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Recent trends in macromolecular design.


This review will focus on four main areas of interest in macromolecular design. These areas include: (1) living and controlled polymerization, (2) multiple hydrogen bonding in supramolecular assemblies, (3) nanocomposites, and (4) biomaterials. Anionic polymerization techniques have traditionally been employed in the synthesis of polymeric materials macromolecules with controlled molecular weights and tailored chemical compositions. However, rigorous laboratory requirements and functional group intolerance have limited the potential extension of this technique to a variety of monomer families. Recently, controlled free radical techniques have received significant attention and offer an alternative method to producing well-defined polymer products. Stable free radical polymerization (SFRP) is one methodology that was utilized in the synthesis of controlled macromolecules with a variety of monomer compositions. Multiple hydrogen bonding interactions, when used in conjunction with living polymerization, provides the ability to tailor polymer architectures, as well as mechanical and physical properties. The ability to control both polymer structure and functionality enables the synthesis of well-defined macromolecules that may be used in a variety of advanced applications, such as nanocomposites and biomaterials. Polymer properties can be tailored for specific applications, such as clay exfoliation and biomedical devices. As the synergy of these macromolecular design strategies become better understood, the number and complexity of potential applications will continue to grow.


An increasingly important area of polymer science is the preparation of macromolecules with controlled and functional architecture. The preparation of controlled architectures has traditionally been achieved using living polymerization pathways such as anionic, cationic, and group-transfer polymerization. Recent years have seen new developments in free radical chemistry that permit the synthesis of these controlled and functional architectures. The following section will highlight select developments in the synthesis of macromolecules with controlled architecture via both living and free radical polymerization methodologies.

A living polymerization is described as a chain polymerization in which the kinetic steps of termination and chain transfer are absent. (1) Polymerizations that proceed in the absence of termination and chain transfer steps provide the ability to synthesize polymers in a well-controlled fashion. Living polymerization offers control of many fundamental polymer variables that ultimately affect the final polymer properties. Experimental criteria have been proposed and used as diagnostic characteristics for living polymerizations. (1) Living polymerizations proceed until all monomer has been consumed and subsequent addition of monomer results in continued polymerization. Addition of a second monomer results in block copolymer formation. In living polymerizations, the number average molecular weight is directly proportional to conversion. The molecular weight can be controlled by the stoichiometry of the reaction and the number of active chain ends is constant throughout the reaction. Rapid initiation relative to propagation results in narrow molecular weight distribution polymers. The presence of the living chain end at the end of the polymerization also provides the ability to prepare chain-end functionalized polymers in a quantitative yield. (1)

In 1929, Ziegler described the polymerization of butadiene using n-butyllithium. Ziegler described a mechanism that consisted of both initiation and propagation, but mentioned that termination and transfer steps did not play a large role. (1) This was the first anionic polymerization system. However, this work was not pursued at the time and focus shifted to non-living polymerization of butadiene. Firestone also reported the use of an insoluble lithium metal in the polymerization of isoprene in 1955 which resulted in poly(isoprene) properties that were similar to natural rubber. (2) In 1956, Szwarc coined the term "living" for these types of polymerizations. (3) In this seminal work, Szwarc described the nature of the living anionic system, in addition to recognizing the usefulness of the living anionic chain end in the synthesis of more complex architectures. Since that time, anionic polymerizations subsequently have been used extensively for the production of polydienes. Isoprene and butadiene constitute th e majority of the monomers utilized commercially and have been the subject of an extensive body of research. These acyclic dienes have been polymerized via a multitude of different reaction conditions. These conditions include the use of various initiators and the incorporation of numerous additives in an attempt to control the polymer stereochemistry, degree of branching, degree of termination, and functionaliztion (4,5). Living anionic polymerization has been used to polymerize various other monomers, including alkyl acrylates (6), alkyl methacrylates, (7) and cyclic dienes (8,9).

While living polymerization mechanisms offer advantages that include the ability to control molecular weight, molecular weight distribution, architecture (such as well-defined block copolymers), and end group functionalization (10), they also typically require stringent reaction conditions such as the complete absence of oxygen and water impurities as well as ultra-pure solvents and reactants. In addition, living chain polymerization methods are incompatible with a number of functionalities. (11) As a consequence of these factors, the commercial application of living chain polymerization has been limited to a few systems. A desirable alternative has been the development of free radical polymerization chemistry that allows for the production of controlled polymer structures similar to what can be achieved using living polymerization techniques. However, a recognized disadvantage of free radical polymerization is the apparent lack of control over polymer features such as molecular weight, molecular weight distr ibution, and topology. A limited degree of control and functionality can be introduced using free radical polymerization via the addition of functionalized initiators or chain-transfer agents. (12) Recent developments in controlled free radical polymerization methods that include stable free radical polymerization (SFRP), (13) atom transfer radical polymerization (ATRP), (14) and radical addition-fragmentation transfer (RAFT)11 have significantly enhanced the ability to produce well-defined polymer architectures using free radical methods.

Otsu and coworkers first extended the idea of living polymerizations to free radical systems. They reported the use of initiator-transfer-agent-terminators, or iniferters, to reduce irreversible chain termination in free radical polymerization processes. (15,16) The reversible radical termination step helped to control irreversible chain termination and resulted in polymerization behavior that resulted in a linear molecular weight increase with time similar to living anionic polymerizations. However, the iniferter mechanism resulted in polydispersities that were inferior compared to those that could be achieved via anionic polymerization. These broad polydispersities resulted from radicals that would initiate new chains throughout the reaction as well as a significant loss of active end groups resulting in chain-chain coupling termination. (17,18)

Following the earlier work of Rizzardo and coworkers in nitroxide mediated stable free radical polymerization of methyl acrylate, (19) Georges et al. first reported the preparation of polystyrene with low polydispersity using bulk free radical polymerization of styrene initiated by a conventional free radical initiator, benzoyl peroxide (BPO), in the presence of the stable nitroxide free radical, 2,2,6,6-tetramethyl- 1 -piperidinyloxy (TEMPO) at 125[degrees]C. (20) The SFRP process involves a desirable reversible equilibrium between nitroxide capped polymer chains and uncapped polymer radicals. The uncapped polymer radicals are then able to chain extend via styrene monomer addition, see Figure 1.

The success of this method arises from the unique feature that the nitroxide radicals will react with carbon radicals at near diffusion controlled rates, but will not react with other oxygen centered radicals or initiate additional polymer chains. Monomers such as styrene, (20-23) acrylates, (24) dienes, (24) and 2-vinylnaphthalene (25) have been polymerized using SFRP. Polystyrene star polymers have also been synthesized using SFRP methodology. (26) Several studies have also been performed to optimize the SFRP procedure, including investigations into the nitroxide/initiator ratio, (27,28) kinetics of the growing and dormant chains, (29) rate enhancers, (30) and chain end functionalization. (21)

ATRP was first reported by Matyjaszewski in 1995. (14) ATRP was developed as an extension of atom transfer radical addition (ATRA). ATRP extends the concept of ATRA to polymers by subjecting the propagating species to a large excess of monomer so that subsequent additions may take place. Termination reactions are prevented in ATRP by the presence of a metal complexing agent, such as CuBr, which reversibly caps the growing radical chain. (14) ATRP has been used to polymerize a variety of monomers, such as styrene, (14,31,32) alkyl acrylates, (32-34) alkyl methacry-lates, (32,33) dienes, (35) and acrylonitrile. (36)

RAFT polymerizations, as first described by Thang (11), utilize a dithioester that acts as the chain capping agent. RAFT offers advantages over other controlled radical polymerizations due to insensitivity to acid, hydroxy, and tertiary amino groups. (37) RAFT has been used to synthesize relatively narrow molecular weight distribution polymers of acrylates and methacrylates, (11,37,38) ethylene oxide, (11,37) acrylamides and methacry-lamides, (39,40) and styrene. (11,37)


The conventional synthesis of macromolecules involves both step-growth and chain polymerization to form repeat units joined by irreversible stable covalent bonds. High molecular weights of these irreversibly connected systems are desired to optimize physical properties and commercial utility. However, as molecular weight increases, the polymer melt viscosity increases as the 3.4 power of the weight average molecular weight. (41) Increased melt viscosity compromises potential solvent-free manufacturing, melt processability, thermal stability, and solubility of the final products. Although high molecular weighty is necessary to obtain desired physical properties, the melt viscosity becomes increasingly prohibitive as molecular weight increases. Thermoreversible polymers present a possible solution to this dilemma. Ideally, at typical use temperatures the reversible interaction would remain stable, but at melt processing temperatures the interaction would become unstable, causing the polymer to dissociate into l ow molecular weight oligomers. This idealized macromolecule would have both the desired physical properties derived from high molecular weight polymers and low melt viscosity during processing and manufacturing. Several systems are currently under study as possible alternatives to the current irreversible synthetic routes, including hydrogen bonding, ionic interactions, as well as systems triggered using light and pH.

Multiple hydrogen bonding in molecular design has recently received significant attention due to the propensity to form new supramolecular structures that exhibit thermoreversible characteristics. In addition, reversible molecular recognition is also attainable due to relatively weak non-covalent bonds compared to covalent bonds. (42-47) Multiple hydrogen bonding scaffolds are classified into two different families: (1) self-complementary multiple hydrogen bonds (SCMHB) which are formed between identical hydrogen bonding units and (2) complementary multiple hydrogen bonds (CMHB) which are formed between dissimilar hydrogen bonding units containing complementary donor and acceptor sites. Meijer et al. recently reported the versatility of SCMHB between 2-ureido-4 [1H]-pyrimidone (UPy) units, which strongly self-dimerize through four hydrogen bonds arranged in donor-donor-acceptor-acceptor sites, see Figure 2.48-56 The aggregated molecules containing SCMHB units exhibited polymer-like properties including shear thinning in the melt phase, viscoelastic behavior in the solid state, and glass transition temperatures.

Supramolecular structures possessing CMHB units have also received significant attention in the literature.57-62 In particular, heterocyclic base pairing between adenine, guanine, thymine, uracil and tyrosine in DNA and RNA are well-known as tailored CMHB pairs in biological systems.

Recent research efforts in this rapidly evolving area have focused on the design of supramolecular structures consisting of tailored CMHB with extremely high dimerization constants. Initial supramolecular systems based on CMHB were reported by Lehn, (63-68) Kato and Frechet, (69-74) and Whitesides. (75-78) These diverse efforts focused on the selective formation of discrete supermolecules from complementary receptor-substrate hydrogen bonding pairs, such as bartituric acid and tinaminopyrimidine units, 2,6-diaminopyridine and 2,6-pyridinedicarbonyl units, and carboxylic acid and pyridine units.

Other recent research efforts have focused on the introduction of SCMHB or CMHB with relatively high dimerization constants into macromolecules to improve mechanical properties. Coates et al. reported the incorporation of SCMHB containing UPy units into elastomeric nonpolar poly(1-hexene) and their reversible nature.79 Strong dimerization was observed in nonpolar solvents. Chino et al. prepared poly(isoprene) thermoplastic elastomers via derivatization of a maleic anhydride modified poly(isoprene) with 3-amino-1,2,4-triazole. (80) Interestingly, the SCMHB networks exhibited similar mechanical properties to vulcanized rubber and thermoreversible dissociation occurred at approximately 1 850C. Our earlier research efforts have involved the synthesis of well-defined SCMHB polymers, such as glassy poly(styrene) (PS), rubbery poly(isoprene) (P1) and microphase separated PS-b-PJ block copolymers with well-defined molecular weights and narrow molecular weight distributions.81-84 The relationship between end group str ucture and physical properties, such as the glass transition temperature, melt viscosity and morphology were reported. (81,82) In addition, the synthesis and thermoreversible nature of poly(alkyl acrylates) comprising pendant SCMHB units were also reported. (83,84)

Macromolecules containing CMHB units based on nucleic acid heterocyclic bases have also received significant attention.85-88 Inaki et al. reported the synthesis and characterization of graft copolymers containing nucleic acid heterocyclic bases and L-amino acids on poly(ethylene imine). (85) The synthesis of poly(alkyl methacrylate)s containing nucleic acid bases and subsequent photochemical processes were reported. (86) Kalra et al. have described the synthesis and characterization of block copolymers based on poly(ethylene glycol) (PEG) and a nucleoside pendant alkyl methacrylate monomer, which was prepared via selective acylation of the nucleoside. (87) Most CMHB containing polymers that were obtained via a combination of nucleic acid bases and synthetic polymers contain hydrophilic units, such as sugars, poly(ethylene imine), PEG, or poly(peptides) due to limited selective synthetic methodologies and limited solubility of nucleic acid bases and resulting polymers. Thus, the introduction of CMHB based on n ucleic acid derived heterocyclic bases into non-polar polymers is particularly interesting because the nonpolar polymer does not interfere with these tailored, directional, hydrogen bonds.


In order to improve mechanical properties of polymeric materials, fillers may be added as reinforcement agents. (89) These materials are used in a variety of different applications, such as transportation, construction, electronics, and consumer products. Most commonly, glass or carbon fibrous fillers are used in a randomly dispersed state. (89) Although these fillers increase the mechanical properties of the parent polymer, there are negative affects as well, including decreased ductileness, poor moldability, and poor surface smoothness. (89) These conventional composites are also not processable as films or fibers. These deleterious side effects are generally attributed to the difference in dimensions between the filler (~micron) and the polymer (~ nanometer). The size difference between the two components results in relatively weak interactions. In order to minimize the size difference and improve the interactions between the components, a new type of composite has been proposed, in which the different com ponents interact at the nanometer level.

These nanocomposites can be comprised of polymer matrix with a clay interdispersion. There are a variety of different structures possible for the polymer-clay nanocomposites. At one end of the spectrum is the well-ordered and stacked multilayers that result from intercalated polymer chains within the silicate clay layers, see Figure] 3a. At the other end are the exfoliated materials in which the clay layers have lost their long-range order and are well-dispersed within the polymer matrix, see Figure 3c. (90)

There are a variety of intermediate structures possible between these two extreme cases. One example as shown in Figure 3b, is where small stacks of polymer intercalated clay layers are dispersed within a polymeric matrix. (90) Exfoliated, or delaminated, nanocomposites have been shown to display acceptable stiffness, strength, and barrier properties with less filler content than conventional composites. (91) Nanocomposites also show increased chemical resistance, increased dimensional stability, and reduced swelling by solvents. (92) The improved performance properties of the nanocomposite materials are due to both the improved interfacial properties and a unique phase morphology. Generally, the larger the extent of exfoliation in the nanocomposite, the greater the enhancement of these properties. (90) This may be attributed to the greater phase homogeneity in the exfoliated systems, as well as the fact that each nanolayer in the exfoliated nanocomposite is able to contribute fully to the interfacial interac tions in the matrix. (92)

In order to effect adequate mixing between the polymer matrix and clay layers to achieve these properties, it is necessary to choose an appropriate clay. Two types of layered silicates are generally used for polymer-clay nanocomposites, smectites and layered silicic acids. Layered materials are well suited to nanocomposite synthesis due to their lamellar structure, which leads to high inplane strength, stiffness, and a high aspect ratio. (92) Smectites, such as montmorillonite, are favored for two reasons. First, they exhibit a rich intercalation chemistry, which allows for chemical modification to facilitate compatiblization with organic polymers, allowing for their dispersion on a nanometer scale. Second, they are found abundantly in nature and can be obtained in a mineralogically pure form at a low cost. (92) The structure of a typical smectite clay nanolayer is comprised of an octahedral complex of a metal [[M(OH).sub.6]] sandwiched between two sheets of silica tetrahedra, see Figure 4. (93)

This basic oxide lattice is shared by talc and mica. (92,95) However, defects located primarily in the octahedral layers give the clay platelets a net negative charge. (93) This negative charge is balanced by exchangeable cations located in the interlayer region separating the platelets. (93) This is an important property of the smectite clays because these cations may be exchanged for organophilic molecules, which allows for intercalation of organic molecules into the clay matrix. (93) The smectite family of clays has been most widely studied for polymer intercalation. However the layered silicic acids may also provide a good matrix for polymer intercalation due to similar properties. Other clays have been reported for use in polymer-clay nanocomposites, such as kaolinite and layered double hydroxides. (94) However, smectite based systems remain the most commonly reported.

Three main methods have been used to intercalate polymers into smectite clay nanolayers, including the monomer intercalation method, the common solvent method, and the polymer melt intercalation method. The monomer intercalation method involves first intercalating the monomer into the clay matrix, which is subsequently polymerized. Because the interlayer of the clay is generally hydrophilic, it is difficult to intercalate most polymers without modification reactions within the clay nanolayers. Generally, an ion exchange reaction is carried out that replaces the metal cation, usually sodium, of the clay nanolayer with a molecule that is organophilic, allowing for intercalation of the desired monomer. The monomer is chosen so that it either polymerizes in the presence of the clay nanolayer, or so that it may be initiated using a free radical technique. Polymers intercalated using this method include Nylon 6, (89,96,97) PET, (98) and polystyrene (99). It has been found that addition of the clay at different time s during the PET polymerization results in different physical properties. (91)

In the common solvent method, both the polymer and the clay are suspended in a solvent that will dissolve both well. In order to accomplish dissolution of these dissimilar materials, the clay again must undergo an ion exchange reaction, generally with an ammonium ion containing between 10-12 carbon atoms. (96) Carbon chains longer than this make the organophilic clay too hydrophobic. In solution, the polymer intercalates into the clay nanolayers and upon evaporation of the solvent, a homogeneous nanocomposite is obtained. Nanocomposites made in this way include polypropylene (100) and poly(ethylene oxide). (99)

The polymer melt intercalation method is also a common method for intercalating polymers into clay nanolayers. This method involves heating the polymer/clay mixture above the melting temperature of the polymer to be intercalated. This allows the polymer to flow into the nanolayers of the clay. Again, the clay needs to undergo an ion-exchange reaction before attempting intercalation of the polymer. Nanocomposites that have been made using this method include poly(ethylene oxide), (101) PET, (98) and polystyrene. (99)

Polymer layered silicate nanocomposites were first reported in 1961 by Blumstein. (102) He reported the polymerization of vinyl monomers that had been intercalated into montmorillonite clay. Investigations into polymer-clay nanocomposites were not widely undertaken until the 1990's, at which time many people became interested in them due to their improved properties over conventional composites. Since that time, research into nanocomposites has expanded quickly, both synthetically and theoretically. Although much remains unknown about nanocomposites and why their properties are so improved, progress continues as research expands to explore these new materials.


Biomaterials are defined as non-viable materials that are used in medical devices for the replacement of living materials. This allows them to repair, restore, or replace damaged or diseased tissue or to interact with biological systems. (103) Aliphatic polyesters such as poly(lactide), poly(glycolide) and their copolymers are biomaterials that have been extensively researched since the 1970's. While they have been researched for packaging and agricultural applications, (104-108) they have been most widely used in the biomedical and pharmaceutical industries as controlled drug delivery systems, the most important and versatile application of these polymers,, (109) and surgical fixation devices (i.e., sutures, clips, bone pins and plates). As controlled drug delivery systems, these polymers have been used not only in medicine but also in the veterinary and agrochemical fields where active ingredients from pesticides to contraceptives can be delivered by sustained release with the ultimate biodegradation of the carrier medium. Amass et al. stated that copolymers of DL-lactide (DL-LA) and glycolide (GA) were most useful as drug carrying systems when the copolymer consisted of 20-50% GA with polymer molecular weights of approximately 25,000 g/mol. They also acknowledged that modem techniques allowed the incorporation of drugs that vary in nature and molecular weight. (109) Additionally, block copolymer micelles of poly(DL-lactic acids) (PLA) and poly(ethylene glycol) (PEG) have been extensively researched as drug delivery carriers in injectable systems (110-113) and as protein resistant systems due to their biodegradability, in addition to the bioresorbable PLA segment and water-soluble protein resistant PEG segment. (113) PEG also boasts outstanding physiochemical and biological properties that include solubility not only in water but also in organic solvents, lack of toxicity, absence of antigenicity and immunogenicity, and its filterability through the kidney at molecular weights lower than 10,000 g/mol. (114,115) Several researchers have concentrated on the synthesis of these polymers for galenic formulations (sustained drug release and targeting of tumors). Wang et al. studied the controlled release of etanidazole, a compound that is cytotoxic to tumor cells and can chemosensitize some alkylating agents by activating their tumor cell killing capabilities, encapsulated in sprayed dried poly(DL-lactide-co-glycolide) (PLGA) microspheres and compressed into implantable discs. (116) Polymer degradation was cited as the dominant mechanism in the release of the drug. Once the initial burst of 1% had been achieved for the first day, the collective release in the first week was found to be less than 2%. A second burst release then occurred after one month followed by a very slow release rate towards the final stage. However, the incorporation of smaller molecular weight PEG (MW 3350 g/mol) produced a sustained release for about 2 months. Blends of poly(DL-lactide) (PDLLA) and poly(1,5-dioxepan-2-one) (PDXO) have also been inv estigated for the sustained release of diclofenac sodium, a non-steroidal anti-inflammatory drug. (117-121) According to Holland and Tighe, the use of poly(glycolide), poly(lactide), and polyglycolide-co-lactide (PGLA) as absorbable synthetic sutures can be attributed to the ability of these polymers to produce strong filaments as well as their spontaneous degradation. (122,123) Dexon, a multifilament PGA and Vicyl, a copolymer consisting of 8% PLLA and 92% PGA, have been cited as the most widely used absorbable sutures. There are several advantages of using these and other biodegradable implants as alternatives to metal implants. For instance, using metal implants required the need for "stress protection." Moreover, the sensitivity of patients to metals such as nickel must be taken into consideration, and lastly, a removal operation had to be performed once the bone heals. (124) Other motivations for the widespread usage of these biodegradable aliphatic polyesters and their copolymers in the biomedical and p harmaceutical industries can be attributed to their "in vitro" and in vivo" hydrolytic degradation to non-toxic pro ducts. (125)

Bos et al. (126) researched the use of osteosynthesis plates and screws comprised of bioabsorbable poly(L-lactide) (PLLA) for the treatment of zygomatic bone fractures. The authors reported that the PLLA plates and screws guaranteed stable osteosynthesis of zygomatic bone fractures for an adequate period of time and allowed undisturbed healing of the fracture. Most importantly, the removal operation was avoided since bioabsorption took approximately 18 months. Other biomedical application of poly(lactide), poly(glycolide) and their copolymers include resorbable prostheses, scaffolds (wound dressing, tubular conformations, skin substitutes), (109) galenic formulations127 and the long-term delivery of antimalarial drugs, contraceptives and eye drugs. (116)






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Author:Elkins, Casey L.; Karikari, Afia S.; Lizotte, Jeremy R.; Onah, Ejembi; Pasquale, Anthony J.; William
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Date:Apr 1, 2003
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