Poly(Lactic Acid)-Based Graft Copolymers: Syntheses Strategies and Improvement of Properties for Biomedical and Environmentally Friendly Applications: A Review

As a potential replacement for petroleum-based plastics, biodegradable bio-based polymers such as poly(lactic acid) (PLA) have received much attention in recent years. PLA is a biodegradable polymer with major applications in packaging and medicine. Unfortunately, PLA is less flexible and has less impact resistance than petroleum-based plastics. To improve the mechanical properties of PLA, PLA-based blends are very often used, but the outcome does not meet expectations because of the non-compatibility of the polymer blends. From a chemical point of view, the use of graft copolymers as a compatibilizer with a PLA backbone bearing side chains is an interesting option for improving the compatibility of these blends, which remains challenging. This review article reports on the various graft copolymers based on a PLA backbone and their syntheses following two chemical strategies: the synthesis and polymerization of modified lactide or direct chemical post-polymerization modification of PLA. The main applications of these PLA graft copolymers in the environmental and biomedical fields are presented.     

Amphiphilic block copolymers by a combination of anionic polymerization and selective post-polymerization functionalization

Anionic polymerization is the oldest known living/controlled polymerization methodology that leads to well defined macromolecules. It has been also used, with considerable success, for the synthesis of amphiphilic block copolymers (AmBC), a class of functional copolymers having interesting self-assembling properties and high potential for applications in various technological fields. The use of mild and effective post-polymerization functionalization/chemical modification reactions on block copolymers has substantially increased the synthetic capabilities of anionic polymerization methodologies, toward the creation of a variety of AmBC. In this feature article we review work done on these directions in the last ten years. Some perspectives and future work on this particular field of polymer science are also discussed.     

Some Recent Advances in Pure and Applied Aspects of Graft Copolymers

In recent years more attention has been given to the preparation and characterization of graft copolymers and an examination of their properties than to developing novel methods of synthesis. In particular, these property studies can be used to develop applications of graft copolymers. The methods used to prepare and characterize better defined grafts are briefly reviewed. The structure of most graft copolymers is such that considerable amounts of one polymer can be grafted to another without affecting greatly the main properties of that polymer. In this way, the properties of one polymer such as higher water absorption can be imparted to another without changing the mechanical properties, for example, of the second polymer.

Some other inherent properties of graft copolymers are discussed, including their compatibility with the parent homopolymers, their possible con-formational changes, and other features. Finally, the use of the special nature of graft copolymers for the modification of the barrier properties of film and membranes is discussed as one possible general field of application.     

Graft copolymers of poly(ethylene glycol) (PEG) and chitosan

Graft copolymers of poly(ethylene glycol) (PEG) on a chitosan backbone (PEG-g-chitosan) have been synthesized and their aqueous solution properties were investigated. At pH 6.5 the graft copolymers are 100% soluble, while chitosan phase separates from solution at those conditions. These interesting graft copolymers may be especially suitable as carriers for delivery of anionic drugs, such as proteins, glycosaminoglycans, and DNA plasmids or oligonucleotides.     

Relationships of graft polymerization of vinyl monomers onto cellulose. Reaction kinetics and polymer structure

Graft polymerization of acrylamide and sulfuric salt of 2-methyl-5 vinylpyridine onto cellulose using CO(III) salts was studied. It was shown that graft polymerisation of water-soluble polymers can be described by general relationships of radical reaction. The relation between elementary reaction constants of the formation and termination of active centers in cellulose, as well as relation between constants of the propagation, transfer and termination of chains were determined for different cellulose materials and monomers. The translational diffusion, sedimentation, viscosity, and flow birefringence of copolymers have been investigated in different solvents. Experimental data showed that the copolymers obtained are graft copolymers with two to five grafted chains onto macromolecules. The minimum distance between the branch points is 100 ± 20 glucoside units. The peculiarity of these copolymers is relatively low polydispersity. The conformation of macromolecules of such copolymers in solution depends on the compatibility of the copolymer components and the thermodynamic properties of the solvent.     

Grafting of Vinyl Monomers onto Nylon

Polymer chemists have been successful in applying polymerization techniques to develop copolymers of natural and synthetic macromolecules [l]. The literature abounds with examples of the successful formation of copolymers from natural and synthetic macromolecules [2–5]. Copolymerization is attractive to chemists as a means of modifying macromolecules since, in general, degradation can be minimized. Despite the heterogeneity and complexity of these copolymers, much has been achieved in their characterization. The desirable properties of the polymer are retained and additional properties are acquired through the added polymer. The desired material may be formed in situ by polymerization of a monomer or monomers, by condensation of reactants, or by the decomposition of a preformed polymer.     

GRAFT COPOLYMERIZATION OF CELLULOSE,CELLULOSE DERIVATIVES,AND LIGNOCELLULOSE

Abstract

The growth of polymer science has led to the development of new materials in direct competition with natural materials, many of which have been in use since earliest times. This has caused researchers to look more critically at both natural and synthetic macromolecules in order to learn more about their underlying structures and their relation to the properties exhibited by the macromolecules. In this regard, chemical modifications have been devised to impart certain desirable properties of both natural and synthetic macromolecules, and their applications have become an integral part of such chemical modifications. Various chemical modifications (e.g., change of functionality, oxidative degradation, inter- and intramolecular gelation, graft copolymerization), have been practiced to add improved properties to the base polymers. However, among all these methods, modification of polymers via graft copolymerization has been the subject of much interest and has made paramount contribution toward improved industrial and biomedical applications.     

Polyfunctional polyisobutenes as building blocks for amphiphilic graft polymers

Polyfunctional polyisobutenes (PIB) have been synthesized by cationic copolymerization of isobutene and chloromethylstyrene. Their potential applications with focus on their use as macroinitiators (MI) for oxazoline polymerization were discussed. The “grafting from” reaction led to tailor-made graft copolymers with various backbones and adjustable graft arm length. Graft copolymers with hydrophobic PIB backbone and poly(2-methyloxazoline) graft arms have unusual viscosity properties due to their amphiphilic character and show aggregate formation.     

Cyclic Polymer Grafts That Lubricate and Protect Damaged Cartilage

Tissue‐reactive graft copolymers were designed to protect the cartilage against enzymatic degradation and restore its lubrication properties during the early stages of osteoarthritis (OA). The copolymers feature a poly(glutamic acid) (PGA) backbone bearing hydroxybenzaldehyde (HBA) functions and cyclic poly(2‐methyl‐2‐oxazoline) (PMOXA) side chains. PGA‐PMOXA‐HBA species chemisorb on the degraded tissue via Schiff bases and expose the biopassive and lubricious PMOXA cyclic grafts at the interface. The smaller hydrodynamic radius by cyclic PMOXA side chains coupled to the intrinsic absence of chain ends generate denser and more lubricious films on cartilage when compared to those produced by copolymers bearing linear PMOXA. Topology effects demonstrate how the introduction of cyclic polymers within tissue‐reactive copolymers substantially improve their tribological and biopassive properties, suggesting a plethora of possible applications for cyclic macromolecules in biomaterials formulations.     

Polyferrocenylsilanes: Metal‐Containing Polymers for Materials Science,Self‐Assembly and Nanostructure Applications

Polyferrocenylsilanes represent a recently established, readily accessible class of transition metal‐containing macromolecules consisting of alternating ferrocene and organosilane units. High molecular weight, soluble samples of these materials were first prepared in the early 1990's by thermal ring‐opening polymerization (ROP) of silicon‐bridged [1]ferrocenophanes ([1]silaferrocenophanes). More recently living anionic and transition metal‐catalyzed ROP methodologies have been developed, which permit unprecedented access to controlled polymer architectures (e.g. block copolymers) with transition metals in the main chain. Polyferrocenylsilane homopolymers and block copolymers offer exciting opportunities in materials and supramolecular science and for nanostructure applications. In this article some recent research areas are discussed which illustrate the broad scope of these interesting new materials.     

Advanced polymers for electrooptics

Polymers have been in use for a long time as passive materials for components in electronics and electrooptics. They combine the possibility of easy processing with an infinite potential of functionalization. They can be conductors or semiconductors, ferroelectrics and can exhibit interesting properties such as photoconductivity, piezo, pyroelectricity or nonlinear optical properties. As a consequence they are now used for merging active components in the field of electrooptics such as displays, sensors or modulators for optical signal treatment. This paper describes the properties and applications of some recent polymers in the field of electroptics. In the first part of this paper the properties of amorphous copolymers with a pending group with large hyperpolarizability are described. These amorphous copolymers show high optical nonlinear coefficients after poling under an electric field. We have used these copolymers for the manufacture of an electrooptic modulator working at 1.3 μm at a frequency of 1 GHz. The second part deals with ferroelectric polymers and their pyroelectric properties. The realization and performance of an IR pyroelectric sensor using copolymers of polyvinylidene fluoride–ethylene trifluride are described. Some new results concerning polymersdispersed liquid crystals are also described.     

Bioinspired mineralization of inorganics from aqueous media controlled by synthetic polymers

The formation of inorganic structures in nature is commonly controlled by biogenic macromolecules. The understanding of mineralization phenomena and the nucleation and growth mechanisms involved is still a challenge in science but also of great industrial interest. This article focuses on the formation and mineralization of two archetypical inorganic materials: zinc oxide and amorphous calcium carbonate (ACC). Zinc oxide is selected as a model compound to investigate the role that polymers play in mineralization. Most of the effort has been devoted to the investigation of the effects of double-hydrophilic block and graft copolymers. Recent work has demonstrated that latex particles synthesized by miniemulsion polymerization, properly functionalized by various chemical groups, have similar effects to conventional block copolymers and are excellently suited for morphology control of ZnO crystals. Latex particles might serve as analogues of natural proteins in biomineralization. The second example presented, ACC, addresses the issue of whether this amorphous phase is an intermediate in the biomineralization of calcite, vaterite, or aragonite. Conditions under which amorphous calcium carbonate can be obtained as nanometer-sized spheres as a consequence of a liquid-liquid phase segregation are presented. Addition of specific block copolymers allows control of the particle size from the micrometer to the submicrometer length scale. The physical properties of novel materials synthesized from concentrated solution and their potential applications as a filler of polymers are also discussed.     

THERMALLY STIMULATED SHAPE MEMORY BEHAVIOR OF POLYMERS WITH PHYSICAL CROSSLINKS*

The shape memory effect of polymers was investigated for the purpose of improving the processingconditions of their preparation and broadening the list of polymers for shape memory applications. Emphasiswas put on the possibility of using polymers with physical crosslinks as shape memory materials and theirstructure-function relationships. Segmented block polyurethanes and polyethylene/nylon 6 graft copolymerswere used as examples of polymers with physical crosslinks. It was found that these copolymers can really beused as thermally stimulated shape memory materials with large recoverable strain and high final recoveryrate. The main advantage of using copolymers is their improved processing conditions as compared withpolymers with chemical crosslinks. As only physical crosslinks are introduced, all conventional processingtechniques for thermal plastics can be used, and the materials become easily reusable. The results indicatethat the high crystallinity of these copolymers at room temperature and the formation of stable physicalcrosslinks are the two prerequisites for these polymers to exhibit shape memory effect. The successful use ofblock and graft copolymers imply the possibility of using polymers of various structure and properties asshape memory materials.     

Modular Toolkit of Multifunctional Block Copoly(2-oxazoline)s for the Synthesis of Nanoparticles

Post-polymerization modification provides an elegant way to introduce chemical functionalities onto macromolecules to produce tailor-made materials with superior properties. This concept was adapted to well-defined block copolymers of the poly(2-oxazoline) family and demonstrated the large potential of these macromolecules as universal toolkit for numerous applications. Triblock copolymers with separated water-soluble, alkyne- and alkene-containing segments were synthesized and orthogonally modified with various low-molecular weight functional molecules by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and thiol-ene (TE) click reactions, respectively. Representative toolkit polymers were used for the synthesis of gold, iron oxide and silica nanoparticles.     

SPECIAL PP'S FOR A DEVELOPING AND FUTURE MARKET

The present article starts with a short overview of the international development of the polyolefin production in the last 10 years and with a forecast for the next 3 to 5 years. The reason for these developments, as the cost/properties relationships will be discussed. The extended properties of the polypropylenes, their molecular and supermolecular structures, and the correlations to the properties play a central role in the paper. The reinforcing and compounding maximized the properties and reduced the weaknesses of the mechanical and thermal properties. The narrower molecular weight distribution and the possible new copolymers with better randomness and new comonomers explain the importance of the metallocene catalysts. Examples demonstrate the future developments of new PP's by orientation on a second generation of single site catalysts.

The super soft PIP's as a new interesting elastomer class, are described. The new developments from the post reactor modification to a new HMS-PP technology are described in detail. The new graft copolymers with styrene and methylmethacrylate are pointed.     

Properties of side chain liquid crystal and amorphous polymers. Applications to non-linear optics

Comb-like liquid-crystalline polymers exhibit many unique properties that challenge not only basic research but also numerous technological opportunities. They combine (partly) the properties of orientation of low molecular weight liquid crystals with the rigidity of polymers. For example, they can be oriented in the mesomorphic state and the structure frozen in a glassy state. These polymers with functionalized pendent groups lead to potential applications in the field of nonlinear optics, or in the domain of electro-optical displays. Other polymers like polysilanes show interesting properties such as photo-conductivity. This paper describes the properties and applications of some new side chain liquid-crystalline polyacrylates and their amorphous copolymers. It also describes the photo-conductive properties of polysilanes and their applications in spatial light modulators with liquid crystals. In the first part of this paper, we describe the properties of liquid crystal copolymers and amorphous polyacrylate copolymers with cyanobiphenyls and/or pendent groups with a large hyperpolarizability. Their different properties are compared with some recent results from the literature. These amorphous copolymers allow one to obtain, after poling in an electric field, high optical non-linear coefficients. We have used these copolymers for the manufacture of an electro-optic modulator working at 1·3 μm in the frequency range of 1 GHz. Applications to second harmonic generation at 1·06 μm are also discussed. In the second part of this paper we describe the photo-conductive properties of polysilanes and the realization and performance of an organic spatial light modulator for optical correlation.     

Copolymers: efficient carriers for intelligent nanoparticulate drug targeting and gene therapy

Copolymers are among the most promising substances used in the preparation of drug/gene delivery systems. Different categories of copolymers, including block copolymers, graft copolymers, star copolymers and crosslinked copolymers, are of interest in drug delivery. A variety of nanostructures, including polymeric micelles, polymersomes and hydrogels, have been prepared from copolymers and tested successfully for their drug delivery potential. The most recent area of interest in this field is smart nanostructures, which benefit from the stimuli-responsive properties of copolymeric moieties to achieve novel targeted drug delivery systems. Different copolymer applications in drug/gene delivery using nanotechnology-based approaches with particular emphasis on smart nanoparticles are reviewed.     

Polymers with tertiary amine groups for drug delivery and bioimaging

Over the past decade, biopolymers have gained great interests especially in biomedicine due to their physical properties and/or chemical structures changes in response to external stimuli in a certain time frame or at a specific location. Among them,poly(β-amino ester)s, methacrylate-based block copolymers and polypeptide with tertiary amine groups have been extensively studied and exhibit pH sensitive properties due to the protonation of tertiary amine groups. The pH values in normal organs,tissues, and subcellular compartments are always different from those in pathological tissues. These interesting properties allow their applications in a variety of fields ranging from diagnosis and therapeutics of diseases. Here, we review the recent progress of poly(β-amino ester)s, methacrylate-based block copolymers and polypeptide with tertiary amine groups and their applications in drug delivery and bioimaging.     

聚二茂铁基硅烷二嵌段共聚物的制备、组装及应用研究

二茂铁分子由于含有独特的芳香结构和过渡金属元素铁,具有特殊的光、电、磁特性,一直以来是科学研究和技术应用的热点之一,在功能高分子材料的制备方面具有广泛的应用前景。聚二茂铁硅烷嵌段共聚物是一类新型的主链含有二茂铁和有机硅单元的聚合物,以其可控的分子量和丰富的自组装形貌,不断得到科研人员的关注。本文主要从聚合、自组装和应用三个方面系统介绍了聚二茂铁硅烷二嵌段共聚物。其中重点介绍了应用广泛的活性阴离子聚合和进一步发展起来的阴离子两步法聚合,解释了活性聚合机理,归纳了目前成功合成的不同种类的二嵌段共聚物;论述了在选择性溶剂中,聚二茂铁硅烷二嵌段共聚物可以自组装形成柱状、管状、球状等常规胶束和片层、刷形等复杂胶束,胶束的形成同聚合一样具有活性特征,并且在一定的刺激条件下,不同类型的胶束可完成相互转变过程,另外可以对胶束局部进行功能化以赋予其特殊的性能;最后介绍了聚二茂铁硅烷二嵌段共聚物在药物缓释、纳米材料、特种催化方面的具体实例,并对其应用前景加以展望。     

The role of miktoarm star copolymers in drug delivery systems

Abstract

Miktoarm star copolymers are relatively considered to be a new and unique class of macromolecules, and are a new topical area due to the unique properties by varying their polymer arms. This macromolecules with the A_mB_n architecture, have m arms of polymer A and n arms of polymer B connected at one central junction point. Over the past decade, miktoarms have been used in biomedical applications such as drug delivery, gene delivery, tissue engineering, diagnosis, and antibacterial/antifouling biomaterials. The intensified interest in miktoarms is attributed to their unique topological structures and attractive physical/chemical properties, including low critical micelle concentration (CMC) in solutions, encapsulation capability, internal and peripheral functionality, and enhanced stimuli-responsiveness. This review outlines the advances in the use of miktoarms in drug delivery for their good performance in biocompatibility, biodegradability and sustained, controlled and targeted drug delivery during the past decade and some unique self-assembly behaviors of miktoarm star copolymers have been reported.