We have seen that the addition of a proton to an alkene linkage gives a carbocationic species, which is a strong electrophile. Such carbocations are highly reactive and quickly react with available nucleophiles. In the case of hydrogen chloride addition, the carbocations react with chloride ion as the nucleophile, to yield the addition product, an alkyl halide. When there is no strong nucleophile present and when the solvent is even weakly nucleophilic (water, alcohols, etc.), the carbocation reacts with the solvent nucleophiles to give an alcohol or an ether.
Consider the case where the solvent is not nucleophilic and there are no other strong nucleophiles present in abundance. If the alkene is present at high concentration, the alkene can act as a nucleophile toward the carbocation, forming a dimeric species containing a new carbon-carbon bond and also a new carbocation center. This process is illustrated below for the case of isobutene. When this second (dimer) carbocation reacts with another molecule of isobutene, still another C-C bond is formed, giving a trimeric species. This can continue until a large number (n) of isobutene molecules is incorporated into the final product, which is called a polymer, specifically polyisobutene.
q This kind of polymerization is called cationic polymerization, because it involves a chain process which is propagated by carbocation intermediates.
q Cationic polymerization of simple alkenes is especially efficient for alkenes which form relatively stable carbocations (note the tertiary carbocation intermediate) and which have one double bond terminus of the alkene unsubstituted, so as to minimize steric effects in the TS for the addition reaction.
q Thus, isobutene is especially amenable to cationic polymerization, but other alkenes with electron donating groups (oxygen and nitrogen-based functionalities) are also efficiently polymerized.
q Polymer Nomenclature: There are several methods of naming polymers. The simplest method is simply to place the name of the starting alkene (called the monomer) in parenthesis and add the prefix poly, as shown above for polyisobutene.
B. Anionic Polymerization
In contrast to cationic polymerization, anionic polymerization is relatively efficient when the intermediate anionic species is relatively stabilized. Since alkyl groups are donor groups, simple alkenes are not polymerized efficiently by the anionic method. An example of an alkene which is readily polymerized anionically is methyl acrylate, an alkene which has a strongly electron withdrawing ester function (carbomethoxy). The intermediate anions will (hopefully) be recognized as ester enolate anions.
You may recall that radical centers are stabilized by either electron donating or withdrawing groups, and especially by conjugating groups. Consequently, radical polymerization is one of the more general methods of polymerization. In fact, even ethylene is readily polymerized to give the familiar polymer poly(ethylene). The mechanism of radical polymerization is illustrated below for the polymerization of chloroethene (vinyl chloride).
q The repeat structure of the final polymer is written as:
q As is typical for initiating radical reactions, an initiator, usually a peroxide is employed. This is because the O-O bonds of peroxides are relatively weak and easily cleaved homolytically under mild thermal conditions (e.g., 60 – 80 o C).
c. [2 pts] Draw the repeat structure of the polymer and provide its formal name.
q The formal name is: poly(vinyl chloride) ; note that the parentheses are necessary because the monomer name is compound.
Polymer Regiochemistry. Consider the three possible structures of the polymer of vinyl chloride given below. The difference between the three structures is essentially a difference in the regiochemistry of the addition reaction. We can analyze the different regiochemistries illustrated below by considering the two non-equivalent carbons of the alkene double bond as head and tail respectively. It doesn’t matter which we consider the head and which the tail, but arbitrarily let’s consider the carbon bearing the chlorine substituent as the head and the unsubstituted carbon as the tail. Then we can see that in the case of A, the tail carbon of on monomer is bonded to the head carbon of the other and conversely. This is called head-to-tail regiochemistry. In poly(vinyl chloride) essentially all of the C-C bonds are formed this way. It is highly regiospecific for the head –to-tail regiochemistry. Structure C represents a polymer formed by bonding the tail of one monomer unit to the tail of the other, and also the head of one monomer unit to the head of another. This is called head-to-head regiochemistry and it is not observed in this polymerization. Finally, structure B illustrates a hypothetical polymer which is formed non-regiospecifically, i.e., some of the new C-C bonds are formed in a head-to-tail manner and some by a head-to-head regiochemistry.
A B C
q The basis for the preference for head-to-tail regiochemistry is revealed by examining TS models for the addition step.
This has radical character This has primary radical character
stabilized by a chlorine substituent
It is therefore preferred over primary
q It should also be noticed that steric effects play an important role in determining the preferred regiochemical sense. The second transition state model has the two more highly substituted alkene carbons bonding to one another, which represents a higher level of steric repulsion than in the first TS.
q The stabilization of the radical site by the chlorine substituent is based upon three electron bonding. A molecular orbital diagram is illustrated below.
q As we have noted previously, when the BMO is double occupied and the ABMO is singly occupied, there is a net stabilizing effect, termed three electron bonding.
Polymerization of other Vinyl Monomers. Recall that the vinyl group is CH2=CH- .
q Most monomers which are efficiently polymerized by radical (or other) means are of this type, i.e., they have a single substituent replacing one hydrogen of ethene. This minimizes steric effects in the TS for the addition reaction. Here are a few well known examples.
q A few monomers have two substituents upon the same carbon (geminal substitution), but it is rare for a monomer which has substituents on both carbons to be efficiently polymerized by these means. The basis for this is steric repulsions (see the TS for the hypothetical head-to-head polymerization of vinyl chloride). Illustrated below are exceptions to this generality.
q The polymerization of a tetrasubstituted monomer is especially unique. It is possible only because: (1) the fluorines, though larger than H, are relatively small substituents and (2) the presence of four very strongly electron withdrawing fluorines on the alkene double bond strongly destabilizes the pi bond and makes it highly reactive.
q Although vinylidene chloride can be polymerized, it is best known for its copolymer with vinyl chloride (Saran wrap). A copolymer is a polymer formed by incorporating two different monomers into the polymer chain.
THE POLYMERIZATION OF ETHENE.
As noted previously, even ethene can be polymerized via the radical method. However, as we will see below, the polymer formed has a somewhat unexpected structure which contains some branches at various points along the polymer backbond.
q We note that the chain-carrying radicals in the polymerization of ethene are of the primary type, and therefore are highly reactive.
q In many cases they add to another molecule of ethene, as would ordinarily be expected for radical polymerization.
q However, some of these intermediate primary radicals abstract a hydrogen atom from nearby points along the polymer chain, thereby forming a more stable, secondary radical.
q We have learned previously that radicals typically are able to undergo abstraction reactions and especially hydrogen abstractions. This is particularly the case when the reaction is intramolecular, as shown in the mechanism given below, and when the resulting radical is more stable.
q One of the favored spots for hydrogen abstraction is illustrated below, but there are other possible locations. The particular spot illustrated is favored because the TS for the abstraction is essentially a six-membered ring.
q This intramolecular hydrogen abstraction reaction is termed “backbiting”.
q The resulting polymer has butyl (and other) groups as branches along the chain.
q The presence of branches on the polymer has a substantial effect upon the physical properties and uses of the resulting polymer, as we shall see further on.
q Note that in the polymerizations of vinyl chloride, styrene, acrylonitrile and virtually all other monomers, the intermediate radical is much more stable than a primary radical, so branching does not occur in these polymers.
A fourth and fundamentally different mechanism for polymerization is the so-called Ziegler-Natta polymerization. In this polymerization mechanism, neither carbocations, radicals, nor carbanions are involved. The key intermediates involved here are organometallic compounds. The mechanism for this polymerization type is illustrated below for ethylene.
q Since no reactive intermediates such as radicals are involved in Z-N polymerization, no backbiting is observed. The polymer obtained from ethene is straight chain (i.e., unbranched) polyethylene.
q Note that the R2Ti groups are end groups in the polymers, and make little difference ot its properties. They can be removed and the titanium recovered if desirable.
q Straight chain (Z-N) polyethylene is a somewhat more rigid and higher melting polymer that the branched chain polyethylene generated by radical polymerization.
q The former is often called high density polyethylene (HDPE) and the latter low density polyethylene (LDPE). The unbranched chains fit much more snugly into the solid state lattice, thus requiring a higher temperature to soften the polymer.
q The uses of these two types of polymers are therefore quite different. About 50% of the polyethylene made in the U.S. is of each type.
Other Uses of Ziegler-Natta Polymerization.
Polymer Stereochemistry. Besides providing the only route to unbranched polyethylene, the invention of Z-N polymerization has provided solutions to other challenges in polymer chemistry. One of these lies in the area of polymer stereochemistry.
q In the polymerization of vinyl monomers, there exists a stereochemical issue which we have not previously discussed. The question relates to the relative stereochemistries at the secondary carbon atoms of the polymer. It is essentially an issue of diastereoisomerism.
q The question boils down to whether the substituents, such as the methyl groups used in the illustration below of polypropylene, are on the same side of the polymer chain (shown in the structure on the left) or on alternating sides of the chain (shown on the right.
q The left hand structure is called isotactic and the right hand one syndiotactic. Both of these are stereoregular polymers, in that the relationships between the various methyl groups are consistent—all on the same side or all alternating.
q Since stereoregular polymers have consistent, repeating structures, they fit more snugly in to a polymer lattice and have higher melting points and greater strength. They are usually more highly desirable than stereorandom polymers.
q Radical and other polymerization methods typically do not furnish stereoregular polymers, i.e., some of the substitutents are on the same side and some alternate. These polymers are stereorandom.
q Z-N polymerization was the first polymerization method to provide stereoregular polymers. Both isotactic and syndiotactic polypropylene have been made using Z-N catalysis. [Depends upon the specific catalyst]
Conjugate Addition to Dienes to Give Elastomeric Polymers (Including Rubber).
One of the challenges of polymer chemistry was to synthesize polymers having structures identical to (and analogous to) that of natural rubber.
q Natural rubber is essentially an addition polymer of isoprene (2-methyl-1,3-butadiene). [Although, in nature, rubber is not synthesized from isoprene]
q But there are two major challenges in polymerizing isoprene to a rubber structure. The rubber structure has a cis or Z double bond, which is the less thermodynamically stable isomer.
q The second challenge is that the rubber structure corresponds to 100% 1,4 or conjugate addition to the diene (addition to the diene termini). If you recall, additions to dienes typically give rise to a mixture of 1,2 and 1,4 additions as a result of reaction so an intermediate allylic species at either one of the two active allylic positions.
q Z-N catalysis elegantly solves both of these problems and provides a structure identical to that of naturally occurring rubber.
q The mechanism for the Z-N polymerization of the simplest diene, 1,3-butadiene is illustrated below:
Polymer Repeat Structure:
q The initially formed coordination complex is formed from the predominant s-trans conformer of the diene. However, rotation around the central C-C bond of the diene is possible, and this gives an even more stable coordination complex because both pi bonds of the diene are able to complex with the titanium atom in the complex of the s-cis-diene.
q When the addition occurs in the conjugate manner, the C=C is generated in the cis configuration. The 1,4 addition mode may be favored because of the six-membered ring TS.
All of the polymerizations (except for the Z-N method) discussed thus far have been chain processes. The growth of the polymer chain is therefore referred to as chain growth. In many other polymerizations a chain process is not involved, but many stable intermediates are formed along the way in stepwise fashion. These are called step growth polymerizations. A particularly important and familiar example of a step growth polymer is that of Nylon.
q Nylon is a copolymer of a dicarboxylic acid (most commonly adipic acid, i.e., hexanedioic acid) and a diamine, commonly 1,6-hexanediamine.
q The first step in Nylon preparation is simply mixing the reagents, which react (as amins have been shown to react with carboxylic acids) to form a salt, sometimes called “Nylon salt”.
q This salt is then heated to form amide bonds between the carboxyl groups and the amine functions (which are formed by reversal of the nylon salt-forming reaction.
q An illustration of the structure of the initial amide formed form one molecule of adipic acid and one of 1,6-hexanediamine is given below:
q Now the importance of having difunctional comonomers appears. When they have been coupled via the formation of one amide bond, there still remains a difunctional molecule which has a carboxylic acid function on one end and an amine function on the other. This “dimer” can then react at either, and eventually both, ends to build up the polymer chain.
q It can do that by reacting at either end first. Below, we have illustrated the “trimer” product which would be obtained by reacting the amino end of the “dimer” (which is the growing polymer chain) with a molecule of adipic acid.
q We can see from the nature of the monomers used why this particular Nylon polymer is called Nylon 66. Of course, other dicarboxylic acids and/or other diamines could be used, but these are two of the most readily available monomers.
q We can also see from the mechanism, which proceeds from monomers to dimer to trimer to tetramer, etc, that this is a step growth polymerization.
Test for Chain Growth vs. Step Growth Polymerization.
Experimentally, the nature of a polymerization, whether step or chain growth can be easily ascertained when we remember the mechanisms of these reactions.
q In a step growth polymerization, no polymer of very high molecular weight (high polymer) is formed until very late in the process, because all of the material has to be brought up in a stepwise fashion through dimers, trimers, tetramers, etc, etc. However in a chain process, high polymer is formed very quickly, because the reactive intermediate proceed all the way to the final polymer product before stopping.
q Therefore, if we stop the polymerization after short reaction times (or better, low monomer conversions, such as 5-10%; meaning when only 5-10% of the monomer has been consumed and 90-95% remains), we can examine whether there is or is not any high polymer formed. It is not so much a matter of reaction time, but of the fact that the conversion is low.
q If the monomer consumed at low conversion is all converted to high polymer, this is a chain growth process.
q If only dimers, trimers, or low oligomers are formed, this is a step growth process.
Nylon is formed using difunctional comonomers which form amide linkages. In a similar way, we can combine a dicarboxylic acid and a diol to give a polyester.
q Dacron is such a polyester, formed from terephthalic acid (1,4-benzenedioic acid) and ethylene glycol (1,2-ethanediol).
q In practice is is somewhat more convenient to use a transesterication reaction that an esterification reaction. That is, instead of terephthalic acid, one uses dimethyl terephthalate and ethylene glycol. In this case, the condensation product is not water but methanol, which has a lower boiling point and is more readily removed, allowing the equilibrium to be pushed toward the product polymer. A transesterification reaction is a reaction in which one ester function is converted into another ester function.