Poly comonomer sequence distribution

Table 13. Comonomer sequence distribution of the poly(CPP-SA) series...

A series of ethylene-vinyl chloride (E-V) copolymers was generated [27] by the reductive dechlorination of poly (vinyl chloride) (PVC) [28]. Each E-V copolymer has the same average chain length (approx. 1000 repeat units) and polydispersity. High-resolution solution NMR revealed [27] their random-like comonomer sequence distributions, which were quantitatively determined to the comonomer sequence triad-level (see chapter 2). E-V copolymers with less than 40% V units were observed [29] to crystallize, and the stabilities, structures and morphologies of their crystals were observed [29,30] and analyzed by DSC, X-ray and electron diffraction, and electron microscopy. 

Recently, Choo and Way mouth performed the copolymerization of ethylene with 1,5-HD using various metallocene catalysts (12, 13, 14, Figure 19.2). 1,5-HD cyclopolymerized exclusively to give MCP units in the copolymers, with only traces of uncyclized 1,2-inserted 1,5-HD. The diaste-reoselectivity of the cyclocopolymerization favored the formation of 1,3-cyclopentane rings for metallocenes (74% trans for 12, 81% trans for 13, and 66% trans for 14). For metallocenes 12 and 14, the ethylene/1,5-HD copolymerization yielded copolymers with similar comonomer compositions and sequence distributions to those observed for ethylene/1-hexene copolymerization with these catalysts. On the other hand, the copolymers derived from metallocene 13 showed very different compositions and sequence distributions. At comparable comonomer feed ratios, the poly(ethylene-c -l,5-HD)s were enriched in the 1,5-HD comonomer and deficient in ethylene as compared to the analogous polymers prepared from ethylene and 1-hexene. The copolymerization behavior of 13 provided support for a dual-site alternating mechanism for 1,5-HD incorporation, wherein one coordination site of the active catalyst center is highly selective for the initial 1,2-inserion of 1,5-HD and the other site is selective for cyclization. 

Copolymers of tetrafluoroethylene with isobutylene are crystalline in the equimolar range.(174) The maximum melting temperature of 203 °C corresponds to the equimolar composition. The x-ray diffraction pattern did not show any correspondence with the homopolymer of poly(tetrafluoroethylene). The prepared copolymers were not crystalline outside a narrow composition range. It is unresolved whether crystallinity could be induced over a wider composition range, or if the sequence distribution is such as to preclude the development of crystallization. In contrast, the copolymers containing propylene did not display any crystallinity over the complete composition range. This result appears to indicate a low level of stereoregularity for the propylene comonomer. 

The sequence distribution [31] of hydrophobic comonomer in hydrophobic poly(sodium acrylates) has also been obtained for low molecular weight/high hydrophobe materials. These polymers have been found to obey Bernoullian statistics. 

The polymerization of cyclotrisiloxane with mixed siloxane units was expected to allow for a synthesis of siloxane-siloxane copolymers with a uniform distribution of units. These copolymers are not accessible by the kinetically controlled copolymerization of two cyclic trisiloxane comonomers, which leads to a microsequential order of siloxane units and a gradient arrangement of monomer units along the chain 16J7), So far the sequencing in poly siloxane, obtained from cyclotrisiloxane having two kinds of siloxane units, have been studied only for the polymerization in the presence of anionic initiators (16,18,19). The use of cationic routes for a controlled synthesis of copolymers with a uniform distribution of units is, therefore, of interest. 

Sequence The difference in reactivity between comonomers affects the composition and also alters the placement of the monomer units along the chain. In the case of living polymerization, sequential monomer addition leads to the formation of block copolymers. However, when a random copolymer is targeted, reactivity differences can lead to nonrandom distribution of monomer units. If the incorporation of a comonomer B is intended to disrupt crystallinity of poly( A), uninteimpted sequences of A can lead to domains of crystallinity. For example, block copolymers of ethylene-propylene are highly aystaUine, while random copolymers are completely amorphous. 

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