Control of polymer microstructure

In the past three decades, industrial polymerization research and development aimed at controlling average polymer properties such as molecular weight averages, melt flow index and copolymer composition. These properties were modeled using either first principle models or empirical models represented by differential equations or statistical model equations. However, recent advances in polymerization chemistry, polymerization catalysis, polymer characterization techniques, and computational tools are making the molecular level design and control of polymer microstructure a reality. 

Clearly, an enormous number of new polymers have and continue to be synthesized using ROMP reactions. The development of the new generation of single-site aUcylidene catalysts has introduced a new level of control over ROMP chemistry. Control of polymer microstructure should, in turn, result in a better understanding of the interplay between microstmcture and macroscopic properties. The use of living ROMP chemistry is still in its infancy. It will be interesting to observe whether or not useful materials can be developed from this chemistry. 

Since the active species are free radicals, it is impossible to entirely suppress bimolecular termination or other mechanisms such as chain transfer. Nonetheless, CRP chemistries allow imprecedented control of polymer microstructure not achievable by conventional FRP. 

The exact control of polymer microstructures (e.g., tacticity and double-bond configuration) resulting from the ROMP of substituted norbomenes and norbomadienes is essential for the development of polymers with well-defined physical properties. Norbomene and norbomadiene-derived polymers could be accessed with... 

Crystal Morphology Crystal morphology determines the mechanical properties, the biodegradability, and the biocompatibility of polymers. Thus, it is necessary to understand the mechanism of polymer crystallization in order to control the polymer microstructure and, thereby, its properties. The crystallinity and cavitation of polymers have been studied using optical and in situ electron microscopy as complementary techniques, among many others. 

MAJOR APPLICATIONS POE is a new family of ethylene a-olefin copolymers produced using metallocene catalyst. The uncross-linked polymers referred to in this chapter are known to have only moderate elastomeric recovery properties (up to 96%). These copolymers are characterized by a narrow molecular weight distribution (MWD) (M /Mn = 2-2.5) and homogeneous comonomer distribution.The control of chain microstructure by the use of metallocene catalyst makes it possible to produce poly(a-olefin) copolymers with considerably lower density, which has not been possible before using the conventional Ziegler-Natta catalyst. Some of the highly branched ethylene copolymers presented in the entry on Polyethylene, metallocene linear low-density, in this handbook may be closely related. 

Various fabrication methods have been developed in order to attain the 3D scaffold characteristics. In the case of synthetic polymer or polymer-matrix composite scaffolds, the methods include [47] solvent casting and particle leaching, phase separation, extrusion, gas foaming, and free form fabrication. Each method presents certain advantages with respect to others, ranging from ease of manufacture to control of the microstructure/nanostructure. Solvent casting and phase separation methods have been studied at our laboratory. 

Figure 3.4 Schematic representations of polymer microstructures accessible through controlled radical polymerization techniques.

It is known that the physical properties of a polymer depend not only on the type of monomer(s) comprising it, but also on the secondary and tertiary structures, i.e., the stereochemistry of the linkage, the chain length and its distribution, its ability to crystallize or remain amorphous under various conditions, and the shape or distribution of the shapes of the chain in the crystalline and amorphous states. Through advances in polymer chemistry, in most cases polymers can be designed with specific properties. Control of the microstructure, e.g., the tacticity and molecular weight distribution of vinyl polymers, has been the focus of a number of papers in the last two decades. 

Polymer Property Advantages. Metallocene catalysts, through variation of catalyst structure, can produce a broad spectrum of polymer microstructures leading to a very wide property envelope which is potentially accessible. Because of the defined molecular structure of the catalyst, once a catalyst is chosen for a given application, the properties can be precisely controlled. 

The final part of this book features tactic polymerizations of functional and nonolefinic (ring-opening) monomers— materials for which many aspects of polymer stereochemistry and microstructure control are very different than for simple polyolefins. Acrylate, epoxide, and lactide polymerizations are addressed in this part, along with tactic olefin/carbon monoxide co- and terpoly-mers. These final chapters provide an expanded view of polymer microstructures and stereocontrol strategies, such as enantiomer-selective polymerization, that may be less familiar to the polyolefin-minded chemist and serve to enhance the reader s overall understanding of stereoregular polymers and polymerization. 

This chapter discussed only a subset of the work conducted in the field of ROMP in the last 10 years, showing that. ROMP is a privileged polymerization method for the preparation of highly functionalized polymers and is used in almost every contemporary polymer research field. ROMP is fast, functional-group tolerant, reliable, flexible, and versatile, and allows the synthesis of a broad spectrum of different polymer architectures. However, precise control of the microstructure of the polymers is still a challenge, particularly in case of ruthenium-based initiators, which, in practice, are the most commonly employed. 

The type of manufacturing process, reaction conditions, and catalyst are the controlling factors for the molecular structure of the polymers [4-8]. The molecular features govern the melt processability and microstructure of the solids. The formation of the microstructure is also affected by the melt-processing conditions set for shaping the polymeric resin [9]. The ultimate properties are, thus, directly related to the microstructural features of the polymeric solid. 

There are methods to manipulate the backbones of polymers in several areas that include control of microstructures such as crystallinity, precise control of molecular weight, copolymerization of additives (flame retardants), antioxidants, stabilizers, etc.), and direct attachment of pigments. A major development with all this type action has been to provide significant reduction in the variability of plastic performances, more processes can run at room temperature and atmospheric pressure, and 80% energy cost reductions. 

Until the early 1970s, the absence of suitable techniques for probing the detailed microstructure of polymers or for examining the selectivity and rates of radical reactions prevented the traditional view front being seriously questioned. In more recent times, it has been established that radical reactions, more often than not, are under kinetic rather than thermodynamic control and the preponderance of... 

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