Polyisoprene Sequences

For the diblock copolymer, which exhibits a flow region at longer times than pure polyisoprene, the relaxation of the isoprene sequence is treated like the relaxation of the arm of a star polymer. We have followed the description proposed by McLeish [17, 18] for star homopolymers. The distribution of relaxation times is given by Eq. (9), where Mb is the molecular weight of one branch (here the molecular weight of the polyisoprene sequence), s ranges be-... 

In order to improve the processing and end-user properties and also to simplify the preparation of the blends described above, we propose to design new copolymers which would have the same block sequences (polyisoprene and polystyrene) but not the same topology [23, 26-28]. We have already pointed out that, to obtain optimized rheological properties for a good tack, we must have a free polyisoprene sequence which explores the network of the trapped polyisoprene sequences of the triblock copolymer and swells this network. It is important to notice that this is the triblock copolymer which causes the soUd-Uke behavior [23]. This configuration can be obtained with a series of block sequences terminated by a polyisoprene sequence [26-28] ... 

The molecular weight of the end polyisoprene (free sequence) must be the same as for the free polyisoprene sequence of the SI diblock. 

The molecular weight of the trapped polyisoprene sequence must be higher than the critical molecular weight for entanglements of polyisoprene. 

Cyclic peroxides are formed preferably in polyisoprene rather than in polybutadiene, because the mesomeric stabilization of allylic radicals is much less enhanced in polybutadienoid than in polyisoprenic sequences, on account of the lack of tertiary carbon in the former. 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

Figures 7 and 8 show the original morphologies of the block copolymers observed by TEM selectively stained P4VP, P2VP, and polyisoprene (PIP) sequences...

Successive 1,4 units in the synthetic polyisoprene chain evidently are preponderantly arranged in head-to-tail sequence, although an appreciable proportion of head-to-head and tail-to-tail junctions appears to be present as well. Apparently the growing radical adds preferentially to one of the two ends of the monomer. Which of the reactions (6) or (7) is the preferred process cannot be decided from these results alone, however. Positive identification of both 1,2 and 3,4 units in the infrared spectrum shows that both addition reactions take place during the polymerization of isoprene. The relative contributions of the alternative addition processes cannot be ascertained from the proportions of these two units, however, inasmuch as the product radicals formed in reactions (6) and (7), may differ markedly in their preference for addition in one or the other of the two resonance forms available to each. We may conclude merely that structural evidence indicates a preference for oriented (i.e., head-to-tail) additions but that the 1,4 units of synthetic polyisoprene are by no means as consistently arranged in head-to-tail sequence as in the naturally occurring poly-isoprenes. 

Unlike polybutadiene, polyisoprene prepared at low temperatures shows little or no inclination to crystallize either on stretching or cooling. This may seem surprising in view of the even greater preponderance of trans-1 4 units in polyisoprene than in poly butadiene. The explanation for the contrasting behavior in this respect between low temperature synthetic polyisoprene, on the one hand, and guttapercha and low temperature polybutadiene, on the other, probably is to be found in the appreciable occurrence of head-to-head and tail-to-tail sequences of 1,4 units of the former. 

In this study, the effects of the variations in block sequence and composition (and thus relative block length) on the material properties of two series of triblock copolymers has been investigated. One of the blocks, the hydrogenated polybutadiene (HB), is semicrystalline, and the other block, the hydrogenated polyisoprene (HI) is rubbery at room temperature. Thus in one series, the HBIB block copolymers, the end blocks are semi-... 

The characteristic ratios of stereoirregular 1,4-poiybutadiene and 1,4-polyisoprene chains are theoretically investigated by the Monte Carlo procedure in accordance with the model proposed by Mark (V 001 and V 003). It is pointed out that the presence of discrete cis units in frans-rich chains significantly reduces the characteristic ratio while that of discrete trans units in c/is-rich chains has little effect on the characteristic ratio. The characteristic ratio and its dependence on both the trans and cis contents and their sequence distribution is calculated for stereoirregular polymers in accordance with the interdependent RIS model proposed by Mark (V 001 and V 003), and Ishikawa and Nagai V 005 and V 007 . 

C-NMR spectroscopy has shown that the polybutadienes prepared using alkyl-lithium initiators have random placement of the different modes of enchainment 222-223). This contrasts with an earlier claim of blocky structures 224). Random sequence distribution has also been established for polyisoprene by 1H-NMR 225) and 13C-NMR 226) spectroscopy. 

Synthetic cis-1,4 polyisoprene has the structural feature similar to PB. As to the NMR analysis of sequence distribution, however, little work has been done after 1978. Detailed assignment may be possible by using modern NMR techniques and magnetic field higher than 100 MHz for 13C-NMR. 

The various regular polymers that can be produced by polymerization of butadiene and isoprene are summarized in reactions (4-3) and (4-4). In addition to the structures shown in these reactions, it should be remembered that 1, 4 polymerization can incorporate the monomer with cis or trans geometry at the double bond and that the carbon atom that carries the vinyl substituent is chiral in 1,2 and 3,4 polymers. It is therefore possible to have isotactic or syndiotactic polybutadiene or polyisoprene in the latter cases. Further, these various monomer residues can alt appear in the same polymer molecule in regular or random sequence. It is remarkable that all these conceivable polymers can be synthesized with the use of suitable catalysts comprising transition metal compounds and appropriate ligands. 

Recent C T, measurements performed on various bulk polymers with low Tg, such as polyisoprene, polyisobutylene, polyvinylmethylether and polypropyleneoxide, have shown that the experimental value of T at the minimum is always much higher than the prediction from Hall-Helfand expression. Thus, this discrepancy cannot be assigned to a specific motional anisotropy of the cis 1,4 sequences of the polybutadiene chain. It seems more likely to assign it to a fast anisotropic motion of the CH bonds inside a cone. In the case of polybutadiene, the corresponding half angle of the cones should be 26° and 36° for CH and CH2... 

The weight fraction of each block sequence was determined by ordinary element analysis and was verified by the infrared absorption band at 1683 cm"1 characteristic of polyisoprene (13) in CC14 solution (0.5 g/1). The findings agreed within experimental error, and the arithmetic average was used as the weight fraction for the EO and Is block segments. 

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