Chapter 1
Trends in Synthetic Strategies for Making (CO)Polymers

Anbanandam Parthiban

1.1 Background and Introduction

Polymers have been an inherent part of human life for well over half a century at present. In spite of the comparably poor mechanical properties of polymers with that of metals, polymers encompass the applications of materials ranging from metals to glass and have replaced them in many applications. Light weight in combination with ease of processing as compared to that of metals and glasses are two of the most favorable characteristics of polymers. These characteristics are of great significance in the present circumstances as efforts are being made to lower energy consumption in various processes, and thus such lesser energy consumption would also accompany lesser emissions of CO2. Polymeric materials with enhanced properties are required in order to meet the ever improving technological needs in various fields. In addition to the demand in technological improvements, health concerns, predominantly about monomers, also bring in legislative changes leading to the disappearance of polymers from the markets, if not completely, in selected sectors of application where, in fact, these polymers were in use for many decades. One recent example is the polycarbonate derived from Bisphenol A. Owing to the suspected nature of Bisphenol A as an endocrine disruptor, its use in drinking water bottles has been banned recently in some of the developed countries. There has been as immense pressure to replace Bisphenol A in other applications as well. Indeed, until now, Bisphenol A is one of the widely employed monomers for making linear polymers like polycarbonates as well as thermosetting resins and adhesives based on bisepoxy compound (Figure 1.1).

Figure 1.1 Chemical structure of bisepoxy compound.

Although the use of Bisphenol-A-based polymers is likely to face continued intense scrutiny, there are interesting developments with some other polymers that date farther back such as polyethylene. Polyethylene is an interesting case. For a long time, it is the largest volume of synthetic polymer falling under the category of commodity plastic. However, there are recent trends that expand the application of polyethylene into selected specialty areas. The efforts for making ultrahigh molecular weight polyethylene (UHMWPE) and make use of these materials in offshore applications are of particular significance in this regard. With the development of processes for converting bioethanol to ethylene, a green label is being attached to polyethylene in addition to other claims like lower carbon footprint in comparison to other polymers. It is interesting to note that the nondegradable nature of polyolefins and in particular polyethylene is of immense concern for a long time because it is the largest volume of synthetic polymer and thus constitutes a major component in landfill.

Synthesis and the development of synthetic methodologies are the life blood of new materials. The challenges posed by changing and ever demanding technologies and also health and environmental concerns can be met by synthetic methods that evolve with time. It is an objective of this chapter to give an overview of interesting synthetic methods developed in the recent past. In published literature, newer methods and polymers made therefrom abound. However, any new development has to meet many if not all of the following requirements in order for the process and/or product to reach industrial scale manufacturing processes and subsequently the market:

1. Reagents and catalysts employed for making new monomers and polymers should be, preferably, available on industrial scale.
2. Requirement of low consumption of energy, which means that the process be of low or moderately high temperature and pressure in nature.
3. Processes that demand unusual machinery, very high rate of mixing, exotic reagents, solvents, and reaction conditions should be avoided.
4. Methods of purification should be simple and straightforward, preferably free from techniques like column chromatography.
5. Use of nontoxic and recyclable solvents is an important criterion to be considered for industrial scale processes.
6. It is also preferable to have processes in which lesser number of steps is involved to make the desired product.
7. Atom economical and high yielding processes are generally preferred.
8. It is also desirable to have processes that do not generate lot of waste water and do not make use of highly corrosive reagents or catalysts.

The developments discussed in following sections may be looked at by keeping the above requirements for a successful process.

Interestingly, some of the stated objectives of the synthetic methodologies developed in the recent past are as follows:

1. To make polymers of well-defined or predetermined molecular weights and low polydipersities.
2. Control over (co)polymer architectures.
3. Sequence-regulated polymerization.
4. Formation of reversible covalent bonds.
5. Self-healing or self-repairing polymers.
6. Recyclable thermoset polymers.
7. Chain-shuttling polymerization.

Although many of the abovementioned developments have generated immense interest only among academic communities and thus resulted in enormous volumes of publications, these are nevertheless worth noting on account of very interesting material characteristics achieved through these developments.

1.2 Significance of Control Over Arrangement of Monomers in Copolymers

Developments in controlled radical polymerization had led to the formation of polymers of varying structures such as block copolymers, cylindrical brushes, gradient copolymers, graft copolymers, hyperbranched polymers, macrocycles, and miktoarm stars. Each of these polymers possessed unique characteristics that were absent in the corresponding linear polymers, although in terms of chemical composition they were alike. An interesting case is the gradient copolymers whose physical properties differed considerably from the corresponding block and random copolymers of similar chemical composition as given in Table 1.1 [1].

Table 1.1 Comparison of Block and Gradient Copolymers of Poly(styrene-co-methylacrylate)

1.3 Chain-Growth Condensation Polymerization

Among the various polymerization techniques, step-growth or condensation polymerization has its own place in making polymeric materials with unique properties. A large majority of condensation polymers are engineering thermoplastics well known for their high temperature properties, crystallinity, excellent mechanical properties, and so on. Polyesters as represented by poly(ethylene and butylene terephthalate)s, aromatic and aliphatic polyamides, polyimides, a wide variety of poly(arylene ether)s such as polyether ether ketone (PEEK) and other poly(ether ketone)s, poly(ether sulfone), and poly(benzimiazole)s are some of the well-known examples of polymers formed by condensation polymerization. Condensation polymerization that typically involves AA- and BB-type monomers or AB-type monomers, where A and B represent different reacting functionalities during polymerization, generally yields polymers with polydispersity of 2 or more. However, recently, Yokozawa et al. [2] have introduced a new concept termed as chain-growth condensation polymerization whereby the molecular weights of condensation polymers such as polyamides, polyesters, and polyethers have been controlled and polydispersity of these polymers is well below the theoretically predicted 2. Some special para-substituted AB-type aromatic monomers were employed for this purpose (Scheme 1.1). By introducing an activated functional group in the AB-type monomer, a preferred reaction site was created that resulted in sequential addition of monomers.

Scheme 1.1 (a) Polyamides (I) and block copolyamides (II) prepared by step-growth polymerization.

(Reprinted with permission from [2l]. Copyright © 2002 American Chemical Society.) (b) Preparation of diblock copolymers, poly(amide-block-ether) by chain-growth condensation polymerization. (Reprinted with permission from [2m]. Copyright © 2009 Wiley Periodicals Inc.)

1.3.1 Sequential Self-Repetitive Reaction (SSRR)

Dai et al. [3] reported a sequential self-repetitive reaction by which the condensation of diisocyanate with diacid in the presence of a carbodiimide catalyst like 1,3-dimethyl-3-phospholene oxide (DMPO) led to the formation of polyamide (Scheme 1.2). The reaction is so called because of the occurrence of repetitive reactions sequentially by the following three steps:

1. Condensation of two isocyanates to yield carbodiimide.
2. Addition of carboxylic acid to carbodiimide leading to the formation of N-acyl urea.
3. Thermal fragmentation of N-acyl urea that results in...