Dienes as monomers

Despite the enormous importance of dienes as monomers in the polymer field, the use of radical addition reactions to dienes for synthetic purposes has been rather limited. This is in contrast to the significant advances radical based synthetic methodology has witnessed in recent years. The major problems with the synthetic use of radical addition reactions to polyenes are a consequence of the nature of radical processes in general. Most synthetically useful radical reactions are chain reactions. In its most simple form, the radical chain consists of only two chain-carrying steps as shown in Scheme 1 for the addition of reagent R—X to a substituted polyene. In the first of these steps, addition of radical R. (1) to the polyene results in the formation of adduct polyenyl radical 2, in which the unpaired spin density is delocalized over several centers. In the second step, reaction of 2 with reagent R—X leads to the regeneration of radical 1 and the formation of addition products 3a and 3b. Radical 2 can also react with a second molecule of diene which leads to the formation of polyene telomers. 

Due to the significant importance of dienes as monomers, absolute as well as relative rate data have been determined for the addition of initiator derived radicals. Photolysis of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) 5 leads to the formation of... 

Alkyl derivatives of the alkaline-earth metals have also been used to initiate anionic polymerization. Organomagnesium compounds are considerably less active than organolithiums, as a result of the much less polarized metal-carbon bond. They can only initiate polymerization of monomers more reactive than styrene and 1,3-dienes, such as 2- and 4-vinylpyridines, and acrylic and methacrylic esters. Organostrontium and organobarium compounds, possessing more polar metal-carbon bonds, are able to polymerize styrene and 1,3-dienes as well as the more reactive monomers. 

Although the polymerization of diene monomers is most familiar for 1,3-dienes, as in the production of rubbers, the polymerization of 1,6-dienes to yield polymers containing six-membered rings ( cyclopolymerization ) has been well established for many years43. Gibson et al.441 have used cyclopolymerization of 1,6-diynes to prepare polymers which are effectively substituted polyacetylenes, the archetype being the polymerization of 1,6-heptadiyne ... 

I, 3-diene polymerization. Monomer molecules are included in chiral channels in the matrix crystals, and the polymerization takes place in chiral environment. The y-ray irradiation polymerization of trans- 1,3-pentadiene included in 13 gives an optically active isotactic polymer with a trans-structure. The polymerization of (Z)-2-methyl-1,3-butadiene using 15 as a matrix leads to a polymer having an optical purity of the main-chain chiral centers of 36% [47]. 

This review does not cover the application of Ln-polymerization catalysis to polar monomers. A comprehensive review on this topic is urgently required. Nevertheless, we hope that this volume will become the future key reference in Ln and especially in Nd-based catalyst systems as well as in Nd-catalyzed polymerization of dienes. As a starting point for future work unsolved and open questions are summarized in a separate chapter of the first part of this volume. We really hope that this list of open questions will inspire and stimulate further research in this interesting field of catalysis. 

In order to have a successful polymerization of such a diene-dieno-philic monomer, both the diene and dienophile portions must be highly reactive. In the examples of the fos-cyclopentadienyl monomers given, this high reactivity is also the basis of most of the troubles associated with the polymerization reaction in that the monomers cannot be suitably purified. Purification in this case involves the separation of a mixture of monomer and other oligomers as well as adducts of monomer and monoalkylated cyclopentadiene. Ideally, a diene-dienophilic monomer which could be held in an inactive state during purification and until ready for the polymerization reaction would represent the perfect monomer for such a polymerization. 

This has been essentially accomplished in a case of the diene-dienophile monomer polymerization (6, 7, 8, 16). This excellent work makes use of the reaction of such dienes as cyclopentadienones, a-pyrones, and thiophene dioxides with dienophiles. The resulting adduct, a protected or inactive diene, loses carbon monoxide, carbon dioxide, or sulfur dioxide at elevated temperatures to form a diene. Thus when a Ws-malei-mide reacts with one mole of a cyclopentadienone, the resulting adduct will lose carbon monoxide at 150—260° in an inert solvent to form an active intermediate diene-dienophilic monomer which readily poly-... 

Catalysts of the Ziegler-Natta type are applied widely to the anionic polymerization of olefins and dienes. Polar monomers deactivate the system and cannot be copolymerized with olefins. J. L. Jezl and coworkers discovered that the living chains from an anionic polymerization can be converted to free radicals by the reaction with organic peroxides and thus permit the formation of block copolymers with polar vinyl monomers. In this novel technique of combined anionic-free radical polymerization, they are able to produce block copolymers of most olefins, such as alkylene, propylene, styrene, or butadiene with polar vinyl monomers, such as acrylonitrile or vinyl pyridine. 

Of great industrial interest are the copolymers of ethene and propene with a molar ratio of 1/0.5, up to 1/2. These EP-polymers show elastic properties and, together with 2-5 wt% of dienes as third monomers, they are used as elastomers (EPDM). Since they have no double bonds in the backbone of the polymer, they are less sensitive to oxidation reactions. As dienes, ethylidenenorbomene, 1,4-hexadiene, and dicyclopentadiene are used. In most technical processes for the production of EP and EPDM rubber in the past, soluble or highly disposed vanadium components are used [69]. Similar elastomers can be obtained with metallocene/MAO catalysts by a much higher activity which are less colored [70-72]. The regiospecificity of the metallocene catalysts toward propene leads exclusively to the formation of head-to-tail enchainments. The ethylidenenor-bornene polymerizes via vinyl polymerization of the cyclic double bond and the tendency to branching is low. The molecular weight distribution of about 2 is narrow [73]. 

Another isohypsic transformation of special significance involves elimination of H-X elements from allylic derivatives to form 1,3-dienes. Besides being extremely important compounds as monomers, 1,3-dienes occupy a unique position in synthetic practice as components in the Diels-Alder reaction. One of the common routes of synthesis of 1,3-dienes also employs a vinyl Grignard addition to carbonyl compounds as the initial step (Scheme 2.55). Allylic alcohols thus formed can easily undergo 1,2-elimination (in some cases it is preferable first to transform the alcohols into their respective acetates). 

Several early attempts at ADMET polymerization were made with classical olefin metathesis catalysts [57-59]. The first successful attempt was the ADMET polymerizations of 1,9-decadiene and 1,5-hexadiene with the WClg/EtAlf l,. catalyst mixture [60]. As mentioned in the introduction, the active catalytic entities in these reactions are ill-defined and not spectroscopically identifiable. Ethylene was trapped from the reaction mixture and identified. In addition to the expected ADMET polymers, intractable materials were observed, which were presumed to be the result of vinyl polymerization of the diene to produce crosslinked polymer. Addition to double bonds is a common side reaction promoted by classical olefin metathesis catalysts. Indeed, reaction of styrene with this catalyst mixture and even wifh WCl, alone led to polystyrene. Years later, classical catalysts were revisited in fhe context of producing tin-containing ADMET polymers wifh tungsten phenoxide catalysts [61], Alkyl tin reagents have long been known to act as co-catalysts in classical metathesis catalyst mixtures, and in this case the tin-containing monomer acted as monomer and cocatalyst [62]. Monomers with less than three methylene spacers between the olefin and tin atoms did not polymerize (Scheme 6.14). 

The following sections detail the literature reports pertaining to the synthesis of block copolymers using nitroxide-mediated polymerization techniques. The sections are organized according to monomer type and generally follow the historical development of the particular subsection. Most literature on nitroxide mediated preparation of block copolymers is found for the styrene-based monomers, and is summarized first. This is followed by acrylates and dienes, as they were the next monomers to be studied. These sections are followed by more recent work with vinyl pyridine, acrylamides, and maleic anhydride. The final section deals with methacrylates. This is presented last to stress the importance of developing new nitroxides that can successfully be used for the homopolymerization of methacrylate-based monomers. 

These polyolefin rubbers are produced in two main types the saturated co-polymers, ethylene propylene rubber (EPM), and the unsaturated ethylene-propylene diene terpolymer (EPDM). The monomers are co-polymerised in ziegler natta type catalysts. The EPDM types are capable of sulfur vulcanisation as they contain, in addition to olefins, a non coagulated diene as the third monomer. 

A related strategy described by Frechet (Figure 13.5b) utilizes a bis-diene amide monomer 21 that possesses an internal acetal/ketal moiety [86]. Monomer 21 is a bis-acrylamide capable of conjugate or Michael addition at unsubstituted double bonds. A diamine such as piperazine 22 can be used to prepare acetal/ketal polyamidoamines 23. One characteristic of polyacetal polyamidoamines 23 is they will be cationic and have been used in stndies to complex oligonucleotide analogues (e.g., siRNA and DNA). 

Formation of AC-2 results in a more substantial transformation of bonds than the formation of AC-1. A different spatial structure of AC-2 is formed, which has the flower shape and permits both mono and bidentate coordination of 1,3-dienes, as well as putting less restrictions on their coordination with a monomer for all the positions of substituent at the C = C bond, in relation to a crystal surface. This is the reason for AC-2 stereospecificity and the aforementioned possibility of stereo block copolymer formation during a-polymerisation in the presence of Ziegler-Natta catalytic systems. AC-2 can orient unsaturated C = C bonds of 1,3-dienes to bidentate coordination on the bidentate AC. 

It was shown that p-hydrogen-containing nitroxides promote the controlled polymerization of not only styrenic monomers but alkyl acrylates and dienes as well. Taking this into account, a novel ttifimcdonal alkoxyamine (Scheme 28, 7) based on N-t Tt-butyl-1 -diethylphosphono-2,2-dimethylpropyl nitroxide (Scheme 28, 8) was developed for the synthesis of 3-arm PS and poly(n-butyl acrylate) (PnBuA) stars along with (PnBuA-b-PS)3 star-block copolymers. ... 

Vinylene-bridged [2]ferrocenophanes [35] or diene-bridged [4]ferrocenophanes [36] have been utilized as monomers in a ring-opening metathesis polymerization by the use of molybdenum catalysts (Fig. 8.10). 

In sequential IPNs, if polymer I is based on a diene, and monomer II (such as styrene) is polymerized via free-radical polymerization, some grafting will occur. Of coin-se, this grafting is important for interfacial bonding in such related materials as high impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) materials. 

EPDM is a terpolymer of ethylene, propylene, and a small amount of an unsaturated diene as a third monomer to provide a cure site. Unlike the elastomers previously discussed, the unsaturation in EPDM is not in the main chain, but it is pendent to the chain. Peroxide-based cure systems afford better aging resistance and low compression set. A comparison of a sulfur-based cure to two different peroxides in EPDM is shown in Table 21 (4). Initial properties for these three compounds are reasonably close. However, after air aging, the advantages of peroxide curing are apparent. Most dramatic is the improved compression set... 

One or Both Monomers Dienes. As in homo-polydienes, very irregular structures can result from cisjtrans or 1,2-addition, as exemplified particularly by a C study of styrene-isoprene copolymer in which the distribution of styrene and m-buta-1,4-, rrans-buta-1,4- and buta-1,2-diene was examined. Similar studies of styrene-butadiene, , ethylene-butadiene, and butadiene-isoprene systems have been reported. 

On the other hand, 5-ethylidenenorbornene (which is about the only third monomer used today to make EPDM) is produced from a reaction of butadiene and cyclopenta-diene as shown in Figure 3.17. 

The events following the astute observation by Jeremy Burroughes of light emission from a layer of PPV held under an electrical potential that led to the description of the use of such materials as the electroluminescent layer in polymer-based LEDs, and the enormous amount of research and development associated with this area has now entered the folk law of the organic electronics field. [2.2]Paracyclophane-1,9-diene, the monomer shown on the left-hand side in Figure 27 (where R=H), is the obvious monomer to use in ROMP for the synthesis of such materials and Thom-Csanyi and coworkers were the first to show that this was possible. 

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