E/NB copolymers

Polynorbornene (PNB) homopolymers of industrial interest are not processible owing to their high TtaS and insolubility in common organic solvents. By copolymerization of norbornene with ethylene, a cycloolefin copolymer (COC) can be produced. These new materials have been the focus of academic and industrial research. Ethylene/norbornene (E/NB) copolymers are usually amorphous and show an excellent transparency and high refractive index, making them suitable for optical applications. ... 

While the copolymerization of norbornene and ethylene by vanadium and titanium catalysts had been investigated in the 1970s, it was not until the invention of metallocene/MAO catalysts and their application to the homopolymerization of norbornene and other cycloolefins by Kaminsky et ai.8,15,19 that the use of early transition metal catalysts for the addition polymerization of norbornene drew new attention. Nevertheless, the industrial relevance of E/NB copolymers, and the nature of the homopolymers, described in the first reports as insoluble in organic solvents, crystalline, and extremely high melting, focused further investigations on copolymers rather than on norbornene homopolymers. 

Half-sandwich complexes of titanium, when activated by a cocatalyst such as MAO, are even more active for the homopolymerization of norbomene than metallocenes. Heitz and Peucker found that chromium-based half-sandwich complexes can also be activated to produce PNBs and E/NB copolymers. Nevertheless, detailed investigations on half-sandwich complexes for the addition polymerization of norbomene do not appear in the literature. 

Another interesting feature of these catalyst systems is that they generate highly branched or even hyper-branched (i.e., containing branches on branches) products when used for ethylene polymerization. E/NB copolymers with low norbomene contents also show these hyper-branches. 

Early attempts to produce E/NB copolymers utilized heterogeneous TiCl4/AlEt2Cl or vanadium catalysts, but real progress was achieved utilizing metallocene catalysts for this purpose. Metallocenes are about ten times more active than vanadium systems, and by choosing the metallocene, the norbornene/ethylene comonomer sequence distribution in the copolymer may be varied from statistical (random) to alternating. 

There is a comonomer feed composition effect seen on the polymerization activity of 19. Activity reaches a maximum at xn = 0.1 (Table 16.6, Run 3) and is seven times higher than that for the homopolymerization of ethylene. At higher Xns, activity decreases. Catalyst 20 does not show such an effect. The E/NB copolymer molecular weights obtained with the Pd catalysts range between 7,000 and 502,000 g/mol. For both catalysts, a comonomer feed composition effect on molecular... 

The amount of norbornene incorporated into the copolymer also influences Tg. The TgS of E/NB copolymers produced with 19 are very high and range from 98 to 217 °C. Copolymers produced at xn = 0.80 and higher, as well as homo-PNB, show no TgS or JmS under 350 °C, and decompose at >350 °C. The TgS of copolymers produced by 20 range from —28 to 120 °C, owing to their lower norbornene content. 

The best method for E/NB copolymer micro- and stereostructural analysis is NMR spectroscopy,which can be used to determine if the polymer chain contains dyads and triads (short norbornene blocks) or only isolated norbornene units. This section compares the different microstructures of E/NB copolymers made with the different types of catalysts discussed in this chapter. Details can be found in the literature. ... 

To begin a discussion of the micro- and stereostructures of E/NB copolymers produced with different types of catalysts, trends for Ci-symmetric metallocenes will first be considered. While C2-symmetric catalysts (e.g., 2,4,6,7) produce random copolymers with small norbornene blocks if the norbornene concentration in the feed is high, C -symmetric catalysts produce more alternating (10) or purely alternating (9) polymers. The catalyst system lO/MAO was found to incorporate norbornene slightly better than 9/MAO (Table 16.4) whereas exclusively isolated and alternating norbornene... 

FIGURE 16.14 Structural units in E/NB copolymers with (a) isolated norbornene units and (b) alternating norbornene/ethylene units. 

Assignment of the Norbornene C1-C7 and Ethylene Q-C Carbon Atoms in E/NB Copolymers with Isolated or Alternating Norbornene Units to Different Groups of C NMR Peaks as Depicted in Figure 16.14... 

The incorporation level of norbornene, as well as its distribution and enchainment orientation, have a great influence on the properties of E/NB copolymers. Norbornene copolymerization with early and late transition metal complexes happens exclusively by double bond addition (2,3-insertion). The norbornene unit has a 2,3-cis-exo-orientation (Figure 16.14a), and two stereocenters are formed in the polymer chain. Copolymers with isolated norbornene units therefore have different possible tacticities than alternating copolymers (Figure 16.14b). 

Table 16.7 gives correlations of NMR peaks to the carbon atoms in the E/NB copolymer chain as shown in Figure 16.14a (signal areas A, B, C,D stand for peak regions of the CNMR spectrum). The signals for general E/NB copolymer NMR spectra (i.e., for both isolated norbornene units and... 

Pentad and Tetrad Assignments of NMR Peaks for E/NB Copolymer Carbon Atoms C2/C3 73,75,76... 

FIGURE 16.15 NMR spectra of E/NB copolymers synthesized using constrained geometry catalysts 16... 

Alternating E/NB copolymers synthesized by CGC catalysts show a range of different microstructures and tacticities (Figure 16.15). Catalysts 16 and 18 are not able to make copolymers with norbornene-norbornene dyads, even with a high molar excess of norbornene in the feed. Small amounts of meso norbornene dyads can be observed if catalyst 17, which contains a relatively flexible cyclododecyl ligand N-substituent, is used. 

FIGURE 16.16 TgS of E/NB copolymers made with different zirconocenes (filled squares = 2 open squares = 7 filled circles = 8 open circles = 3. 

At a norbornene content of 50 mol%, the Tg of a statistically random or alternating E/NB copolymer is about 150 °C. A material with 75 mol% norbornene has a Tg of about 200 °C. Alternating copolymers are semicrystalline if the norbornene content is above 37 mol%. They feature TgS of 100-130 °C and T s of 270-320 °C. Alternating copolymers have an even greater solvent resistance than statistical copolymers, although they are still transparent owing to the small size of their crystalline regions (5 nm). 

A commercial plant for the production of COC material (E/NB copolymer) was built in 2000 by Ticona in Oberhausen, Germany, with a capacity of 30,000 t/a (tons per annum). Mitsui produces E/NB copolymers using vanadium-based catalysts. The industrially produced copolymers have norbornene contents between 30 and 60 mol% and TgS of 120-180 "C. The copolymer densities are low and near 1. For many applications, these COC materials show better mechanical properties than comparable amorphous thermoplastics, and are processible by all conventional methods. E/NB copolymers are proving valuable as materials for high capacity CDs and DVDs, lenses, blister foils, medical equipment, capacitors, and packaging. ... 

E/NB copolymers can be obtained as random amorphous materials with high TgS or as alternating, partially crystalline materials with high TmS. Amorphous copolymers have short blocks of norbornene units (dyads or triads), which account for their high TgS and excellent optical properties. All norbornene homo- and copolymers made by single site catalysts are characterized by narrow molecular weight distributions, which make technical processing easier. The first commercial norbornene copolymer products are already available. 

In addition to linear a-olefins, Shiono and coworkers reported that 15/MAO (Figure 9) catalyzed the living copolymerization of ethylene and NB. For example, at 0 °C 15/MAO can furnish poly(E-co-NB) with 53mol.% NB and Mn = 78000gmol" with Mw/M = 1.16. Furthermore, a linear increase in M with reaction time was observed for this system. When activated with MAO at 40 °C, a similar compound, 20 (Figure 9), also provided ethylene-NB copolymers with fairly narrow PDIs (mJm = 1.21-1.27). ... 

First, different data sets (away from and at binodal) must be scaled to the identical temperature and concentrations. This is possible for a few blends with (assumed) identical interaction parameter but characterized by different chain lengths (Na, Nb) and hence different phase diagrams. This method is used for isotopic polystyrene mixtures. If the parameter ySANs((t)) is linear with 1/T for each concentration < > then %SANS(=const, T) can be reasonably extrapolated to regions at or inside coexistence curve. We use this solution for olefinic blends composed of random copolymers E EE. Here the self-same mixtures are used in both bulk SANS samples and in profiled thin films. 

Figure 6.2 Block copolymers at the interface between a good solvent for the A blocks and a good solvent for the B blocks. The degree of polymerization of the two blocks are Na and Nb respectively, and E is the area per molecule at the interface.

Equation (103) demonstrates that the CMC for quasi-neutral micelles is a strongly increasing function of a, i.e., it is strongly affected by the value of the bulk pH. When -A Aln(l - at) > y Nb/(P), quasi-neutral micelles do not form at any copolymer concentration in the solution. 

The transition from morphology i to i + 1 (i.e., lamella to cylinder or cylinder to sphere) occurs upon an increase in the degree of ionization of the coronal blocks, a, and/or an increase in A(A/decrease in Nb- These molecular parameters are specified by the block copolymer composition. Therefore, to detect the predicted structural transformations, one has to use a series of block copolymer with finely tuned molecular weights of the blocks. As follows from (138), the relative width of the corridor, delineating the stability range of cylindrical micelles in a low salt solution. 

Figure 18a demonstrates the diagram of states in Nb, Oio coordinates for block copolymer with the length of pH-sensitive block Na = 50, and pH = pATa (i.e., for ah = 0.5). Solid lines indicate the binodals calculated according to (148) and (142). A smaller value of oct = 0.1 is used in Fig. 18b The diagrams localize the stability regions of three main morphologies of block copolymer aggregates spherical, S (i = 3), and cylindrical C (/ = 2) micelles, and lamellae L (i = 1). The latter can further associate due to Van der Waals forces, and precipitate from the solution.

Comparison of diagrams of Fig. 18a, b indicates that morphology i of aggregate formed by pH-sensitive block copolymers can be tuned by variations in both concentration of added salt, Oion and pH in solution. For example, when pH < pAa (e.g., at ttfc = 0.1, Fig. 18b), a copolymer with lengths of the blocks, Na = 50 and Nb = 125, retains the cylindrical (C) morphology at any salt concentration, Oion. In contrast, when pH = pAa (a, = 0.5, Fig. 18a), the same copolymer makes cylindrical (C) micelles only at low salt concentrations, and associates into spherical (S) micelles upon a further increase in Oion. 

Simulations on a cubic lattice have been used to study the self-assembly into micelles by diblock copolymers with Na segments of block A and Nb segments of block B. Previous work has shown these simulations can be used to measure the critical micelle concentration (cmc). In the present work, the pairwise interactions cause the medium to behave as a poor solvent for block A and a good solvent for block B. The purpose is to determine the relative importance of the three pairwise interactions between different species, denoted by Eas E ab and Egg, for the cmc. Here ExY denotes the dimensionless pairwise interaction, Uxv/ Ty where UxY is the pairwise interaction energy of segments of species X and Y. The three species involved here are segments of A, segments of B, or solvent (S). Hence we examine all E xy where X Y. The results are compared with previous work that focused attention on the dominant pairwise interaction parameter, E as The effects of E ab bs important for asym-... 

In addition to cyclopentene, copolymers from ethylene and norbornene have also been made using 31/MAO (Yoon et al, 2006). With this catalyst, a high molecular weight, low PDI poly(E-co-NB) sample was prepared (M = 238 000 g/mol, MJM = 1.05) containing 62 mol% ethylene and a Fg of 86.5 °C. In addition, 31/MAO was also used to synthesize a high molecular weight poly(E-co-P)-Moc/ -poly(E-co-NB) sample (M = 576 000 g/mol, Mw/Mn = 1.13). 

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