湘大高分子专业英语Charpter-2-3

Charpter2-32.3.FreeRadical(Addition)Polymerization2.3.1InitiatorsforFreeRadicalPolymerizationFreeradicalinitiationcanoccurthroughapplicationofheat(thermal),(ultravioletandvisible)light(photochemical),(ionizing)light,redoxreagents,electricity(electrochemical),etc.,thatisanyprocessthatcreatestheessentialfreeradicals.Whileapplicationofheatcanrupturethepibondinthevinylmonomercausingtheformationofatwo-headedfreeradicalthatcanactasafreeradicalinitiator,peroxidesanddinitrilesaregenerallyemployed.ThisisaconsequenceofthegeneralbonddissociationenergytrendofC-HC-CC-NO-O.Dinitrileorazocompoundssuchas2,2’-azo-bis-isobutyronitrile,AIBN,requiretemperaturesofabout70–80oCtoproducedecompositionwithfreeradicalformation.2.3.自由基（加成）聚合2.3.1自由基聚合的引发自由基引发可以通过应用热（热），（紫外可见光）光（光化学），（电离）光，氧化还原试剂，电力（电化学）等发生，即任何一个都可以制造自由基过程必需的条件。而施加热可以切断乙烯基单体的π键引起的双头自由基，可作为自由基引发剂的形成，过氧化物和二氰基化合物通常采用（这种方式）。一般键离解能趋势排序结果C-HC-CC-NO-O。二氰基化合物或偶氮化合物，如2,2‘-偶氮二异丁腈，AIBN，要求温度约70–80oC才能分解产生自由基。Peroxidessuchasbenzoylperoxide,BPO,requiretemperaturesintherangeof60–140oCfordecompositionandfreeradicalformation.WhilethedissociationbondenergyforC-NisgenerallygreaterthanforO-O,theformationofastableN2moleculeisthethermodynamicdrivingforceduetoanentropyeffectallowingdissociationtooccurattypicallylowertemperatures.Lightintheultravioletandvisiblerangecandisruptselectedbondsformingfreeradicals.Suchdisruptionoccursasweareexposedtosunlight.Suntantreatmentsoftencontaincertaincompoundsthatcanacceptthisdamagingradiation.Relatedcompoundsarealsousedinfoodstogivethemlongershelflife.Theyaregenerallyknownasantioxidants.Syntheticantioxidantsincludebenzophenones,benzils,andcertainorganicketones.Thus,diphenylketonedecomposesonexposureofultravioletradiationoftheappropriatewavelengthformingtwofreeradicals.Theadvantageofusingsuchphotochemicalinitiationsisthatpolymerizationcanbecarriedoutatroomtemperature.过氧化物，如过氧化苯甲酰，BPO，需要温度在60–140oC的范围下分解和自由基的形成。而解离的键能为C-N一般比O-O大，（这是由于）形成一个稳定的N2分子形成了由于熵效应产生热力学驱动力，（这导致）在通常较低的温度（也可以）发生解离。在紫外可见光范围内的光可以破坏自由基选定的键形成。发生这种干扰，因为我们暴露在阳光下。防晒处理往往含有某些化合物，可以接受这种伤害性辐射。有关的化合物也可用于食品中，使它们得到的更长的保质期。它们一般被称为抗氧化剂。合成抗氧化剂包括二苯甲酮，苯偶酰，和某些有机酮。因此，在适当的波长紫外线辐射的照射下二苯基酮分解形成两个自由基。使用这样的光引发剂的优点是，聚合反应可在室温下进行。Whenmoleculesareexposedtolightofhigherenergy,shorterwavelengthorhigherfrequency,electronscanberemovedoraddeddependingonthespecificconditions.Usualformsofionizingradiationemployedindustriallyandexperimentallyincludeneutrons,X-rays,protons,andalphaandbetaparticles.Asimplifiedschemeillustratingfreeradicalformationisgivenfollowing.Intruth,theprecisemechanismisgenerallymorecomplexbecausevariousradicals,cations,andanionswillbeformed.Ionizingradiationinducedpolymerizationsaregenerallycarriedoutatroomtemperatureandhigher.Here,thereactantsareselectedsothatthefreeradicalsaremorestablethanthecationsandanionsallowingalargelyfreeradicalpolymerizationtooccur.当分子被暴露于更高的能量，更短的波长或频率较高的光下时，根据具体条件下电子可以被移除或加入。电离辐射在工业和实验采用的通常的形式包括中子，X射线，质子，α和β粒子。下面给出了一个简化的方案来说明自由基的形成。事实上，精确的机理一般比较复杂，因为各种自由基，阳离子和阴离子将形成。电离辐射诱导的聚合反应是通常在室温和更高温度下完成。此处，对反应物进行选择，使得自由基是比阳离子和阴离子更稳定，允许主要发生自由基聚合。Oxidation-reduction,redox,reactionsarealsooftenemployedtoinitiatefreeradicalpolymerizationsinsolutionorheterogeneousorganic–aqueoussystems.Freeradicalscanbecreatedbypassingacurrentthroughareactionsystemsufficienttoinitiatefreeradicalpolymerizations.Thus,solutionscontainingsaltsofcarboxylicacidsoxidizeattheanodeofanelectrochemicalcellformingfreeradicalswhencurrentisapplied.Suchfreeradicalscanbeusedtoinitiatefreeradicalpolymerizationsat,below,oraboveroomtemperature.Herewewillfocusonthesimpleheatinduceddecompositionofclassicalfreeradicalinitiators.氧化-还原，氧化还原，反应也常用来引发自由基聚合反应在溶液或多相有机-水系统。自由基可以产生，通过使电流通过一个反应系统足以引发自由基聚合反应。因此，含羧酸盐的溶液在氧化的电化学电池形成自由基的阳极当施加电流时。这样的自由基，可用于在低于或高于室温（的条件下）引发自由基聚合反应。在这里，我们将重点放在传统的自由基引发剂的简单的热诱导分解。Therateofdecompositionofinitiatorsusuallyfollowsfirst-orderkineticsandisdependentonthesolventpresentandthetemperatureofpolymerization.Therateisusuallyexpressedasahalf-lifetime(t1/2)wheret1/2=In2/kd=0.693/kd.Therateconstant(kd)changeswithtemperatureinaccordancewiththeArrheniusequationasshownbelow:TherateconstantsforseveralcommoninitiatorsarelistedinTable.TableRateConstantsforCommonInitiatorsinVariousSolventsa引发剂的分解速率通常遵循一级动力学，且取决于所使用的溶剂和聚合的温度。分解速率通常表示为一个半衰期时间（t1/2），其中t1/2=In2/kd=0.693/kd。速率常数（kd）随温度变化符合如下图所示阿伦尼乌斯方程：:几种常用的引发剂的速率常数列于表。表常用的引发剂在各种溶剂中的速率常数Typicalequationsforthedissociationof2,2’-azobisisobutyronitrile(AIBN)andbenzoylperoxide(BPO)areshownbelow.Itshouldbepointedoutthatbecauseofrecombination,whichissolvent-dependent,andothersidereactionsofthefreeradicals(R⋅),theinitiatorefficiencyisseldom100%.Hence,anefficiencyfactor(f)isemployedtoshowthepercentageofeffectivefreeradicalsproduced.Theprecisestructureoftheinitiatingagentandinitialadditiontothemonomervariesaccordingtothereactionconditions,monomer,andinitiator.Forillustrationtheperoxidebondinbenzoylperoxide,BPO,breaksformingtwobenzoylfreeradicals.以下为2,2‘-偶氮二异丁腈（AIBN）和过氧化苯甲酰（BPO）解离的典型公式。应当指出，由于重组，这是溶剂依赖性，和其它副反应自由基(R⋅)，引发剂效率是很少的100％。因此，效率因子(f)是用来显示所产生的有效的自由基的百分比。引发剂精的确结构初始除了各种各样的单体外