enchainment of the monomer units was essentially isotactic and head-tail. Resonances observed for the homopolymers, assignments made previously by Fish and Dannenberg [78], the Grant-Paul relationships and variations in resonance intensity with copolymer composition were used to make assignments for the resonances of methine, methylene and methyl carbon atoms. Triad and some tetrad sequence distribution measured from the spectra were consistent with Bernoullian distributions over the entire range of copolymer compositions examined. 

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