Keywords

8.1 Introduction

Pesticides are the substances used to kill, suppress, or control any form of plant or organisms that can cause damage to human health or property. Agricultural products such as cereals, vegetables, and fruits are an essential part of the diet which provides a variety of nutrients that are required for metabolic reactions of the body. During the production and storage, these products are attacked by a variety of pests. Pests cause damaging of the agricultural products and reduce the quality and quantity as well. Various pesticides along with other pest management techniques are used to reduce the losses of these products and maintain their quality [1]. The uses of pesticides have attracted much attention as they have rapid action and require less labor than other pest control methods. Pesticides can also improve the nutritional value and quality of food [2]. Despite their usefulness and popularity, pesticides can cause many health risks arising from their exposure and from residues in food and drinking water [3]. Pesticide residues are the main source of soil and water pollution. The greater amount of pesticides is used for a longer period of time in conventional agrochemical application methods [4].

After the application of traditional pesticide formulations to the crops, the effective availabilities are usually less than 30% due to losses. More than 70% of pesticide does not reach to the target organisms and spreads in a wide area through water, soil, and air [5, 6]. These losses occur by wash off by rainwater, leaching, precipitation, and volatilization in the environment. Waxy cuticle and root surfaces of the plant also absorb pesticides, and they enter into the transport system of the plant. Degradation due to photolysis in the presence of sunlight, etc., is among the routes that reduce the amount of unused pesticide, and they also leave their residues in the environment and pose health hazards. The presence of pesticide residues in the environment is of great concern for researchers as pesticides have the potential to pose harmful effects and cause diseases to humans and other non-targeted organisms. Pesticides can also interfere with the reproductive systems thus initiating the fetal development. Pesticides also have the potential to cause cancer and asthma [7]. Some of the pesticides are sustained and remain in the body causing long-term exposure.

In the current scenario, the plant protection is based on the proper utilization of pesticide, and it is the most economical way of getting a high yield of good quality food but not compromising with the environment. The use of pesticides indiscriminately would result in harmful effects to the environment, people, and animals [8]. Researchers have been constantly trying to produce new formulations for the controlled release of pesticides for the protection of the environment and human health. Various mixtures of chemical agents to control pests are known as pesticide formulations. Special formulations of pesticides improve effectiveness, storage, safety, and handling. A pesticide formulation has an important role in its effectiveness and safety concerns. An appropriate formulation for a particular application can be obtained by considering type of pesticide, applicators safety, pest biology, available equipment for application, and final product cost. Various classes of polymers such as plastics, elastomers, and fibers are largely used in agriculture for increasing water-holding capacity and achieving controlled release of pesticides and nutrients [9, 10]. Polymers, in the form of micro-nanocarriers, beads, granules and gels, are very important materials for the development of controlled release formulations (CRF). CRF provide slow and controlled release of pesticides and also enhance the water-holding capacity of the soil [11]. After degradation, these formulations convert into compost and enhance soil nutrients [12]. CRF of pesticides are defined as depot systems which continuously release pesticide into the environment for a long time (months to years). According to this definition, such formulations can be successfully employed where a chronic exposure to biologically active compounds is required over a longer period. The use of polymeric formulations to solve the problem of pesticide loss and their accumulation in the environment is shown in Fig. 8.1.

Fig. 8.1
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Use of polymeric formulations to solve the problems of pesticide loss and their accumulation in environment

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A variety of polymers have been extensively used to control the release rates, molecular motilities, and the period of effectiveness depending on their end application. Polymeric dispersants offer superior stability by strong adsorption to the surface along with multiple anchoring points. The resultant effect is that the dispersant will not be disrupted by the incorporation of an adjuvant leading to a stable and highly efficacious formulation. Polymer-based controlled release systems offer various advantages over conventional formulations to avoid excessive use of active agrochemicals thus providing the most suitable technical solution of the pesticide residues [13]. The major benefit of the CRF is that if fewer amounts of agrochemicals are used for the protection of plants for a predetermined period, then it will reduce volatilization, leaching, and degradation of pesticide.

In 2016, the global market for controlled release pesticide size was estimated to be 1.7 billion USD, and up to 2025, it is expected to grow at a CAGR of 7.3%. The increasing demand of food for growing population is expected to drive more quanta of research in the field of CRF in the future [14].

8.2 Types of Polymers Used in Formulation

For the release of pesticides, many natural and synthetic polymers have been largely used in designing their formulations. Natural polymers are environment-friendly and easily degradable whereas synthetic polymers provide better stability to pesticide release carriers. Major classes of some natural and synthetic polymers used in making pesticide formulations are shown in Fig. 8.2.

Fig. 8.2
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Major classes of some natural and synthetic polymers used in making pesticide formulations

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List of some natural and synthetic polymers which are used in formulation for pesticides are given in Table 8.1.

Table 8.1 Some natural and synthetic polymers used in pesticide formulations
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8.2.1 Natural Polymers

Nowadays, natural polymers are gaining increasing attention over synthetic polymers in pesticide release formulations because of their easy availability, eco-friendly nature, cost-effectiveness, and biodegradability [35]. The following are major classes of natural polymers which are used in polymeric formulations of pesticides.

8.2.1.1 Polysaccharides

Polysaccharides are most widely used in controlled release pesticide formulations. Due to their ready availability, fast degradation, low cost and wide variety polysaccharides facilitate large-scale production of these formulations. Some of the known examples are amylose, cellulose, pectin, alginate, etc. On the basis of degradation behavior, polysaccharides are classified into (a) starch and systems based on amylase which are readily degradable, and (b) other polysaccharides such as cellulose and derivatives, dextran, chitin, chitosan, alginate, and guar gum which degrade slowly as compared to starches [36]. These macromolecules can be cyclic (cyclodextrin) or linear (chitosan) with positive or negative charges or even neutral [37]. Many polysaccharides can be used for the controlled release of agrochemicals by ionotropic gelation process by using multivalent metal ions [38,39,40]. Polysaccharides are suitable for all type of formulations including hydrogel, micro/nanoparticles, micro/nanocapsules, beads, and emulsions. Polysaccharides are hydrophilic in nature and are most suitable for hydrophilic pesticides as a matrix system. Hydrophobic pesticides can also be loaded into cross-linked micro/nanocapsules or used in the form of nano-emulsions.

8.2.1.2 Proteins

Proteins are another major class of natural polymers which are used in controlled release formulations of pesticides. Gelatin [29], casein, and albumin [31] are some of the common proteins which are largely used in these formulations. A herbicide [(4-chloro-2-methylphenoxy) acetic acid] formulation with gelatin clay composite was prepared and release of herbicide was investigated [41]. Proteins are also used in combination with polysaccharide to give structural stability.

8.2.1.3 Phospholipids

Many natural phospholipids from plants and animal origin such as lecithin are used in the preparation of liposomal delivery systems for pesticides. Natural phospholipids are preferred over synthetic phospholipids for the preparation of liposomes as they are cheap and largely available with reproducible results. Natural phospholipids obtained from vegetable sources such as soybeans, canola seed, wheat germ, sunflower, and flaxseed, and from animal sources, such as milk and egg yolk, can be used to produce low-cost liposomes for controlled release of pesticides. Natural phospholipids are also well accepted by regulatory authorities and are produced using less chemicals and solvents at higher yields [42].

8.2.1.4 Other Natural Polymers

Lignin which is a complex organic molecule found in support tissues of vesicular plants, and some algae are also widely used in agriculture for controlled release formulations [43]. Polylactic acid (PLA) which is a FDA-approved material is used as carriers for active agents [44, 45]. Liu et al. used polylactic acid for preparing carriers for controlled release of Lambda-Cyhalothrin via premix membrane emulsification. They prepared three types of carriers including microspheres, microcapsules, and porous microcapsules which are shown in Fig. 8.3.

Fig. 8.3
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SEM images of the a microspheres, b microcapsules, and c porous microspheres before and after cut by a super thin blade [21]

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8.2.2 Synthetic Polymers

Many biodegradable synthetic polymers such as polyvinyl alcohol, polyacrylamide [46] are frequently used in the fabrication of slow-release formulations for pesticides. Although polyacrylamide gels are frequently used for encapsulation of pesticides, other polymers such as polyethylene, divynilbenzene, copolymers of acrylic acid and copolymers of cyclopentadiene with a glyceryl ester of an unsaturated fatty acid have also been used by researchers for designing pesticide release applications [47]. Some pesticides containing monomers are copolymerized with acrylamide and other hydrophilic co-monomers such as 4 vinyl pyridine [48]. Many pesticides such as pentachlorophenol (PCP), 2, 4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid are used as pendant groups in polymers [49]. Vinyl monomers containing pentachlorophenol pesticide via an ester linkage was prepared and homo- and copolymerized with 4-vinylpridine and styrene to induce hydrophilic and hydrophobic nature to the polymers [50]. Researchers combined chloropyriphos with polyethylene which can control mosquito larvae for about 18 months by one application itself [51]. Poly (vinyl alcohol) was widely used in agriculture for studying controlled release of agrochemicals [52]. Furthermore, the pesticides Azadirachtin A was physically bounded to both the PVA and poly (vinyl acetate) for its controlled release [53].

8.3 Diffusion of Pesticide into Polymer

Pesticides can diffuse into a polymer forming monolithic or reservoir-type formulations. In monolithic type, the pesticide is uniformly dispersed in a polymer matrix whereas in reservoir type the pesticide is covered by polymeric membrane as in case of capsules, laminates, liposome, coatings, etc.

8.3.1 Reservoir System

Reservoir-type formulations essentially consist of an enclosure system which exploits the diffusion of pesticides through a non-porous membrane as wall material for adjustment of the migration rates. Encapsulation ranges from macro to micro and nanoscales. In these systems, pesticides in the form of solid particles, liquids, or dispersions of solid particles in liquids, form a nucleus which is enclosed in a polymeric film. In reservoir-type formulation, the migration is controlled by membranes in which the transport of pesticides is controlled by Fick’s law

$$J = {\text{d}}Mt/A{\text{d}}t = ( - D{\text{dcm}})/{\text{d}}x$$

where J  = Flux in g/cm

cm= Concentration of pesticide in g/cm2 of the polymeric membrane

dcm/dx = Concentration gradient

D  = Diffusion coefficient of the pesticide in cm2/s in polymeric membrane

A  = Surface area in cm2, through which diffusion takes place

M  = Mass of agent releaseddM/

dt  = steady-state release rate at time t.

8.3.1.1 Laminates

In laminate systems, a special type of membrane-controlled pesticide formulations can be produced. Laminates are obtained by the reciprocal bonding of different layers of sheet-like materials. For example in a three-layer formulation, the middle layer act as a reservoir containing pesticide in it and the other two outer layers act as barriers for controlling migration of pesticide. At the surfaces, the pesticides are released by leaching, evaporation, degradation, moisture wind dust, or mechanical contact by human or animals [54]. Laminates are used for the release of insect pheromones and insect attractants for insect control. A typical laminate system is shown in Fig. 8.4.

Fig. 8.4
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Illustration of laminates-type controlled release system

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8.3.1.2 Capsules

Capsules are core–shell type structure where core is made up of pesticide which is surrounded by polymeric shell. They are usually in the range of nano to microsize. In the past decade, various pesticides were encapsulated into micro and nanocapsules [18, 33, 55]. Capsules can be mononuclear or polynuclear [56].

In the mononuclear capsules, only one depot of pesticide is present in core, whereas in multinuclear one, many small depots of pesticide are present. Chuxiang Sun et al. prepared nanocapsules of cross-linked carboxymethyl chitosan for the encapsulation of hydrophilic pesticide methomyl [18]. SEM and tem images of prepared nanocapsules are shown in Fig. 8.5 which clearly reveal core–shell nature of the capsules.

Fig. 8.5
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TEM (left) and SEM (right) images of cross-linked nanocapsules

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Latheef et al. prepared microcapsules of poly (methyl methacrylate) (PMMA), ethyl cellulose, poly (α-methylstyrene) and cellulose acetate with butyrate insecticide sulprofos contained in the core. The authors found the best results with ethyl cellulose formulations against eggs and larvae of the tobacco budworm Heliothis virescens in cotton plants [57].

Controlled release capsules of chitosan, alginate, and gelatin mixture were prepared by cross-linking biopolymers with glutaraldehyde. The capsules were loaded with the insecticides temephos [29], malathion and spinosad [54]  and their pesticticidal activities were studied against Culex pipiens larvae.

8.3.1.3 Liposomes

Liposomes are lipid-based nonmaterial with hydrophic core encapsulated in a lipid bilayer [58]. Liposomes are also known as fatty acid vesicles [59], and it has been reported that the pesticides encapsulated in liposomes demonstrate superior action due to the prolonged persistence at reduced damage to other non-targeted organisms [60]. Liposomes can carry both hydrophilic and hydrophobic pesticide [61]. Hydrophobic pesticide can be encapsulated between lipid bilayer and hydrophilic in the core. Both types of liposomes can be made sticky so that they remain on leaves for longer times and do not wash off to the ground. Inexpensive liposomes can also be produced from synthetic lipids and natural lipids for the controlled release of pesticides. Hwang et al. prepared liposome of chitosan-coated lecithin for the controlled release of etofenprox [62].

8.3.1.4 Coatings

Mesoporous silica nanoparticles with polymer coatings have been developed to achieve controlled release of pesticides. The tunable pore size, low cost, large surface area, good-loading capacity, and low cost make them ideal carriers for controlled release of pesticides [63, 64]. Lidong et al. successfully coated a water-soluble chitosan (CS) derivative (N-(2-hydroxyl) propyl-3-trimethyl ammonium CS chloride on the surfaces of the mesoporous silica nanoparticle loaded with pyraclostrobin. The loading efficiency was greatly improved by the coating and the material showed excellent fungicidal activity against Phomopsis asparagi (Sacc.) in half dose [65]. Sato et al. developed a slow-release system with a certain lag time for Imidacloprid. They mixed Imidacloprid with bentonite and clay for the preparation of core granules. The core granules were coated with a mixture of high-density polythene and talc by using spouted bead coating system [66].

8.3.2 Matrix System

In matrix-type formulations, the pesticide is heterogeneously dispersed or dissolved in a solid polymeric matrix, and this can be either biodegradable or nonbiodegradable. The release of pesticide is generally controlled by diffusion through the matrix, chemical, or biological erosion, or through a combination of diffusion and erosion. The release by erosion is surface area-dependent, and the general expression that describes the rate of release Rr by an erosion mechanism is given as,

$${\text{Rr}} = {\text{d}}M/{\text{d}}t = {\text{KECo}}\,A$$

where KE= erosion rate constantA = exposed surface area

Co=concentration of loaded pesticide in the matrix.

Matrix systems are easy to produce than the reservoir systems, but the zero-order release cannot be obtained by these systems. These systems are easy to produce than the reservoir devices [67]. A major factor affecting the release process is the porosity or free volumes within the matrix of the plastic material the magnitude of which is controlled by the processing conditions and incorporated additives [68]. Volova and coworkers used poly(3-hydroxybutyrate) as a matrix for slow-release formulations of the herbicide metribuzin. Physical mixtures of polymer and pesticide in the form of solutions, powders, and emulsions were used to construct different metribuzin formulations in the form of granules, pellets, and microparticles [23].

8.3.2.1 Particles

Micro and nanoparticle prepared from a variety of natural and synthetic polymers have attracted attention of researchers in the area of controlled release of pesticide. The size of particles has great influence on the release behavior. Grillo et al. developed biodegradable polymeric microparticles of poly (hydroxybutyrate) (PHB) or poly(hydroxybutyrate-valerate) (PHBV), and the authors noticed that the herbicidal activity in microparticles formulation was found better as compared to the conventional formulations [69]. Faria et al. prepared calcium alginate microparticles for controlled release of tebuthiuron [70]. Liu et al. incorporated tebuconazole and chlorothalonil fungicides in polymeric nanoparticles prepared from polyvinyl pyridine and polyvinyl(pyridine-co-styrene) with 10 and 30% styrene, respectively. The mean diameter of particles was found in the range of 100–250 nm, and it increases with increase in the styrene content [71].

8.3.2.2 Pellets

PVA and 1,8-cineole mixture were prepared by dry mixing method. The aim of the present study was the evaluation of controlled release time and efficiency of 1 g insecticide pellets prepared using the eucalyptol, botanic constituent, and poly (vinyl alcohol), a biodegradable polymer, under laboratory condition [72].

8.3.2.3 Beads

Beads are the simplest formulations for the controlled release of pesticides. They are normally prepared by cross-linking of preformed polymers. The beads of polyacrylamide-g-guar gum and sodium alginate were prepared by cross-linking the IPN of grafted polymer and alginate with gluteraldehyde. The as-prepared beads were loaded with chlorpyrifos and liquid fenvelarate for their controlled release [16]. Hydrogel beads of calcium alginate and nickel alginate were prepared and assessed for the controlled release of carbaryl insecticide [15].

8.4 Formulation Methods

The release rate of pesticide from the polymer matrix is highly influenced by the following factors:

Chemical nature of the pesticide—polymer bond such as esters, amides, and acetals.

  • The distance of the pesticide molecule from the polymer backbone

  • Rate of breakdown of the bond between pesticide and the polymer by chemical, biological, or environmental agents.

  • Biodegradation of polymer.

  • Structure and dimension such as degree of polymerization, solubility, degree of cross-linking, co-monomers, and the stereochemistry of the polymer.

In order to control these parameters, the pesticides are either chemically bounded or physically incorporated into a polymer matrix by different techniques. The migration of the substances is, therefore, preceded by chemical reactions for bond cleavage or physical transport processes in and through polymers.

8.4.1 Physical Combination of Polymer and Pesticide

The active agent is dissolved, dispersed, or encapsulated within the polymeric matrix or coating. The release takes place through diffusion or biological/chemical degradation of the polymer. The polymer acts as a rate-controlling device in physical combination [73]. The physical combination broadly comprises laminated, reservoir, and monolithic systems [54].

8.4.1.1 Interfacial Polymerization

Morgan and co-researchers first described the principles of interfacial polymerization [74]. In interfacial polymerization, the polymerization reaction occurs at the interface of the two immiscible liquids [75]. For interfacial polymerization, two reactive monomers are dissolved in two immiscible solvents to facilitate their contact only at interface. In this way, the polymerization occurs only at the interface forming a polymeric film. Interfacial polymerization is largely employed for the formation of microcapsules consisting of liquid pesticide inside the polymeric membrane. In the formulation, generally one phase is an aqueous and the other phase is organic. If the aqueous phase is dispersed in organic phase, the core will be hydrophilic and can be used for encapsulation of hydrophilic pesticide whereas inverting the phases would result in a hydrophobic core which can be used for hydrophobic pesticides. Microcapsules of polyamide, polyurathanes, polyurease, and polyesters can be prepared by interfacial polymerization [76]. Moghbeli et al. prepared polyurea microcapsule containing ethion pesticide by interfacial polycondensation [77].

8.4.1.2 Coacervation

Encapsulation of pesticide by the coacervation phase-separation technique works in the liquid phase and generally consists of three steps.

  • Formation of three immiscible chemical phases

  • Deposition of coating

  • Hardening of the coating.

The pesticide and the polymer materials are initially present as different phases in the form of an emulsion or suspension. During mixing, the polymer encloses pesticide, through adsorption, around a nucleus of the pesticide. The polymer phase is subsequently precipitated (solidified) by means of thermal treatment, cross-linking reaction or desolvation [78]. The solvent used in this process must not dissolve the pesticide to obtain discrete capsules. Particle sizes of capsules are significantly affected by the mixing rate. The best example of simple coacervation is the encapsulation of core material in gelation [79].

The coacervation-phase separation can be divided into two categories, simple coacervation and complex coacervation. In simple coacervation, a strongly hydrophilic substance is added to a colloidal solution which results in the formation of two phases. The complex coacervation is a pH-dependent process where the microcapsules are produced by manipulating acidity or basicity of the system. The microcapsule formation takes place only above a critical pH. Complex coacervation deals with the system containing more than one colloid [80].

8.4.1.3 Ionic Gelation

Ionic gelation involves the interaction of an ionic polymer with oppositely charged ion to initiate cross-linking [81]. Pesticide-loaded matrix of various size ranges can be prepared by ionic gelation method in which the pesticide containing polymer solution is poured dropwise into the solution of metal ions. The best example of ionic gelation is the solution of sodium alginate and pesticide added drop-wise into calcium chloride solution. This method is very useful for the preparation of hydrogel-type release system where release of pesticide depends on swelling of polymer matrix. Isıklan encapsulated carbaryl in the alginate beads by the ionotropic gelation of sodium alginate (NaAlg) with calcium and nickel ions [15]. Kumbar prepared interpenetrating polymer network beads by ionic gelation technique. Solution of polyacrylamide-grafted-guar gum with sodium alginate containing chlorpyrophos/fenvelarate pesticides was dropped into the aqueous solution of gluteraldehyde and hydrochloric acid [82]. Micro and nanoparticles of polysaccharides were also prepared by ionic gelation process, but some defects such as improper surface morphology, high dispersibility index, and fragile particulate system can be encountered [83, 84].

8.4.1.4 Spray Drying and Spray Congealing

Spray drying is a single-step microencapsulation technique for different ingredients in various applications such as cosmetics, food materials, and agrochemicals [85,86,87]. In this technique, a liquid is rapidly transformed into a dried powder [88]. The four important steps of spray drying are atomization, contact of droplet with hot gas, evaporation, and powder separation. The main advantage of this technique is that one can use different forms such as emulsion, suspension, solution, and slurries via this technique [89, 90].

Spray drying has been extensively used for many years for microencapsulation of active agents. Both synthetic and natural polymers such as ethyl cellulose, gums, maltodextrin, polylactic acid (PLA), poly(lactic-co-glycolic acid) (PLGA), chitosan, and poly(ɛ-caprolactone) (PCL), are widely used materials to encapsulate different pesticides [91, 92]. For microencapsulation of pesticide, the pesticide is dispersed in a liquid-coating material forming an emulsion. The emulsion is then sprayed into the heated chamber of a spray drier where rapid solidification of the coating takes place [93]. The size of the encapsulated particles depends upon the droplet size of emulsion. Spray drying process is beneficial for microencapsulation of biopesticides as spray-dried encapsulated biopesticides can be preserved for long time [94, 95].

Basic principal and equipment used for both spray drying and spray congealing are same and the only difference between the two methods is coating solidification. Coating solidification in the case of spray drying the polymeric material used for coating is dissolved in a suitable solvent whereas in spray congealing molten coating material is used. Waxes, fatty acids, and alcohols, polymers which are solids at room temperature but meltable at reasonable temperature are applicable to spray congealing [96].

8.4.2 Chemical Combination of Polymer and Pesticide

In chemical combination of polymer and pesticide, the pesticide is chemically attached to polymeric chain through covalent or ionic bonds. Here the pesticide either constitutes a part of the macromolecular backbone or is attached to it as a pendent group as shown in Fig. 8.6.

Fig. 8.6
figure 6

Illustration of chemical attachments of pesticide molecule with polymer backbone as pendent group (left) and as a part of backbone (Right)

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The polymer acts as a carrier for the pesticide in chemical combination [97] and the release of pesticide is dependent on the biological or chemical degradation by cleavage of the bonds between the polymer and the pesticide molecules [98]. The release of the pesticide is dependent on environmental conditions that break the linkages via chemical attack (hydrolytic by moisture; thermal/photo by sunlight) or biological degradation (enzymatic by microorganisms). pH of the medium, electrolyte concentration, temperature and ionic strength of the dissolution medium have great influence on degradation rate and hence on the release of pesticide also.

Only the pesticides containing a structural moiety with at least one reactive functional group can be used in this technique. For chemical combination, two approaches are utilized; in the first approach, a preformed polymer is used to form chemical linkage with pesticide; while in the second approach, a pesticide containing monomer unit or a pesticide and monomer mixture is polymerized. Amide, ester, and anhydride linkages are most common linkages formed between polymer and pesticide in their chemical combination [99].

8.4.2.1 Chemical Attachment of Pesticide with Preformed Polymer

It involves chemical modification of a preformed polymer with the pesticide via a chemical bond, leading to a polymer having the pesticide linked to the main chain as a pendant group. Pesticides containing carboxylic acid groups can be easily attached to a polymer containing labile hydrogen as found in polymers with alcohol or amine side chain. Here the pesticide is bonded to polymeric main chain as pendent group. This type of reaction is given in equation,

$${\text{Polymer}}\text{-}{\text{X}}\text{-}{\text{H}} + {\text{HCOOR}} \to {\text{Polymer}}\text{-}{\text{X}}\text{-}{\text{COR}} + {\text{H}}_{2} {\text{O}}$$

X can be sulfur, oxygen, or nitrogen.

8.4.2.2 Chemical Attachment of Pesticide with Polymerization of Monomer

This type of reaction leads to the formation of both type of polymers, either having pesticide group in main chain or pesticide group attached with main chain as pendent group. The main advantages of this method are the ability to control the molecular weight of the polymer, the weight ratio of polymer and pesticide, the hydrophobic-hydrophilic balance via appropriate co-monomers, and the distribution of groups along the backbone.

  1. A.

    Pesticide attached to polymer chain as pendent group

Copolymers containing pendant pentachlorophenol have been prepared by free radical copolymerization of the corresponding monomer. A series of vinyl monomers containing PCP via an ester linkage have been prepared which are shown in Scheme 8.1. These monomers have been homo-and copolymerized with styrene and 4-vinylpridine [100].

Scheme 8.1
scheme 1

Examples of polymerization of biologically active monomers. [97]

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  1. B.

    Polymers having the pesticide in the main polymeric chain

Heptachlor is a chlorinated dicyclopentadiene insecticide which is used to control soil insects and termites. Three copolymers of heptachlor with methacrylic acid (MAA), ethylene glycol dimethacrylate (EDMA), 4-vinyl pyridine (4-VP), divinyl-benzene (DVB) and styrene were prepared by Singh et al. The prepared copolymers are heptachlor-co-MAA-co-EDMA, heptachlor-co-4-VP-co-DVB, and heptachlor-co-Styrene-co-DVB [101].